From 3deb86bafdb2f622b9099c405e0e861f70c36a45 Mon Sep 17 00:00:00 2001
From: Jacob Young <jacobly@ziglang.org>
Date: Fri, 12 Jun 2026 13:11:56 -0400
Subject: [PATCH] AstGen: make local align expression implicitly comptime

Closes #35730
---
 lib/std/zig/AstGen.zig                      |  4 +-
 test/behavior/align.zig                     | 25 +++++++++++
 test/cases/compile_errors/runtime_align.zig | 46 +++++++++++++++++++++
 3 files changed, 73 insertions(+), 2 deletions(-)
 create mode 100644 test/cases/compile_errors/runtime_align.zig

diff --git a/lib/std/zig/AstGen.zig b/lib/std/zig/AstGen.zig
index b9de0ee6ee2eead54e661e927e3d726033156a29..b9084412f3b71875e3257efa08a6633839a37e2a 100644
--- a/lib/std/zig/AstGen.zig
+++ b/lib/std/zig/AstGen.zig
@@ -3130,7 +3130,7 @@ fn varDecl(
     }
 
     const align_inst: Zir.Inst.Ref = if (var_decl.ast.align_node.unwrap()) |align_node|
-        try expr(gz, scope, coerced_align_ri, align_node)
+        try comptimeExpr(gz, scope, coerced_align_ri, align_node, .@"align")
     else
         .none;
 
@@ -3477,7 +3477,7 @@ fn assignDestructureMaybeDecls(
                 const this_variable_comptime = is_comptime or (is_const and value_is_comptime);
 
                 const align_inst: Zir.Inst.Ref = if (full_var_decl.ast.align_node.unwrap()) |align_node|
-                    try expr(gz, scope, coerced_align_ri, align_node)
+                    try comptimeExpr(gz, scope, coerced_align_ri, align_node, .@"align")
                 else
                     .none;
 
diff --git a/test/behavior/align.zig b/test/behavior/align.zig
index 3de3dd090670331c27e4856e90246483546a2432..9d4886ee954d42be8f7684091577305b568056be 100644
--- a/test/behavior/align.zig
+++ b/test/behavior/align.zig
@@ -616,3 +616,28 @@ test "function pointer align mask" {
     const aligned: *align(16) const fn () callconv(.c) void = @alignCast(unaligned);
     try expect(@intFromPtr(aligned) == int);
 }
+
+test "align expression is implicitly comptime" {
+    const S = struct {
+        fn alignment() usize {
+            return 4;
+        }
+
+        var global: [3]u8 align(alignment()) = @splat(0);
+        fn check() !void {
+            try std.testing.expect(@intFromPtr(&global) % alignment() == 0);
+            var local: [3]u8 align(alignment()) = @splat(0);
+            try std.testing.expect(@intFromPtr(&local) % alignment() == 0);
+            var de: [3]u8 align(alignment()), var structure: [3]u8 align(alignment()) = .{ @splat(0), @splat(0) };
+            try std.testing.expect(@intFromPtr(&de) % alignment() == 0);
+            try std.testing.expect(@intFromPtr(&structure) % alignment() == 0);
+            var @"struct": struct { field: [3]u8 align(alignment()) } = .{ .field = @splat(0) };
+            try std.testing.expect(@intFromPtr(&@"struct".field) % alignment() == 0);
+            var @"union": union { field: [3]u8 align(alignment()) } = .{ .field = @splat(0) };
+            try std.testing.expect(@intFromPtr(&@"union".field) % alignment() == 0);
+            const ptr: *align(alignment()) [3]u8 = &global;
+            try std.testing.expect(@intFromPtr(ptr) % alignment() == 0);
+        }
+    };
+    try S.check();
+}
diff --git a/test/cases/compile_errors/runtime_align.zig b/test/cases/compile_errors/runtime_align.zig
new file mode 100644
index 0000000000000000000000000000000000000000..d9356f7a4169f16e37d2155c5a68869a3738eb12
--- /dev/null
+++ b/test/cases/compile_errors/runtime_align.zig
@@ -0,0 +1,46 @@
+var alignment: u29 = 4;
+
+var global: u8 align(alignment) = 0;
+export fn globalWithRuntimeAlign() u8 {
+    return global;
+}
+
+export fn localWithRuntimeAlign() u8 {
+    const local: u8 align(alignment) = 0;
+    return local;
+}
+
+export fn destructureWithRuntimeAlign() u8 {
+    const de: u8 align(alignment), const structure align(alignment) = .{ 0, 0 };
+    return de | structure;
+}
+
+export fn structFieldWithRuntimeAlign() u8 {
+    const @"struct": struct { field: u8 align(alignment) } = .{ .field = 0 };
+    return @"struct".field;
+}
+
+export fn unionFieldWithRuntimeAlign() u8 {
+    const @"union": union { field: u8 align(alignment) } = .{ .field = 0 };
+    return @"union".field;
+}
+
+export fn pointerWithRuntimeAlign() u8 {
+    const ptr: *align(alignment) u8 = &global;
+    return ptr.*;
+}
+
+// error
+//
+// :3:22: error: unable to resolve comptime value
+// :3:22: note: alignment must be comptime-known
+// :9:27: error: unable to resolve comptime value
+// :9:27: note: alignment must be comptime-known
+// :14:24: error: unable to resolve comptime value
+// :14:24: note: alignment must be comptime-known
+// :19:47: error: unable to resolve comptime value
+// :19:47: note: alignment must be comptime-known
+// :24:45: error: unable to resolve comptime value
+// :24:45: note: alignment must be comptime-known
+// :29:23: error: unable to resolve comptime value
+// :29:23: note: alignment must be comptime-known
-- 
2.54.0