From 5202c977d95bf65775e733264b7d37ed940a2997 Mon Sep 17 00:00:00 2001
From: mlugg <mlugg@mlugg.co.uk>
Date: Sat, 25 Jan 2025 04:47:33 +0000
Subject: [PATCH 1/4] std: add `fs.path.fmtJoin`

This allows joining paths without allocating using a `Writer`.
---
 lib/std/fs/path.zig | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/lib/std/fs/path.zig b/lib/std/fs/path.zig
index 3376771313d641d52aa5ac3dab218c0eb5b19090..b2f95a937f0537b77407eb21f942b56997a2e11e 100644
--- a/lib/std/fs/path.zig
+++ b/lib/std/fs/path.zig
@@ -18,7 +18,6 @@ const debug = std.debug;
 const assert = debug.assert;
 const testing = std.testing;
 const mem = std.mem;
-const fmt = std.fmt;
 const ascii = std.ascii;
 const Allocator = mem.Allocator;
 const math = std.math;
@@ -147,6 +146,36 @@ pub fn joinZ(allocator: Allocator, paths: []const []const u8) ![:0]u8 {
     return out[0 .. out.len - 1 :0];
 }
 
+pub fn fmtJoin(paths: []const []const u8) std.fmt.Formatter(formatJoin) {
+    return .{ .data = paths };
+}
+
+fn formatJoin(paths: []const []const u8, comptime fmt: []const u8, options: std.fmt.FormatOptions, w: anytype) !void {
+    _ = fmt;
+    _ = options;
+
+    const first_path_idx = for (paths, 0..) |p, idx| {
+        if (p.len != 0) break idx;
+    } else return;
+
+    try w.writeAll(paths[first_path_idx]); // first component
+    var prev_path = paths[first_path_idx];
+    for (paths[first_path_idx + 1 ..]) |this_path| {
+        if (this_path.len == 0) continue; // skip empty components
+        const prev_sep = isSep(prev_path[prev_path.len - 1]);
+        const this_sep = isSep(this_path[0]);
+        if (!prev_sep and !this_sep) {
+            try w.writeByte(sep);
+        }
+        if (prev_sep and this_sep) {
+            try w.writeAll(this_path[1..]); // skip redundant separator
+        } else {
+            try w.writeAll(this_path);
+        }
+        prev_path = this_path;
+    }
+}
+
 fn testJoinMaybeZUefi(paths: []const []const u8, expected: []const u8, zero: bool) !void {
     const uefiIsSep = struct {
         fn isSep(byte: u8) bool {
-- 
2.54.0


From f47b8de2ad706616b648c52f5036102cb804e65d Mon Sep 17 00:00:00 2001
From: mlugg <mlugg@mlugg.co.uk>
Date: Sat, 25 Jan 2025 04:48:16 +0000
Subject: [PATCH 2/4] incremental: handle `@embedFile`

Uses of `@embedFile` register dependencies on the corresponding
`Zcu.EmbedFile`. At the start of every update, we iterate all embedded
files and update them if necessary, and invalidate the dependencies if
they changed.

In order to properly integrate with the lazy analysis model, failed
embed files are now reported by the `AnalUnit` which actually used
`@embedFile`; the filesystem error is stored in the `Zcu.EmbedFile`.

An incremental test is added covering incremental updates to embedded
files, and I have verified locally that dependency invalidation is
working correctly.
---
 src/Compilation.zig                |  83 +++-------
 src/InternPool.zig                 |   9 ++
 src/Sema.zig                       |  20 ++-
 src/Zcu.zig                        |  33 ++--
 src/Zcu/PerThread.zig              | 233 ++++++++++++++++++-----------
 test/incremental/change_embed_file |  46 ++++++
 tools/incr-check.zig               |   8 +-
 7 files changed, 264 insertions(+), 168 deletions(-)
 create mode 100644 test/incremental/change_embed_file

diff --git a/src/Compilation.zig b/src/Compilation.zig
index 1a056c1c04a40e44e236bfc19a2cdec2557c41da..019aa498e1c99db87127a36954dc7d94169cf1c6 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -154,10 +154,6 @@ win32_resource_work_queue: if (dev.env.supports(.win32_resource)) std.fifo.Linea
 /// since the last compilation, as well as scan for `@import` and queue up
 /// additional jobs corresponding to those new files.
 astgen_work_queue: std.fifo.LinearFifo(Zcu.File.Index, .Dynamic),
-/// These jobs are to inspect the file system stat() and if the embedded file has changed
-/// on disk, mark the corresponding Decl outdated and queue up an `analyze_decl`
-/// task for it.
-embed_file_work_queue: std.fifo.LinearFifo(*Zcu.EmbedFile, .Dynamic),
 
 /// The ErrorMsg memory is owned by the `CObject`, using Compilation's general purpose allocator.
 /// This data is accessed by multiple threads and is protected by `mutex`.
@@ -1465,7 +1461,6 @@ pub fn create(gpa: Allocator, arena: Allocator, options: CreateOptions) !*Compil
             .c_object_work_queue = std.fifo.LinearFifo(*CObject, .Dynamic).init(gpa),
             .win32_resource_work_queue = if (dev.env.supports(.win32_resource)) std.fifo.LinearFifo(*Win32Resource, .Dynamic).init(gpa) else .{},
             .astgen_work_queue = std.fifo.LinearFifo(Zcu.File.Index, .Dynamic).init(gpa),
-            .embed_file_work_queue = std.fifo.LinearFifo(*Zcu.EmbedFile, .Dynamic).init(gpa),
             .c_source_files = options.c_source_files,
             .rc_source_files = options.rc_source_files,
             .cache_parent = cache,
@@ -1932,7 +1927,6 @@ pub fn destroy(comp: *Compilation) void {
     comp.c_object_work_queue.deinit();
     comp.win32_resource_work_queue.deinit();
     comp.astgen_work_queue.deinit();
-    comp.embed_file_work_queue.deinit();
 
     comp.windows_libs.deinit(gpa);
 
@@ -2247,11 +2241,6 @@ pub fn update(comp: *Compilation, main_progress_node: std.Progress.Node) !void {
             }
         }
 
-        // Put a work item in for checking if any files used with `@embedFile` changed.
-        try comp.embed_file_work_queue.ensureUnusedCapacity(zcu.embed_table.count());
-        for (zcu.embed_table.values()) |embed_file| {
-            comp.embed_file_work_queue.writeItemAssumeCapacity(embed_file);
-        }
         if (comp.file_system_inputs) |fsi| {
             const ip = &zcu.intern_pool;
             for (zcu.embed_table.values()) |embed_file| {
@@ -3235,9 +3224,6 @@ pub fn getAllErrorsAlloc(comp: *Compilation) !ErrorBundle {
                 try addZirErrorMessages(&bundle, file);
             }
         }
-        for (zcu.failed_embed_files.values()) |error_msg| {
-            try addModuleErrorMsg(zcu, &bundle, error_msg.*);
-        }
         var sorted_failed_analysis: std.AutoArrayHashMapUnmanaged(InternPool.AnalUnit, *Zcu.ErrorMsg).DataList.Slice = s: {
             const SortOrder = struct {
                 zcu: *Zcu,
@@ -3812,9 +3798,10 @@ fn performAllTheWorkInner(
                 }
             }
 
