zig cc: support reading from stdin via "-x LANG -"

src/Compilation.zig

@@ -4597,6 +4597,27 @@ pub const FileExt = enum {
             => false,
         };
     }
+
+    pub fn canonicalName(ext: FileExt, target: Target) [:0]const u8 {
+        return switch (ext) {
+            .c => ".c",
+            .cpp => ".cpp",
+            .cu => ".cu",
+            .h => ".h",
+            .m => ".m",
+            .mm => ".mm",
+            .ll => ".ll",
+            .bc => ".bc",
+            .assembly => ".s",
+            .assembly_with_cpp => ".S",
+            .shared_library => target.dynamicLibSuffix(),
+            .object => target.ofmt.fileExt(target.cpu.arch),
+            .static_library => target.staticLibSuffix(),
+            .zig => ".zig",
+            .def => ".def",
+            .unknown => "",
+        };
+    }
 };
 
 pub fn hasObjectExt(filename: []const u8) bool {

src/main.zig

@@ -3020,6 +3020,32 @@ fn buildOutputType(
         break :l global_cache_directory;
     };
 
+    for (c_source_files.items) |*src| {
+        if (!mem.eql(u8, src.src_path, "-")) continue;
+
+        const ext = src.ext orelse
+            fatal("-E or -x is required when reading from a non-regular file", .{});
+
+        // "-" is stdin. Dump it to a real file.
+        const sep = fs.path.sep_str;
+        const sub_path = try std.fmt.allocPrint(arena, "tmp" ++ sep ++ "{x}-stdin{s}", .{
+            std.crypto.random.int(u64), ext.canonicalName(target_info.target),
+        });
+        try local_cache_directory.handle.makePath("tmp");
+        // Note that in one of the happy paths, execve() is used to switch
+        // to clang in which case any cleanup logic that exists for this
+        // temporary file will not run and this temp file will be leaked.
+        // Oh well. It's a minor punishment for using `-x c` which nobody
+        // should be doing. Therefore, we make no effort to clean up. Using
+        // `-` for stdin as a source file always leaks a temp file.
+        var f = try local_cache_directory.handle.createFile(sub_path, .{});
+        defer f.close();
+        try f.writeFileAll(io.getStdIn(), .{});
+
+        // Convert `sub_path` to be relative to current working directory.
+        src.src_path = try local_cache_directory.join(arena, &.{sub_path});
+    }
+
     if (build_options.have_llvm and emit_asm != .no) {
         // LLVM has no way to set this non-globally.
         const argv = [_][*:0]const u8{ "zig (LLVM option parsing)", "--x86-asm-syntax=intel" };