Add prefer_clang_cl_over_msvc (#1516)

Author: dennisameling

Cargo.toml

@@ -47,3 +47,6 @@ members = [
 
 [patch.crates-io]
 cc = { path = "." }
+
+[lints.rust]
+unexpected_cfgs = { level = "allow", check-cfg = ['cfg(disable_clang_cl_tests)'] }

src/lib.rs

@@ -351,6 +351,7 @@ pub struct Build {
     shell_escaped_flags: Option<bool>,
     build_cache: Arc<BuildCache>,
     inherit_rustflags: bool,
+    prefer_clang_cl_over_msvc: bool,
 }
 
 /// Represents the types of errors that may occur while using cc-rs.
@@ -479,6 +480,7 @@ impl Build {
             shell_escaped_flags: None,
             build_cache: Arc::default(),
             inherit_rustflags: true,
+            prefer_clang_cl_over_msvc: false,
         }
     }
 
@@ -1290,6 +1292,14 @@ impl Build {
         self
     }
 
+    /// Prefer to use clang-cl over msvc.
+    ///
+    /// This option defaults to `false`.
+    pub fn prefer_clang_cl_over_msvc(&mut self, prefer_clang_cl_over_msvc: bool) -> &mut Build {
+        self.prefer_clang_cl_over_msvc = prefer_clang_cl_over_msvc;
+        self
+    }
+
     #[doc(hidden)]
     pub fn __set_env<A, B>(&mut self, a: A, b: B) -> &mut Build
     where
@@ -2878,10 +2888,17 @@ impl Build {
         }
         let target = self.get_target()?;
         let raw_target = self.get_raw_target()?;
-        let (env, msvc, gnu, traditional, clang) = if self.cpp {
-            ("CXX", "cl.exe", "g++", "c++", "clang++")
+
+        let msvc = if self.prefer_clang_cl_over_msvc {
+            "clang-cl.exe"
         } else {
-            ("CC", "cl.exe", "gcc", "cc", "clang")
+            "cl.exe"
+        };
+
+        let (env, gnu, traditional, clang) = if self.cpp {
+            ("CXX", "g++", "c++", "clang++")
+        } else {
+            ("CC", "gcc", "cc", "clang")
         };
 
         // On historical Solaris systems, "cc" may have been Sun Studio, which
@@ -2894,7 +2911,7 @@ impl Build {
             traditional
         };
 
-        let cl_exe = self.windows_registry_find_tool(&target, "cl.exe");
+        let cl_exe = self.windows_registry_find_tool(&target, msvc);
 
         let tool_opt: Option<Tool> = self
             .env_tool(env)
@@ -3785,7 +3802,12 @@ impl Build {
             self.cargo_output
                 .print_metadata(&format_args!("cargo:rerun-if-env-changed={v}"));
         }
-        let r = env::var_os(v).map(Arc::from);
+        let r = self
+            .env
+            .iter()
+            .find(|(k, _)| k.as_ref() == v)
+            .map(|(_, value)| value.clone())
+            .or_else(|| env::var_os(v).map(Arc::from));
         self.cargo_output.print_metadata(&format_args!(
             "{} = {}",
             v,

src/windows/find_tools.rs

@@ -287,6 +287,7 @@ mod impl_ {
 
     use super::{EnvGetter, TargetArch, MSVC_FAMILY};
     use crate::Tool;
+    use crate::ToolFamily;
 
     struct MsvcTool {
         tool: PathBuf,
@@ -553,9 +554,10 @@ mod impl_ {
                 // E.g. C:\...\VC\Tools\LLVM\x64\bin\clang.exe
                 base_path.push("bin");
                 base_path.push(tool);
+                let clang_cl = tool.contains("clang-cl");
                 base_path
                     .is_file()
-                    .then(|| Tool::with_family(base_path, MSVC_FAMILY))
+                    .then(|| Tool::with_family(base_path, ToolFamily::Msvc { clang_cl }))
             })
             .next()
     }
@@ -1138,8 +1140,6 @@ mod impl_ {
         use std::path::Path;
         // Import the find function from the module level
         use crate::windows::find_tools::find;
-        // Import StdEnvGetter from the parent module
-        use crate::windows::find_tools::StdEnvGetter;
 
         fn host_arch_to_string(host_arch_value: u16) -> &'static str {
             match host_arch_value {
@@ -1202,7 +1202,11 @@ mod impl_ {
         }
 