-            while (comp.embed_file_work_queue.readItem()) |embed_file| {
-                comp.thread_pool.spawnWg(&astgen_wait_group, workerCheckEmbedFile, .{
-                    comp, embed_file,
+            for (0.., zcu.embed_table.values()) |ef_index_usize, ef| {
+                const ef_index: Zcu.EmbedFile.Index = @enumFromInt(ef_index_usize);
+                comp.thread_pool.spawnWgId(&astgen_wait_group, workerCheckEmbedFile, .{
+                    comp, ef_index, ef,
                 });
             }
         }
@@ -4377,33 +4364,33 @@ fn workerUpdateBuiltinZigFile(
     };
 }
 
-fn workerCheckEmbedFile(comp: *Compilation, embed_file: *Zcu.EmbedFile) void {
-    comp.detectEmbedFileUpdate(embed_file) catch |err| {
-        comp.reportRetryableEmbedFileError(embed_file, err) catch |oom| switch (oom) {
-            // Swallowing this error is OK because it's implied to be OOM when
-            // there is a missing `failed_embed_files` error message.
-            error.OutOfMemory => {},
-        };
-        return;
+fn workerCheckEmbedFile(tid: usize, comp: *Compilation, ef_index: Zcu.EmbedFile.Index, ef: *Zcu.EmbedFile) void {
+    comp.detectEmbedFileUpdate(@enumFromInt(tid), ef_index, ef) catch |err| switch (err) {
+        error.OutOfMemory => {
+            comp.mutex.lock();
+            defer comp.mutex.unlock();
+            comp.setAllocFailure();
+        },
     };
 }
 
-fn detectEmbedFileUpdate(comp: *Compilation, embed_file: *Zcu.EmbedFile) !void {
+fn detectEmbedFileUpdate(comp: *Compilation, tid: Zcu.PerThread.Id, ef_index: Zcu.EmbedFile.Index, ef: *Zcu.EmbedFile) !void {
     const zcu = comp.zcu.?;
-    const ip = &zcu.intern_pool;
-    var file = try embed_file.owner.root.openFile(embed_file.sub_file_path.toSlice(ip), .{});
-    defer file.close();
+    const pt: Zcu.PerThread = .activate(zcu, tid);
+    defer pt.deactivate();
 
-    const stat = try file.stat();
+    const old_val = ef.val;
+    const old_err = ef.err;
 
-    const unchanged_metadata =
-        stat.size == embed_file.stat.size and
-        stat.mtime == embed_file.stat.mtime and
-        stat.inode == embed_file.stat.inode;
+    try pt.updateEmbedFile(ef, null);
 
-    if (unchanged_metadata) return;
+    if (ef.val != .none and ef.val == old_val) return; // success, value unchanged
+    if (ef.val == .none and old_val == .none and ef.err == old_err) return; // failure, error unchanged
 
-    @panic("TODO: handle embed file incremental update");
+    comp.mutex.lock();
+    defer comp.mutex.unlock();
+
+    try zcu.markDependeeOutdated(.not_marked_po, .{ .embed_file = ef_index });
 }
 
 pub fn obtainCObjectCacheManifest(
@@ -4802,30 +4789,6 @@ fn reportRetryableWin32ResourceError(
     }
 }
 
-fn reportRetryableEmbedFileError(
-    comp: *Compilation,
-    embed_file: *Zcu.EmbedFile,
-    err: anyerror,
-) error{OutOfMemory}!void {
-    const zcu = comp.zcu.?;
-    const gpa = zcu.gpa;
-    const src_loc = embed_file.src_loc;
-    const ip = &zcu.intern_pool;
-    const err_msg = try Zcu.ErrorMsg.create(gpa, src_loc, "unable to load '{}/{s}': {s}", .{
-        embed_file.owner.root,
-        embed_file.sub_file_path.toSlice(ip),
-        @errorName(err),
-    });
-
-    errdefer err_msg.destroy(gpa);
-
-    {
-        comp.mutex.lock();
-        defer comp.mutex.unlock();
-        try zcu.failed_embed_files.putNoClobber(gpa, embed_file, err_msg);
-    }
-}
-
 fn updateCObject(comp: *Compilation, c_object: *CObject, c_obj_prog_node: std.Progress.Node) !void {
     if (comp.config.c_frontend == .aro) {
         return comp.failCObj(c_object, "aro does not support compiling C objects yet", .{});
diff --git a/src/InternPool.zig b/src/InternPool.zig
index a92e93705cc9301c2aee4abf0ac2f22a1c0a69fa..514fb1b63f9645d82120231024d21ffc070f8ba4 100644
--- a/src/InternPool.zig
+++ b/src/InternPool.zig
@@ -42,6 +42,10 @@ nav_ty_deps: std.AutoArrayHashMapUnmanaged(Nav.Index, DepEntry.Index),
 /// * a container type requiring resolution (invalidated when the type must be recreated at a new index)
 /// Value is index into `dep_entries` of the first dependency on this interned value.
 interned_deps: std.AutoArrayHashMapUnmanaged(Index, DepEntry.Index),
+/// Dependencies on an embedded file.
+/// Introduced by `@embedFile`; invalidated when the file changes.
+/// Value is index into `dep_entries` of the first dependency on this `Zcu.EmbedFile`.
+embed_file_deps: std.AutoArrayHashMapUnmanaged(Zcu.EmbedFile.Index, DepEntry.Index),
 /// Dependencies on the full set of names in a ZIR namespace.
 /// Key refers to a `struct_decl`, `union_decl`, etc.
 /// Value is index into `dep_entries` of the first dependency on this namespace.
@@ -90,6 +94,7 @@ pub const empty: InternPool = .{
     .nav_val_deps = .empty,
     .nav_ty_deps = .empty,
     .interned_deps = .empty,
+    .embed_file_deps = .empty,
     .namespace_deps = .empty,
     .namespace_name_deps = .empty,
     .memoized_state_main_deps = .none,
@@ -824,6 +829,7 @@ pub const Dependee = union(enum) {
     nav_val: Nav.Index,
     nav_ty: Nav.Index,
     interned: Index,
+    embed_file: Zcu.EmbedFile.Index,
     namespace: TrackedInst.Index,
     namespace_name: NamespaceNameKey,
     memoized_state: MemoizedStateStage,
@@ -875,6 +881,7 @@ pub fn dependencyIterator(ip: *const InternPool, dependee: Dependee) DependencyI
         .nav_val => |x| ip.nav_val_deps.get(x),
         .nav_ty => |x| ip.nav_ty_deps.get(x),
         .interned => |x| ip.interned_deps.get(x),
+        .embed_file => |x| ip.embed_file_deps.get(x),
         .namespace => |x| ip.namespace_deps.get(x),
         .namespace_name => |x| ip.namespace_name_deps.get(x),
         .memoized_state => |stage| switch (stage) {
@@ -945,6 +952,7 @@ pub fn addDependency(ip: *InternPool, gpa: Allocator, depender: AnalUnit, depend
                 .nav_val => ip.nav_val_deps,
                 .nav_ty => ip.nav_ty_deps,
                 .interned => ip.interned_deps,
+                .embed_file => ip.embed_file_deps,
                 .namespace => ip.namespace_deps,
                 .namespace_name => ip.namespace_name_deps,
                 .memoized_state => comptime unreachable,
@@ -6612,6 +6620,7 @@ pub fn deinit(ip: *InternPool, gpa: Allocator) void {
     ip.nav_val_deps.deinit(gpa);
     ip.nav_ty_deps.deinit(gpa);
     ip.interned_deps.deinit(gpa);
+    ip.embed_file_deps.deinit(gpa);
     ip.namespace_deps.deinit(gpa);
     ip.namespace_name_deps.deinit(gpa);
 
