Prefer clang if linker-plugin-lto specified (#1573)

* Add test-linker-plugin-lto

Author: NobodyXu

.github/workflows/main.yml

@@ -217,6 +217,26 @@ jobs:
       - name: check clean Git workting tree
         uses: ./.github/actions/check-clean-git-working-tree
 
+  test-linker-plugin-lto:
+     name: Test linker-plugin-lto
+     runs-on: ubuntu-latest
+     steps:
+       - uses: actions/checkout@v5
+       - name: Install Rust (rustup)
+         run: |
+           set -euxo pipefail
+           rustup toolchain install stable --no-self-update --profile minimal
+           rustup default stable
+         shell: bash
+       - run: cargo update
+       - uses: Swatinem/rust-cache@v2
+       - run: cargo test --workspace --release
+         env:
+           RUSTCFLAGS: -C linker-plugin-lto
+       # check that there are no uncommitted changes to prevent bugs like https://github.com/rust-lang/cc-rs/issues/1411
+       - name: check clean Git workting tree
+         uses: ./.github/actions/check-clean-git-working-tree
+
   # This is separate from the matrix above because there is no prebuilt rust-std component for these targets.
   check-build-std:
     name: Test build-std
@@ -421,6 +441,7 @@ jobs:
     name: Tests pass
     needs:
       - test
+      - test-linker-plugin-lto
       - check-build-std
       - check-wasm
       - test-wasi

Cargo.toml

@@ -52,3 +52,8 @@ cc = { path = "." }
 
 [lints.rust]
 unexpected_cfgs = { level = "allow", check-cfg = ['cfg(disable_clang_cl_tests)'] }
+
+[profile.release]
+opt-level = 3 # Or "s" or "z" for different optimization goals
+lto = true
+

src/lib.rs

@@ -2808,6 +2808,14 @@ impl Build {
         cmd
     }
 
+    fn prefer_clang(&self) -> bool {
+        if let Some(env) = self.getenv("CARGO_ENCODED_RUSTFLAGS") {
+            env.to_string_lossy().contains("linker-plugin-lto")
+        } else {
+            false
+        }
+    }
+
     fn get_base_compiler(&self) -> Result<Tool, Error> {
         let out_dir = self.get_out_dir().ok();
         let out_dir = out_dir.as_deref();
@@ -2835,14 +2843,19 @@ impl Build {
             ("CC", "gcc", "cc", "clang")
         };
 
-        // On historical Solaris systems, "cc" may have been Sun Studio, which
-        // is not flag-compatible with "gcc".  This history casts a long shadow,
-        // and many modern illumos distributions today ship GCC as "gcc" without
-        // also making it available as "cc".
+        let fallback = Cow::Borrowed(Path::new(traditional));
         let default = if cfg!(target_os = "solaris") || cfg!(target_os = "illumos") {
-            gnu
+            // On historical Solaris systems, "cc" may have been Sun Studio, which
+            // is not flag-compatible with "gcc".  This history casts a long shadow,
+            // and many modern illumos distributions today ship GCC as "gcc" without
+            // also making it available as "cc".
+            Cow::Borrowed(Path::new(gnu))
+        } else if self.prefer_clang() {
+            self.which(Path::new(clang), None)
+                .map(Cow::Owned)
+                .unwrap_or(fallback)
         } else {
-            traditional
+            fallback
         };
 
         let cl_exe = self.find_msvc_tools_find_tool(&target, msvc);