From 7b890ff9789caf010639277168d5f5852a9d852e Mon Sep 17 00:00:00 2001
From: Ryan Liptak <squeek502@hotmail.com>
Date: Thu, 13 Aug 2026 21:44:18 -0700
Subject: [PATCH] Fix Compilation.Path checks when one path is a root path

Previously, it was assumed that the byte after the cwd/outer path had to be a path separator, but that's not true when the cwd/outer path is a root with no components (e.g. `/` or `C:\`)

Fixes #32005
---
 src/Compilation.zig | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/Compilation.zig b/src/Compilation.zig
index f6b30ef3f9c1c91abf29381d6b9d2b273ef6a424..c79afb0923996fb4c41afb1d42fe80809149d130 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -484,8 +484,13 @@ pub const Path = struct {
         assert(fs.path.isAbsolute(sub_path));
         if (!std.mem.startsWith(u8, sub_path, cwd_path)) return sub_path;
         if (sub_path.len == cwd_path.len) return "."; // the strings are equal
-        if (sub_path[cwd_path.len] != fs.path.sep) return sub_path; // last component before cwd differs
-        return sub_path[cwd_path.len + 1 ..]; // remove '/path/to/cwd/' prefix
+        const path_sep_index = path_sep_index: {
+            // cwd is just a root, e.g. / or C:\
+            if (cwd_path[cwd_path.len - 1] == fs.path.sep) break :path_sep_index cwd_path.len - 1;
+            if (sub_path[cwd_path.len] != fs.path.sep) return sub_path; // last component before cwd differs
+            break :path_sep_index cwd_path.len;
+        };
+        return sub_path[path_sep_index + 1 ..]; // remove '/path/to/cwd/' prefix
     }
 
     /// From an unresolved path (which can be made of multiple not-yet-joined strings), construct a
@@ -672,8 +677,13 @@ pub const Path = struct {
         if (!mem.startsWith(u8, inner.sub_path, outer.sub_path)) return .no;
         if (inner.sub_path.len == outer.sub_path.len) return .no;
         if (outer.sub_path.len == 0) return .{ .yes = inner.sub_path };
-        if (inner.sub_path[outer.sub_path.len] != fs.path.sep) return .no;
-        return .{ .yes = inner.sub_path[outer.sub_path.len + 1 ..] };
+        const path_sep_index = path_sep_index: {
+            // outer is just a root, e.g. / or C:\
+            if (outer.sub_path[outer.sub_path.len - 1] == fs.path.sep) break :path_sep_index outer.sub_path.len - 1;
+            if (inner.sub_path[outer.sub_path.len] != fs.path.sep) return .no;
+            break :path_sep_index outer.sub_path.len;
+        };
+        return .{ .yes = inner.sub_path[path_sep_index + 1 ..] };
     }
 
     /// Returns whether this `Path` is illegal to have as a user-imported `Zcu.File` (including
-- 
2.54.0