diff --git a/src/Sema.zig b/src/Sema.zig
index 833b05413f3333f2ef8ef0163e161964d3b4f6ca..0b06eba519ab35398d24dad3ed384fff7fc73aa2 100644
--- a/src/Sema.zig
+++ b/src/Sema.zig
@@ -13949,6 +13949,8 @@ fn zirEmbedFile(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!A
     defer tracy.end();
 
     const pt = sema.pt;
+    const zcu = pt.zcu;
+
     const inst_data = sema.code.instructions.items(.data)[@intFromEnum(inst)].un_node;
     const operand_src = block.builtinCallArgSrc(inst_data.src_node, 0);
     const name = try sema.resolveConstString(block, operand_src, inst_data.operand, .{ .simple = .operand_embedFile });
@@ -13957,18 +13959,24 @@ fn zirEmbedFile(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!A
         return sema.fail(block, operand_src, "file path name cannot be empty", .{});
     }
 
-    const val = pt.embedFile(block.getFileScope(pt.zcu), name, operand_src) catch |err| switch (err) {
+    const ef_idx = pt.embedFile(block.getFileScope(zcu), name) catch |err| switch (err) {
         error.ImportOutsideModulePath => {
             return sema.fail(block, operand_src, "embed of file outside package path: '{s}'", .{name});
         },
-        else => {
-            // TODO: these errors are file system errors; make sure an update() will
-            // retry this and not cache the file system error, which may be transient.
-            return sema.fail(block, operand_src, "unable to open '{s}': {s}", .{ name, @errorName(err) });
+        error.CurrentWorkingDirectoryUnlinked => {
+            // TODO: this should be some kind of retryable failure, in case the cwd is put back
+            return sema.fail(block, operand_src, "unable to resolve '{s}': working directory has been unlinked", .{name});
         },
+        error.OutOfMemory => |e| return e,
     };
+    try sema.declareDependency(.{ .embed_file = ef_idx });
 
-    return Air.internedToRef(val);
+    const result = ef_idx.get(zcu);
+    if (result.val == .none) {
+        return sema.fail(block, operand_src, "unable to open '{s}': {s}", .{ name, @errorName(result.err.?) });
+    }
+
+    return Air.internedToRef(result.val);
 }
 
 fn zirRetErrValueCode(sema: *Sema, inst: Zir.Inst.Index) CompileError!Air.Inst.Ref {
diff --git a/src/Zcu.zig b/src/Zcu.zig
index c75cd5d40cb6438c1caf772b60d4ef727bab93fa..120188cedfa28570ef9a52b76e8a56eb772e4f5f 100644
--- a/src/Zcu.zig
+++ b/src/Zcu.zig
@@ -143,8 +143,6 @@ compile_log_sources: std.AutoArrayHashMapUnmanaged(AnalUnit, extern struct {
 /// Using a map here for consistency with the other fields here.
 /// The ErrorMsg memory is owned by the `File`, using Module's general purpose allocator.
 failed_files: std.AutoArrayHashMapUnmanaged(*File, ?*ErrorMsg) = .empty,
-/// The ErrorMsg memory is owned by the `EmbedFile`, using Module's general purpose allocator.
-failed_embed_files: std.AutoArrayHashMapUnmanaged(*EmbedFile, *ErrorMsg) = .empty,
 failed_exports: std.AutoArrayHashMapUnmanaged(Export.Index, *ErrorMsg) = .empty,
 /// If analysis failed due to a cimport error, the corresponding Clang errors
 /// are stored here.
@@ -893,13 +891,23 @@ pub const File = struct {
 };
 
 pub const EmbedFile = struct {
+    /// Module that this file is a part of, managed externally.
+    owner: *Package.Module,
     /// Relative to the owning module's root directory.
     sub_file_path: InternPool.NullTerminatedString,
-    /// Module that this file is a part of, managed externally.
-    owner: *Package.Module,
-    stat: Cache.File.Stat,
+
+    /// `.none` means the file was not loaded, so `stat` is undefined.
     val: InternPool.Index,
-    src_loc: LazySrcLoc,
+    /// If this is `null` and `val` is `.none`, the file has never been loaded.
+    err: ?(std.fs.File.OpenError || std.fs.File.StatError || std.fs.File.ReadError || error{UnexpectedEof}),
+    stat: Cache.File.Stat,
+
+    pub const Index = enum(u32) {
+        _,
+        pub fn get(idx: Index, zcu: *const Zcu) *EmbedFile {
+            return zcu.embed_table.values()[@intFromEnum(idx)];
+        }
+    };
 };
 
 /// This struct holds data necessary to construct API-facing `AllErrors.Message`.
@@ -2459,11 +2467,6 @@ pub fn deinit(zcu: *Zcu) void {
         }
         zcu.failed_files.deinit(gpa);
 
-        for (zcu.failed_embed_files.values()) |msg| {
-            msg.destroy(gpa);
-        }
-        zcu.failed_embed_files.deinit(gpa);
-
         for (zcu.failed_exports.values()) |value| {
             value.destroy(gpa);
         }
@@ -3882,6 +3885,14 @@ fn formatDependee(data: struct { dependee: InternPool.Dependee, zcu: *Zcu }, com
             .func => |f| return writer.print("ies('{}')", .{ip.getNav(f.owner_nav).fqn.fmt(ip)}),
             else => unreachable,
         },
+        .embed_file => |ef_idx| {
+            const ef = ef_idx.get(zcu);
+            return writer.print("embed_file('{s}')", .{std.fs.path.fmtJoin(&.{
+                ef.owner.root.root_dir.path orelse "",
+                ef.owner.root.sub_path,
+                ef.sub_file_path.toSlice(ip),
+            })});
+        },
         .namespace => |ti| {
             const info = ti.resolveFull(ip) orelse {
                 return writer.writeAll("namespace(<lost>)");
diff --git a/src/Zcu/PerThread.zig b/src/Zcu/PerThread.zig
index c6a16ee00cacf30326e4e2d0732b7d9af7ce9777..9e21b655d1d205010e8fbc0158a550c977456a49 100644
--- a/src/Zcu/PerThread.zig
+++ b/src/Zcu/PerThread.zig
@@ -2117,32 +2117,32 @@ pub fn embedFile(
     pt: Zcu.PerThread,
     cur_file: *Zcu.File,
     import_string: []const u8,
-    src_loc: Zcu.LazySrcLoc,
-) !InternPool.Index {
+) error{
+    OutOfMemory,
+    ImportOutsideModulePath,
+    CurrentWorkingDirectoryUnlinked,
+}!Zcu.EmbedFile.Index {
     const zcu = pt.zcu;
     const gpa = zcu.gpa;
 
