Allow windows resource compiler to be overridden

It is now required to provide a resource compiler on windows when
compiling rust. This allows toolchain builders to explicitly provide a
path to an alternative, such as llvm-rc, instead of the one that's
provided by the Windows SDK.

Author: erickt

bootstrap.example.toml

@@ -325,6 +325,9 @@
 # Defaults to the Python interpreter used to execute x.py.
 #build.python = "python"
 
+# The path to (or name of) the resource compiler executable to use on Windows.
+#build.windows-rc = "rc.exe"
+
 # The path to the REUSE executable to use. Note that REUSE is not required in
 # most cases, as our tooling relies on a cached (and shrunk) copy of the
 # REUSE output present in the git repository and in our source tarballs.

compiler/rustc_windows_rc/src/lib.rs

@@ -35,8 +35,11 @@ pub fn compile_windows_resource_file(
     resources_dir.push("resources");
     fs::create_dir_all(&resources_dir).unwrap();
 
-    let resource_compiler =
-        find_resource_compiler(&env::var("CARGO_CFG_TARGET_ARCH").unwrap()).expect("found rc.exe");
+    let resource_compiler = if let Ok(path) = env::var("RUSTC_WINDOWS_RC") {
+        path.into()
+    } else {
+        find_resource_compiler(&env::var("CARGO_CFG_TARGET_ARCH").unwrap()).expect("found rc.exe")
+    };
 
     let rc_path = resources_dir.join(file_stem.with_extension("rc"));
 

src/bootstrap/src/core/builder/cargo.rs

@@ -1221,6 +1221,11 @@ impl Builder<'_> {
             rustflags.arg("-Zehcont-guard");
         }
 
+        // Optionally override the rc.exe when compiling rustc on Windows.
+        if let Some(windows_rc) = &self.config.windows_rc {
+            cargo.env("RUSTC_WINDOWS_RC", windows_rc);
+        }
+
         // For `cargo doc` invocations, make rustdoc print the Rust version into the docs
         // This replaces spaces with tabs because RUSTDOCFLAGS does not
         // support arguments with regular spaces. Hopefully someday Cargo will

src/bootstrap/src/core/config/config.rs

@@ -272,6 +272,7 @@ pub struct Config {
     pub gdb: Option<PathBuf>,
     pub lldb: Option<PathBuf>,
     pub python: Option<PathBuf>,
+    pub windows_rc: Option<PathBuf>,
     pub reuse: Option<PathBuf>,
     pub cargo_native_static: bool,
     pub configure_args: Vec<String>,
@@ -449,6 +450,7 @@ impl Config {
             nodejs: build_nodejs,
             npm: build_npm,
             python: build_python,
+            windows_rc: build_windows_rc,
             reuse: build_reuse,
             locked_deps: build_locked_deps,
             vendor: build_vendor,
@@ -1339,6 +1341,7 @@ impl Config {
                 .unwrap_or(rust_debug == Some(true)),
             vendor,
             verbose_tests,
+            windows_rc: build_windows_rc.map(PathBuf::from),
             // tidy-alphabetical-end
         }
     }

src/bootstrap/src/core/config/toml/build.rs

@@ -37,6 +37,7 @@ define_config! {
         nodejs: Option<String> = "nodejs",
         npm: Option<String> = "npm",
         python: Option<String> = "python",
+        windows_rc: Option<String> = "windows-rc",
         reuse: Option<String> = "reuse",
         locked_deps: Option<bool> = "locked-deps",
         vendor: Option<bool> = "vendor",

src/bootstrap/src/utils/change_tracker.rs

@@ -546,4 +546,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Info,
         summary: "The default value of the `gcc.download-ci-gcc` option has been changed to `true`.",
     },
+    ChangeInfo {
+        change_id: 146663,
+        severity: ChangeSeverity::Info,
+        summary: "New option `build.windows-rc` that will override which resource compiler on Windows will be used to compile Rust.",
+    },
 ];