stage2: better error message for root zig source file not found

closes #6777
closes #6893

src/Cache.zig

@@ -194,6 +194,9 @@ pub const Manifest = struct {
     files: std.ArrayListUnmanaged(File) = .{},
     hex_digest: [hex_digest_len]u8,
     debug_bin_digest: DebugBinDigest = null_debug_bin_digest,
+    /// Populated when hit() returns an error because of one
+    /// of the files listed in the manifest.
+    failed_file_index: ?usize = null,
 
     /// Add a file as a dependency of process being cached. When `hit` is
     /// called, the file's contents will be checked to ensure that it matches
@@ -255,6 +258,8 @@ pub const Manifest = struct {
     pub fn hit(self: *Manifest) !bool {
         assert(self.manifest_file == null);
 
+        self.failed_file_index = null;
+
         const ext = ".txt";
         var manifest_file_path: [self.hex_digest.len + ext.len]u8 = undefined;
 
@@ -366,7 +371,10 @@ pub const Manifest = struct {
             };
             defer this_file.close();
 
-            const actual_stat = try this_file.stat();
+            const actual_stat = this_file.stat() catch |err| {
+                self.failed_file_index = idx;
+                return err;
+            };
             const size_match = actual_stat.size == cache_hash_file.stat.size;
             const mtime_match = actual_stat.mtime == cache_hash_file.stat.mtime;
             const inode_match = actual_stat.inode == cache_hash_file.stat.inode;
@@ -382,7 +390,10 @@ pub const Manifest = struct {
                 }
 
                 var actual_digest: BinDigest = undefined;
-                try hashFile(this_file, &actual_digest);
+                hashFile(this_file, &actual_digest) catch |err| {
+                    self.failed_file_index = idx;
+                    return err;
+                };
 
                 if (!mem.eql(u8, &cache_hash_file.bin_digest, &actual_digest)) {
                     cache_hash_file.bin_digest = actual_digest;
@@ -407,7 +418,10 @@ pub const Manifest = struct {
             self.manifest_dirty = true;
             while (idx < input_file_count) : (idx += 1) {
                 const ch_file = &self.files.items[idx];
-                try self.populateFileHash(ch_file);
+                self.populateFileHash(ch_file) catch |err| {
+                    self.failed_file_index = idx;
+                    return err;
+                };
             }
             return false;
         }

src/Compilation.zig

@@ -1629,7 +1629,7 @@ pub fn performAllTheWork(self: *Compilation) error{ TimerUnsupported, OutOfMemor
                 unreachable;
 
             self.updateStage1Module(main_progress_node) catch |err| {
-                fatal("unable to build stage1 zig object: {}", .{@errorName(err)});
+                fatal("unable to build stage1 zig object: {s}", .{@errorName(err)});
             };
         },
     };
@@ -3001,7 +3001,12 @@ fn updateStage1Module(comp: *Compilation, main_progress_node: *std.Progress.Node
     const prev_hash_state = man.hash.peekBin();
     const input_file_count = man.files.items.len;
 
-    if (try man.hit()) {
+    const hit = man.hit() catch |err| {
+        const i = man.failed_file_index orelse return err;
+        const file_path = man.files.items[i].path orelse return err;
+        fatal("unable to build stage1 zig object: {s}: {s}", .{ @errorName(err), file_path });
+    };
+    if (hit) {
         const digest = man.final();
 
         // We use an extra hex-encoded byte here to store some flags.