-    if (cur_file.mod.deps.get(import_string)) |pkg| {
+    if (cur_file.mod.deps.get(import_string)) |mod| {
         const resolved_path = try std.fs.path.resolve(gpa, &.{
-            pkg.root.root_dir.path orelse ".",
-            pkg.root.sub_path,
-            pkg.root_src_path,
+            mod.root.root_dir.path orelse ".",
+            mod.root.sub_path,
+            mod.root_src_path,
         });
-        var keep_resolved_path = false;
-        defer if (!keep_resolved_path) gpa.free(resolved_path);
+        errdefer gpa.free(resolved_path);
 
         const gop = try zcu.embed_table.getOrPut(gpa, resolved_path);
-        errdefer {
-            assert(std.mem.eql(u8, zcu.embed_table.pop().key, resolved_path));
-            keep_resolved_path = false;
+        errdefer assert(std.mem.eql(u8, zcu.embed_table.pop().key, resolved_path));
+
+        if (gop.found_existing) {
+            gpa.free(resolved_path); // we're not using this key
+            return @enumFromInt(gop.index);
         }
-        if (gop.found_existing) return gop.value_ptr.*.val;
-        keep_resolved_path = true;
 
-        const sub_file_path = try gpa.dupe(u8, pkg.root_src_path);
-        errdefer gpa.free(sub_file_path);
-
-        return pt.newEmbedFile(pkg, sub_file_path, resolved_path, gop.value_ptr, src_loc);
+        gop.value_ptr.* = try pt.newEmbedFile(mod, mod.root_src_path, resolved_path);
+        return @enumFromInt(gop.index);
     }
 
     // The resolved path is used as the key in the table, to detect if a file
@@ -2154,17 +2154,15 @@ pub fn embedFile(
         "..",
         import_string,
     });
-
-    var keep_resolved_path = false;
-    defer if (!keep_resolved_path) gpa.free(resolved_path);
+    errdefer gpa.free(resolved_path);
 
     const gop = try zcu.embed_table.getOrPut(gpa, resolved_path);
-    errdefer {
-        assert(std.mem.eql(u8, zcu.embed_table.pop().key, resolved_path));
-        keep_resolved_path = false;
+    errdefer assert(std.mem.eql(u8, zcu.embed_table.pop().key, resolved_path));
+
+    if (gop.found_existing) {
+        gpa.free(resolved_path); // we're not using this key
+        return @enumFromInt(gop.index);
     }
-    if (gop.found_existing) return gop.value_ptr.*.val;
-    keep_resolved_path = true;
 
     const resolved_root_path = try std.fs.path.resolve(gpa, &.{
         cur_file.mod.root.root_dir.path orelse ".",
@@ -2172,101 +2170,156 @@ pub fn embedFile(
     });
     defer gpa.free(resolved_root_path);
 
-    const sub_file_path = p: {
-        const relative = try std.fs.path.relative(gpa, resolved_root_path, resolved_path);
-        errdefer gpa.free(relative);
-
-        if (!isUpDir(relative) and !std.fs.path.isAbsolute(relative)) {
-            break :p relative;
-        }
-        return error.ImportOutsideModulePath;
+    const sub_file_path = std.fs.path.relative(gpa, resolved_root_path, resolved_path) catch |err| switch (err) {
+        error.Unexpected => unreachable,
+        else => |e| return e,
     };
     defer gpa.free(sub_file_path);
 
-    return pt.newEmbedFile(cur_file.mod, sub_file_path, resolved_path, gop.value_ptr, src_loc);
+    if (isUpDir(sub_file_path) or std.fs.path.isAbsolute(sub_file_path)) {
+        return error.ImportOutsideModulePath;
+    }
+
+    gop.value_ptr.* = try pt.newEmbedFile(cur_file.mod, sub_file_path, resolved_path);
+    return @enumFromInt(gop.index);
+}
+
+pub fn updateEmbedFile(
+    pt: Zcu.PerThread,
+    ef: *Zcu.EmbedFile,
+    /// If not `null`, the interned file data is stored here, if it was loaded.
+    /// `newEmbedFile` uses this to add the file to the `whole` cache manifest.
+    ip_str_out: ?*?InternPool.String,
+) Allocator.Error!void {
+    pt.updateEmbedFileInner(ef, ip_str_out) catch |err| switch (err) {
+        error.OutOfMemory => |e| return e,
+        else => |e| {
+            ef.val = .none;
+            ef.err = e;
+            ef.stat = undefined;
+        },
+    };
 }
 
-/// https://github.com/ziglang/zig/issues/14307
-fn newEmbedFile(
+fn updateEmbedFileInner(
     pt: Zcu.PerThread,
-    pkg: *Module,
-    sub_file_path: []const u8,
-    resolved_path: []const u8,
-    result: **Zcu.EmbedFile,
-    src_loc: Zcu.LazySrcLoc,
-) !InternPool.Index {
+    ef: *Zcu.EmbedFile,
+    ip_str_out: ?*?InternPool.String,
+) !void {
+    const tid = pt.tid;
     const zcu = pt.zcu;
     const gpa = zcu.gpa;
     const ip = &zcu.intern_pool;
 
-    const new_file = try gpa.create(Zcu.EmbedFile);
-    errdefer gpa.destroy(new_file);
-
-    var file = try pkg.root.openFile(sub_file_path, .{});
+    var file = try ef.owner.root.openFile(ef.sub_file_path.toSlice(ip), .{});
     defer file.close();
 
-    const actual_stat = try file.stat();
-    const stat: Cache.File.Stat = .{
-        .size = actual_stat.size,
-        .inode = actual_stat.inode,
-        .mtime = actual_stat.mtime,
-    };
-    const size = std.math.cast(usize, actual_stat.size) orelse return error.Overflow;
+    const stat: Cache.File.Stat = .fromFs(try file.stat());
 
-    const strings = ip.getLocal(pt.tid).getMutableStrings(gpa);
-    const bytes = try strings.addManyAsSlice(try std.math.add(usize, size, 1));
-    const actual_read = try file.readAll(bytes[0][0..size]);
-    if (actual_read != size) return error.UnexpectedEndOfFile;
-    bytes[0][size] = 0;
-
-    const comp = zcu.comp;
-    switch (comp.cache_use) {
-        .whole => |whole| if (whole.cache_manifest) |man| {
-            const copied_resolved_path = try gpa.dupe(u8, resolved_path);
-            errdefer gpa.free(copied_resolved_path);
-            whole.cache_manifest_mutex.lock();
-            defer whole.cache_manifest_mutex.unlock();
-            try man.addFilePostContents(copied_resolved_path, bytes[0][0..size], stat);
-        },
-        .incremental => {},
+    if (ef.val != .none) {
+        const old_stat = ef.stat;
+        const unchanged_metadata =
+            stat.size == old_stat.size and
+            stat.mtime == old_stat.mtime and
+            stat.inode == old_stat.inode;
+        if (unchanged_metadata) return;
     }
 
-    const array_ty = try pt.intern(.{ .array_type = .{
+    const size = std.math.cast(usize, stat.size) orelse return error.FileTooBig;
+    const size_plus_one = std.math.add(usize, size, 1) catch return error.FileTooBig;
+
+    // The loaded bytes of the file, including a sentinel 0 byte.
+    const ip_str: InternPool.String = str: {
+        const strings = ip.getLocal(tid).getMutableStrings(gpa);
+        const old_len = strings.mutate.len;
+        errdefer strings.shrinkRetainingCapacity(old_len);
+        const bytes = (try strings.addManyAsSlice(size_plus_one))[0];
+        const actual_read = try file.readAll(bytes[0..size]);
+        if (actual_read != size) return error.UnexpectedEof;
+        bytes[size] = 0;
+        break :str try ip.getOrPutTrailingString(gpa, tid, @intCast(bytes.len), .maybe_embedded_nulls);
+    };
+    if (ip_str_out) |p| p.* = ip_str;
+
+    const array_ty = try pt.arrayType(.{
         .len = size,
         .sentinel = .zero_u8,
         .child = .u8_type,
-    } });
+    });
+    const ptr_ty = try pt.singleConstPtrType(array_ty);
+
     const array_val = try pt.intern(.{ .aggregate = .{
-        .ty = array_ty,
-        .storage = .{ .bytes = try ip.getOrPutTrailingString(gpa, pt.tid, @intCast(bytes[0].len), .maybe_embedded_nulls) },
+        .ty = array_ty.toIntern(),
+        .storage = .{ .bytes = ip_str },
     } });
-
-    const ptr_ty = (try pt.ptrType(.{
-        .child = array_ty,
-        .flags = .{
-            .alignment = .none,
-            .is_const = true,
-            .address_space = .generic,
-        },
-    })).toIntern();
     const ptr_val = try pt.intern(.{ .ptr = .{
-        .ty = ptr_ty,
+        .ty = ptr_ty.toIntern(),
         .base_addr = .{ .uav = .{
             .val = array_val,
-            .orig_ty = ptr_ty,
+            .orig_ty = ptr_ty.toIntern(),
         } },
         .byte_offset = 0,
     } });
 
-    result.* = new_file;
+    ef.val = ptr_val;
+    ef.err = null;
+    ef.stat = stat;
+}
+
+fn newEmbedFile(
+    pt: Zcu.PerThread,
+    mod: *Module,
+    /// The path of the file to embed relative to the root of `mod`.
+    sub_file_path: []const u8,
+    /// The resolved path of the file to embed.
+    resolved_path: []const u8,
+) !*Zcu.EmbedFile {
+    const zcu = pt.zcu;
+    const comp = zcu.comp;
+    const gpa = zcu.gpa;
+    const ip = &zcu.intern_pool;
+
+    if (comp.file_system_inputs) |fsi|
+        try comp.appendFileSystemInput(fsi, mod.root, sub_file_path);
+
+    const new_file = try gpa.create(Zcu.EmbedFile);
+    errdefer gpa.destroy(new_file);
+
     new_file.* = .{
+        .owner = mod,
         .sub_file_path = try ip.getOrPutString(gpa, pt.tid, sub_file_path, .no_embedded_nulls),
-        .owner = pkg,
-        .stat = stat,
-        .val = ptr_val,
-        .src_loc = src_loc,
+        .val = .none,
+        .err = null,
+        .stat = undefined,
     };
-    return ptr_val;
+
+    var opt_ip_str: ?InternPool.String = null;
+    try pt.updateEmbedFile(new_file, &opt_ip_str);
+
+    // Add the file contents to the `whole` cache manifest if necessary.
+    cache: {
+        const whole = switch (zcu.comp.cache_use) {
+            .whole => |whole| whole,
+            .incremental => break :cache,
+        };
+        const man = whole.cache_manifest orelse break :cache;
+        const ip_str = opt_ip_str orelse break :cache;
+
+        const copied_resolved_path = try gpa.dupe(u8, resolved_path);
+        errdefer gpa.free(copied_resolved_path);
+
+        const array_len = Value.fromInterned(new_file.val).typeOf(zcu).childType(zcu).arrayLen(zcu);
+
+        whole.cache_manifest_mutex.lock();
+        defer whole.cache_manifest_mutex.unlock();
+
+        man.addFilePostContents(copied_resolved_path, ip_str.toSlice(array_len, ip), new_file.stat) catch |err| switch (err) {
+            error.Unexpected => unreachable,
+            else => |e| return e,
+        };
+    }
+
+    return new_file;
 }
 
 pub fn scanNamespace(
diff --git a/test/incremental/change_embed_file b/test/incremental/change_embed_file
new file mode 100644
index 0000000000000000000000000000000000000000..3cb29be2edb31d97f3ad57aa92907245689b138d
--- /dev/null
+++ b/test/incremental/change_embed_file
@@ -0,0 +1,46 @@
+#target=x86_64-linux-selfhosted
+#target=x86_64-linux-cbe
+#target=x86_64-windows-cbe
+#target=wasm32-wasi-selfhosted
+#update=initial version
+#file=main.zig
+const std = @import("std");
+const string = @embedFile("string.txt");
+pub fn main() !void {
+    try std.io.getStdOut().writeAll(string);
+}
+#file=string.txt
+Hello, World!
+#expect_stdout="Hello, World!\n"
+
+#update=change file contents
+#file=string.txt
+Hello again, World!
+#expect_stdout="Hello again, World!\n"
+
+#update=delete file
+#rm_file=string.txt
+#expect_error=ignored
+
+#update=remove reference to file
+#file=main.zig
+const std = @import("std");
+const string = @embedFile("string.txt");
+pub fn main() !void {
+    try std.io.getStdOut().writeAll("a hardcoded string\n");
+}
+#expect_stdout="a hardcoded string\n"
+
+#update=re-introduce reference to file
+#file=main.zig
+const std = @import("std");
+const string = @embedFile("string.txt");
+pub fn main() !void {
+    try std.io.getStdOut().writeAll(string);
+}
+#expect_error=ignore
+
+#update=recreate file
+#file=string.txt
+We're back, World!
+#expect_stdout="We're back, World!\n"
diff --git a/tools/incr-check.zig b/tools/incr-check.zig
index 144e05ccc41786dfdb96ff37c16a6deb6cb3b5da..bbdef190431f016df63b2f5a279ac70b3ff3350e 100644
--- a/tools/incr-check.zig
+++ b/tools/incr-check.zig
@@ -608,6 +608,7 @@ const Case = struct {
         var targets: std.ArrayListUnmanaged(Target) = .empty;
         var updates: std.ArrayListUnmanaged(Update) = .empty;
         var changes: std.ArrayListUnmanaged(FullContents) = .empty;
+        var deletes: std.ArrayListUnmanaged([]const u8) = .empty;
         var it = std.mem.splitScalar(u8, bytes, '\n');
         var line_n: usize = 1;
         var root_source_file: ?[]const u8 = null;
@@ -647,13 +648,14 @@ const Case = struct {
                     if (updates.items.len > 0) {
                         const last_update = &updates.items[updates.items.len - 1];
                         last_update.changes = try changes.toOwnedSlice(arena);
+                        last_update.deletes = try deletes.toOwnedSlice(arena);
                     }
                     try updates.append(arena, .{
                         .name = val,
                         .outcome = .unknown,
                     });
                 } else if (std.mem.eql(u8, key, "file")) {
-                    if (updates.items.len == 0) fatal("line {d}: expect directive before update", .{line_n});
+                    if (updates.items.len == 0) fatal("line {d}: file directive before update", .{line_n});
 
                     if (root_source_file == null)
                         root_source_file = val;
@@ -674,6 +676,9 @@ const Case = struct {
                         .name = val,
                         .bytes = src,
                     });
+                } else if (std.mem.eql(u8, key, "rm_file")) {
+                    if (updates.items.len == 0) fatal("line {d}: rm_file directive before update", .{line_n});
+                    try deletes.append(arena, val);
                 } else if (std.mem.eql(u8, key, "expect_stdout")) {
                     if (updates.items.len == 0) fatal("line {d}: expect directive before update", .{line_n});
                     const last_update = &updates.items[updates.items.len - 1];
@@ -701,6 +706,7 @@ const Case = struct {
         if (changes.items.len > 0) {
             const last_update = &updates.items[updates.items.len - 1];
             last_update.changes = changes.items; // arena so no need for toOwnedSlice
+            last_update.deletes = deletes.items;
         }
 
         return .{
-- 
2.54.0


From 7ef345f342534f93a7e61d9ecd096523f048871d Mon Sep 17 00:00:00 2001
From: mlugg <mlugg@mlugg.co.uk>
Date: Sat, 25 Jan 2025 08:11:00 +0000
Subject: [PATCH 3/4] incr-check: deal with Windows stupidity

The real problem here is that Git for Windows has horrendous defaults
which convert LF to CRLF. However, rather than changing this
configuration on the CI runners, it's worth supporting inexplicable CRLF
in these files so that anyone else cloning Zig on Windows doesn't get
unexpected test failures.
---
 tools/incr-check.zig | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/tools/incr-check.zig b/tools/incr-check.zig
index bbdef190431f016df63b2f5a279ac70b3ff3350e..8ef3fddc56f2fc7db0cd909481bde95800db1e87 100644
--- a/tools/incr-check.zig
+++ b/tools/incr-check.zig
@@ -660,21 +660,28 @@ const Case = struct {
                     if (root_source_file == null)
                         root_source_file = val;
 
-                    const start_index = it.index.?;
-                    const src = while (true) : (line_n += 1) {
+                    // Because Windows is so excellent, we need to convert CRLF to LF, so
+                    // can't just slice into the input here. How delightful!
+                    var src: std.ArrayListUnmanaged(u8) = .empty;
+
+                    while (true) {
                         const old = it;
-                        const next_line = it.next() orelse fatal("line {d}: unexpected EOF", .{line_n});
+                        const next_line_raw = it.next() orelse fatal("line {d}: unexpected EOF", .{line_n});
+                        const next_line = std.mem.trimRight(u8, next_line_raw, "\r");
                         if (std.mem.startsWith(u8, next_line, "#")) {
-                            const end_index = old.index.?;
-                            const src = bytes[start_index..end_index];
                             it = old;
-                            break src;
+                            break;
                         }
-                    };
+                        line_n += 1;
+
+                        try src.ensureUnusedCapacity(arena, next_line.len + 1);
+                        src.appendSliceAssumeCapacity(next_line);
+                        src.appendAssumeCapacity('\n');
+                    }
 
                     try changes.append(arena, .{
                         .name = val,
-                        .bytes = src,
+                        .bytes = src.items,
                     });
                 } else if (std.mem.eql(u8, key, "rm_file")) {
                     if (updates.items.len == 0) fatal("line {d}: rm_file directive before update", .{line_n});
-- 
2.54.0


From fcf8d5ada28c02722575cc78d41171692a227061 Mon Sep 17 00:00:00 2001
From: mlugg <mlugg@mlugg.co.uk>
Date: Sat, 25 Jan 2025 09:32:40 +0000
Subject: [PATCH 4/4] incr-check: check compile errors against expected

Also modifies all incremental cases using `#expect_error` to include the
errors and notes which are expected.
---
 test/incremental/add_decl                     |   2 +-
 test/incremental/add_decl_namespaced          |   3 +-
 test/incremental/change_embed_file            |   4 +-
 test/incremental/change_enum_tag_type         |   2 +-
 test/incremental/compile_error_then_log       |   7 +-
 test/incremental/delete_comptime_decls        |   7 +-
 test/incremental/fix_astgen_failure           |   4 +-
 test/incremental/fix_many_errors              |  21 ++-
 test/incremental/remove_enum_field            |   3 +-
 .../remove_invalid_union_backing_enum         |   3 +-
 test/incremental/temporary_parse_error        |   2 +-
 test/incremental/unreferenced_error           |   2 +-
 tools/incr-check.zig                          | 137 +++++++++++++++---
 13 files changed, 162 insertions(+), 35 deletions(-)

diff --git a/test/incremental/add_decl b/test/incremental/add_decl
index b842593056c692abe307a646e2eb84b776a76d7d..87f33b1c51839963e2a48c78b4a63f5285383753 100644
--- a/test/incremental/add_decl
+++ b/test/incremental/add_decl
@@ -39,7 +39,7 @@ pub fn main() !void {
 }
 const foo = "good morning\n";
 const bar = "good evening\n";
-#expect_error=ignored
+#expect_error=main.zig:3:37: error: use of undeclared identifier 'qux'
 
 #update=add missing declaration
 #file=main.zig
diff --git a/test/incremental/add_decl_namespaced b/test/incremental/add_decl_namespaced
index a42e5600bc299fdc0f484db8be7e88370ac4374c..84472effb5463af138b076331d1a8cf55facbf2e 100644
--- a/test/incremental/add_decl_namespaced
+++ b/test/incremental/add_decl_namespaced
@@ -39,7 +39,8 @@ pub fn main() !void {
 }
 const foo = "good morning\n";
 const bar = "good evening\n";
-#expect_error=ignored
+#expect_error=main.zig:3:44: error: root source file struct 'main' has no member named 'qux'
+#expect_error=main.zig:1:1: note: struct declared here
 
 #update=add missing declaration
 #file=main.zig
diff --git a/test/incremental/change_embed_file b/test/incremental/change_embed_file
index 3cb29be2edb31d97f3ad57aa92907245689b138d..171e4d31780cf7c969df8696658c12226c8cf964 100644
--- a/test/incremental/change_embed_file
+++ b/test/incremental/change_embed_file
@@ -20,7 +20,7 @@ Hello again, World!
 
 #update=delete file
 #rm_file=string.txt
-#expect_error=ignored
+#expect_error=main.zig:2:27: error: unable to open 'string.txt': FileNotFound
 
 #update=remove reference to file
 #file=main.zig
@@ -38,7 +38,7 @@ const string = @embedFile("string.txt");
 pub fn main() !void {
     try std.io.getStdOut().writeAll(string);
 }
-#expect_error=ignore
+#expect_error=main.zig:2:27: error: unable to open 'string.txt': FileNotFound
 
 #update=recreate file
 #file=string.txt
diff --git a/test/incremental/change_enum_tag_type b/test/incremental/change_enum_tag_type
index e66728e08d7d752e7318bd8d299219fc3b5f3b9b..d3f6c85c3743c4f14cd077c781b91e2df340bc61 100644
--- a/test/incremental/change_enum_tag_type
+++ b/test/incremental/change_enum_tag_type
@@ -39,7 +39,7 @@ comptime {
     std.debug.assert(@TypeOf(@intFromEnum(Foo.e)) == Tag);
 }
 const std = @import("std");
-#expect_error=ignored
+#expect_error=main.zig:7:5: error: enumeration value '4' too large for type 'u2'
 #update=increase tag size
 #file=main.zig
 const Tag = u3;
diff --git a/test/incremental/compile_error_then_log b/test/incremental/compile_error_then_log
index 9ab844bb4aa350cd767f427260aac53242a222c0..800fb92dfec05fd788878b29ead328b8e0145289 100644
--- a/test/incremental/compile_error_then_log
+++ b/test/incremental/compile_error_then_log
@@ -4,19 +4,22 @@
 #target=wasm32-wasi-selfhosted
 #update=initial version with compile error
 #file=main.zig
+pub fn main() void {}
 comptime {
     @compileError("this is an error");
 }
 comptime {
     @compileLog("this is a log");
 }
-#expect_error=ignored
+#expect_error=main.zig:3:5: error: this is an error
+
 #update=remove the compile error
 #file=main.zig
+pub fn main() void {}
 comptime {
     //@compileError("this is an error");
 }
 comptime {
     @compileLog("this is a log");
 }
-#expect_error=ignored
+#expect_error=main.zig:6:5: error: found compile log statement
diff --git a/test/incremental/delete_comptime_decls b/test/incremental/delete_comptime_decls
index 45c77048eb4fd8d9b453bd490be58fc10f108948..b5ccd438a23083220586a7f073c646cefb0d38eb 100644
--- a/test/incremental/delete_comptime_decls
+++ b/test/incremental/delete_comptime_decls
@@ -26,7 +26,10 @@ comptime {
     const slice = array[3..2];
     _ = slice;
 }
-#expect_error=ignored
+#expect_error=main.zig:5:32: error: end index 6 out of bounds for slice of length 4 +1 (sentinel)
+#expect_error=main.zig:10:28: error: end index 6 out of bounds for array of length 4 +1 (sentinel)
+#expect_error=main.zig:15:28: error: end index 5 out of bounds for array of length 4
+#expect_error=main.zig:20:25: error: start index 3 is larger than end index 2
 
 #update=delete and modify comptime decls
 #file=main.zig
@@ -38,4 +41,4 @@ comptime {
     const y = x[0..runtime_len];
     _ = y;
 }
-#expect_error=ignored
+#expect_error=main.zig:6:16: error: slice of null pointer
diff --git a/test/incremental/fix_astgen_failure b/test/incremental/fix_astgen_failure
index a57b1ebde321c9d1c0b03f40f28a0b2e191fa4aa..51972e9232baf90877500932b3b3d6e8d088edf2 100644
--- a/test/incremental/fix_astgen_failure
+++ b/test/incremental/fix_astgen_failure
@@ -11,7 +11,7 @@ pub fn main() !void {
 pub fn hello() !void {
     try std.io.getStdOut().writeAll("Hello, World!\n");
 }
-#expect_error=ignored
+#expect_error=foo.zig:2:9: error: use of undeclared identifier 'std'
 #update=fix the error
 #file=foo.zig
 const std = @import("std");
@@ -25,7 +25,7 @@ const std = @import("std");
 pub fn hello() !void {
     try std.io.getStdOut().writeAll(hello_str);
 }
-#expect_error=ignored
+#expect_error=foo.zig:3:37: error: use of undeclared identifier 'hello_str'
 #update=fix the new error
 #file=foo.zig
 const std = @import("std");
diff --git a/test/incremental/fix_many_errors b/test/incremental/fix_many_errors
index 0722cdfced41ef74c5706a98d2948a8be94d6b51..1d9446022ccf073fb3382615790b97b85279ea8f 100644
--- a/test/incremental/fix_many_errors
+++ b/test/incremental/fix_many_errors
@@ -24,7 +24,26 @@ export fn f6() void { @compileError("f6"); }
 export fn f7() void { @compileError("f7"); }
 export fn f8() void { @compileError("f8"); }
 export fn f9() void { @compileError("f9"); }
-#expect_error=ignored
+#expect_error=main.zig:2:12: error: c0
+#expect_error=main.zig:3:12: error: c1
+#expect_error=main.zig:4:12: error: c2
+#expect_error=main.zig:5:12: error: c3
+#expect_error=main.zig:6:12: error: c4
+#expect_error=main.zig:7:12: error: c5
+#expect_error=main.zig:8:12: error: c6
+#expect_error=main.zig:9:12: error: c7
+#expect_error=main.zig:10:12: error: c8
+#expect_error=main.zig:11:12: error: c9
+#expect_error=main.zig:12:23: error: f0
+#expect_error=main.zig:13:23: error: f1
+#expect_error=main.zig:14:23: error: f2
+#expect_error=main.zig:15:23: error: f3
+#expect_error=main.zig:16:23: error: f4
+#expect_error=main.zig:17:23: error: f5
+#expect_error=main.zig:18:23: error: f6
+#expect_error=main.zig:19:23: error: f7
+#expect_error=main.zig:20:23: error: f8
+#expect_error=main.zig:21:23: error: f9
 #update=fix all the errors
 #file=main.zig
 pub fn main() !void {}
diff --git a/test/incremental/remove_enum_field b/test/incremental/remove_enum_field
index a1e5e20fd33f2d08a508ecad90580fc1b55206ba..8d3796b7c34af0dfead84f08b4234dd0182d3403 100644
--- a/test/incremental/remove_enum_field
+++ b/test/incremental/remove_enum_field
@@ -23,4 +23,5 @@ pub fn main() !void {
     try std.io.getStdOut().writer().print("{}\n", .{@intFromEnum(MyEnum.foo)});
 }
 const std = @import("std");
-#expect_error=ignored
+#expect_error=main.zig:6:73: error: enum 'main.MyEnum' has no member named 'foo'
+#expect_error=main.zig:1:16: note: enum declared here
diff --git a/test/incremental/remove_invalid_union_backing_enum b/test/incremental/remove_invalid_union_backing_enum
index 4308899f9aef0255c197ff75aa5e0d05bd6b8e73..1f7ee69b144e50ce39d4763181f16aab3881f77f 100644
--- a/test/incremental/remove_invalid_union_backing_enum
+++ b/test/incremental/remove_invalid_union_backing_enum
@@ -15,7 +15,8 @@ pub fn main() void {
     const u: U = .{ .a = 123 };
     _ = u;
 }
-#expect_error=ignored
+#expect_error=main.zig:6:5: error: no field named 'd' in enum 'main.E'
+#expect_error=main.zig:1:11: note: enum declared here
 #update=remove invalid backing enum
 #file=main.zig
 const U = union {
diff --git a/test/incremental/temporary_parse_error b/test/incremental/temporary_parse_error
index 675232ea94bb8e60a7d12da1f841bdc95ae91c88..0933546d337a50bc5da381bbebc05e1abeb1978c 100644
--- a/test/incremental/temporary_parse_error
+++ b/test/incremental/temporary_parse_error
@@ -11,7 +11,7 @@ pub fn main() !void {}
 #update=introduce parse error
 #file=main.zig
 pub fn main() !void {
-#expect_error=ignored
+#expect_error=main.zig:2:1: error: expected statement, found 'EOF'
 
 #update=fix parse error
 #file=main.zig
diff --git a/test/incremental/unreferenced_error b/test/incremental/unreferenced_error
index 29a9a34d97829b60390127edf2a3354a8d42cfc2..6025f3fdae9ee4a7d43b829c61bf7e407631a154 100644
--- a/test/incremental/unreferenced_error
+++ b/test/incremental/unreferenced_error
@@ -18,7 +18,7 @@ pub fn main() !void {
     try std.io.getStdOut().writeAll(a);
 }
 const a = @compileError("bad a");
-#expect_error=ignored
+#expect_error=main.zig:5:11: error: bad a
 
 #update=remove error reference
 #file=main.zig
diff --git a/tools/incr-check.zig b/tools/incr-check.zig
index 8ef3fddc56f2fc7db0cd909481bde95800db1e87..93d16e1e583b7226de89970106296f39756bb8e8 100644
--- a/tools/incr-check.zig
+++ b/tools/incr-check.zig
@@ -340,19 +340,63 @@ const Eval = struct {
     }
 
     fn checkErrorOutcome(eval: *Eval, update: Case.Update, error_bundle: std.zig.ErrorBundle) !void {
-        switch (update.outcome) {
+        const expected_errors = switch (update.outcome) {
             .unknown => return,
-            .compile_errors => |expected_errors| {
-                for (expected_errors) |expected_error| {
-                    _ = expected_error;
-                    @panic("TODO check if the expected error matches the compile errors");
-                }
-            },
+            .compile_errors => |expected_errors| expected_errors,
             .stdout, .exit_code => {
                 const color: std.zig.Color = .auto;
                 error_bundle.renderToStdErr(color.renderOptions());
                 eval.fatal("update '{s}': unexpected compile errors", .{update.name});
             },
+        };
+
+        var expected_idx: usize = 0;
+
+        for (error_bundle.getMessages()) |err_idx| {
+            if (expected_idx == expected_errors.len) {
+                const color: std.zig.Color = .auto;
+                error_bundle.renderToStdErr(color.renderOptions());
+                eval.fatal("update '{s}': more errors than expected", .{update.name});
+            }
+            eval.checkOneError(update, error_bundle, expected_errors[expected_idx], false, err_idx);
+            expected_idx += 1;
+
+            for (error_bundle.getNotes(err_idx)) |note_idx| {
+                if (expected_idx == expected_errors.len) {
+                    const color: std.zig.Color = .auto;
+                    error_bundle.renderToStdErr(color.renderOptions());
+                    eval.fatal("update '{s}': more error notes than expected", .{update.name});
+                }
+                eval.checkOneError(update, error_bundle, expected_errors[expected_idx], true, note_idx);
+                expected_idx += 1;
+            }
+        }
+    }
+
+    fn checkOneError(
+        eval: *Eval,
+        update: Case.Update,
+        eb: std.zig.ErrorBundle,
+        expected: Case.ExpectedError,
+        is_note: bool,
+        err_idx: std.zig.ErrorBundle.MessageIndex,
+    ) void {
+        const err = eb.getErrorMessage(err_idx);
+        if (err.src_loc == .none) @panic("TODO error message with no source location");
+        if (err.count != 1) @panic("TODO error message with count>1");
+        const msg = eb.nullTerminatedString(err.msg);
+        const src = eb.getSourceLocation(err.src_loc);
+        const filename = eb.nullTerminatedString(src.src_path);
+
+        if (expected.is_note != is_note or
+            !std.mem.eql(u8, expected.filename, filename) or
+            expected.line != src.line + 1 or
+            expected.column != src.column + 1 or
+            !std.mem.eql(u8, expected.msg, msg))
+        {
+            const color: std.zig.Color = .auto;
+            eb.renderToStdErr(color.renderOptions());
+            eval.fatal("update '{s}': compile error did not match expected error", .{update.name});
         }
     }
 
@@ -595,11 +639,11 @@ const Case = struct {
     };
 
     const ExpectedError = struct {
-        file_name: ?[]const u8 = null,
-        line: ?u32 = null,
-        column: ?u32 = null,
-        msg_exact: ?[]const u8 = null,
-        msg_substring: ?[]const u8 = null,
+        is_note: bool,
+        filename: []const u8,
+        line: u32,
+        column: u32,
+        msg: []const u8,
     };
 
     fn parse(arena: Allocator, bytes: []const u8) !Case {
@@ -665,13 +709,11 @@ const Case = struct {
                     var src: std.ArrayListUnmanaged(u8) = .empty;
 
                     while (true) {
-                        const old = it;
-                        const next_line_raw = it.next() orelse fatal("line {d}: unexpected EOF", .{line_n});
+                        const next_line_raw = it.peek() orelse fatal("line {d}: unexpected EOF", .{line_n});
                         const next_line = std.mem.trimRight(u8, next_line_raw, "\r");
-                        if (std.mem.startsWith(u8, next_line, "#")) {
-                            it = old;
-                            break;
-                        }
+                        if (std.mem.startsWith(u8, next_line, "#")) break;
+
+                        _ = it.next();
                         line_n += 1;
 
                         try src.ensureUnusedCapacity(arena, next_line.len + 1);
@@ -699,7 +741,24 @@ const Case = struct {
                     if (updates.items.len == 0) fatal("line {d}: expect directive before update", .{line_n});
                     const last_update = &updates.items[updates.items.len - 1];
                     if (last_update.outcome != .unknown) fatal("line {d}: conflicting expect directive", .{line_n});
-                    last_update.outcome = .{ .compile_errors = &.{} };
+
+                    var errors: std.ArrayListUnmanaged(ExpectedError) = .empty;
+                    try errors.append(arena, parseExpectedError(val, line_n));
+                    while (true) {
+                        const next_line = it.peek() orelse break;
+                        if (!std.mem.startsWith(u8, next_line, "#")) break;
+                        var new_line_it = std.mem.splitScalar(u8, next_line, '=');
+                        const new_key = new_line_it.first()[1..];
+                        const new_val = std.mem.trimRight(u8, new_line_it.rest(), "\r");
+                        if (new_val.len == 0) break;
+                        if (!std.mem.eql(u8, new_key, "expect_error")) break;
+
+                        _ = it.next();
+                        line_n += 1;
+                        try errors.append(arena, parseExpectedError(new_val, line_n));
+                    }
+
+                    last_update.outcome = .{ .compile_errors = errors.items };
                 } else {
                     fatal("line {d}: unrecognized key '{s}'", .{ line_n, key });
                 }
@@ -749,3 +808,43 @@ fn waitChild(child: *std.process.Child, eval: *Eval) void {
         .Signal, .Stopped, .Unknown => eval.fatal("compiler terminated unexpectedly", .{}),
     }
 }
+
+fn parseExpectedError(str: []const u8, l: usize) Case.ExpectedError {
+    // #expect_error=foo.zig:1:2: error: the error message
+    // #expect_error=foo.zig:1:2: note: and a note
+
+    const fatal = std.process.fatal;
+
+    var it = std.mem.splitScalar(u8, str, ':');
+    const filename = it.first();
+    const line_str = it.next() orelse fatal("line {d}: incomplete error specification", .{l});
+    const column_str = it.next() orelse fatal("line {d}: incomplete error specification", .{l});
+    const error_or_note_str = std.mem.trim(
+        u8,
+        it.next() orelse fatal("line {d}: incomplete error specification", .{l}),
+        " ",
+    );
+    const message = std.mem.trim(u8, it.rest(), " ");
+    if (filename.len == 0) fatal("line {d}: empty filename", .{l});
+    if (message.len == 0) fatal("line {d}: empty error message", .{l});
+    const is_note = if (std.mem.eql(u8, error_or_note_str, "error"))
+        false
+    else if (std.mem.eql(u8, error_or_note_str, "note"))
+        true
+    else
+        fatal("line {d}: expeted 'error' or 'note', found '{s}'", .{ l, error_or_note_str });
+
+    const line = std.fmt.parseInt(u32, line_str, 10) catch
+        fatal("line {d}: invalid line number '{s}'", .{ l, line_str });
+
+    const column = std.fmt.parseInt(u32, column_str, 10) catch
+        fatal("line {d}: invalid column number '{s}'", .{ l, column_str });
+
+    return .{
+        .is_note = is_note,
+        .filename = filename,
+        .line = line,
+        .column = column,
+        .msg = message,
+    };
+}
-- 
2.54.0