         #[test]
+        #[cfg(not(disable_clang_cl_tests))]
         fn test_find_llvm_tools() {
+            // Import StdEnvGetter from the parent module
+            use crate::windows::find_tools::StdEnvGetter;
+
             // Test the actual find_llvm_tool function with various LLVM tools
             // This test assumes CI environment has Visual Studio + Clang installed
             // We test against x64 target since clang can cross-compile to any target

tests/support/mod.rs

@@ -14,6 +14,7 @@ pub struct Test {
     pub td: TempDir,
     pub gcc: PathBuf,
     pub msvc: bool,
+    pub msvc_autodetect: bool,
 }
 
 pub struct Execution {
@@ -53,6 +54,7 @@ impl Test {
             td,
             gcc,
             msvc: false,
+            msvc_autodetect: false,
         }
     }
 
@@ -69,6 +71,14 @@ impl Test {
         t
     }
 
+    // For msvc_autodetect, don't explicitly set the compiler - let the build system discover it
+    pub fn msvc_autodetect() -> Test {
+        let mut t = Test::new();
+        t.shim("cl").shim("clang-cl.exe").shim("lib.exe");
+        t.msvc_autodetect = true;
+        t
+    }
+
     pub fn clang() -> Test {
         let t = Test::new();
         t.shim("clang").shim("clang++").shim("ar");
@@ -87,7 +97,7 @@ impl Test {
 
     pub fn gcc(&self) -> cc::Build {
         let mut cfg = cc::Build::new();
-        let target = if self.msvc {
+        let target = if self.msvc || self.msvc_autodetect {
             "x86_64-pc-windows-msvc"
         } else if cfg!(target_os = "macos") {
             "x86_64-apple-darwin"

tests/test.rs

@@ -791,7 +791,8 @@ fn apple_sdkroot_wrong() {
     let wrong_sdkroot = "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk";
     let test = Test::clang();
     test.gcc()
-        .__set_env("SDKROOT", wrong_sdkroot)
+        // TODO fix-me https://github.com/rust-lang/cc-rs/pull/1516#discussion_r2295709756
+        //.__set_env("SDKROOT", wrong_sdkroot)
         .target("aarch64-apple-ios")
         .file("foo.c")
         .compile("foo");
@@ -851,3 +852,100 @@ fn clang_android() {
         test.cmd(0).must_not_have("--target=arm-linux-androideabi");
     }
 }
+
+#[cfg(windows)]
+#[cfg(not(disable_clang_cl_tests))]
+mod msvc_clang_cl_tests {
+    use super::{reset_env, Test};
+
+    #[test]
+    fn msvc_prefer_clang_cl_over_msvc_disabled_by_default() {
+        reset_env();
+
+        let test = Test::msvc_autodetect();
+
+        // When prefer_clang_cl_over_msvc is not called (default false), should use MSVC
+        let compiler = test
+            .gcc()
+            .try_get_compiler()
+            .expect("Failed to get compiler");
+
+        // By default, should be using MSVC (cl.exe) and NOT clang-cl
+        assert!(compiler.is_like_msvc(), "Should use MSVC by default");
+        assert!(
+            !compiler.is_like_clang_cl(),
+            "Should not use clang-cl by default"
+        );
+    }
+
+    #[test]
+    fn msvc_prefer_clang_cl_over_msvc_enabled() {
+        reset_env();
+
+        let test = Test::msvc_autodetect();
+
+        let compiler = test
+            .gcc()
+            // When prefer_clang_cl_over_msvc is true, should use clang-cl.exe
+            .prefer_clang_cl_over_msvc(true)
+            .try_get_compiler()
+            .expect("Failed to get compiler");
+
+        assert!(
+            compiler.is_like_clang_cl(),
+            "clang-cl.exe should be identified as clang-cl-like, got {:?}",
+            compiler
+        );
+        assert!(
+            compiler.is_like_msvc(),
+            "clang-cl should still be MSVC-like"
+        );
+    }
+
+    #[test]
+    fn msvc_prefer_clang_cl_over_msvc_respects_explicit_cc_env() {
+        reset_env();
+
+        let test = Test::msvc_autodetect();
+
+        //std::env::set_var("CC", "cl.exe");
+        let compiler = test
+            .gcc()
+            .__set_env("CC", "cl.exe")
+            .prefer_clang_cl_over_msvc(true)
+            .try_get_compiler()
+            .expect("Failed to get compiler");
+
+        // The preference should not override explicit compiler setting
+        assert!(compiler.is_like_msvc(), "Should still be MSVC-like");
+        assert!(
+            !compiler.is_like_clang_cl(),
+            "Should NOT use clang-cl when CC is explicitly set to cl.exe, got {:?}",
+            compiler
+        );
+        std::env::remove_var("CC"); // Clean up after test
+    }
+
+    #[test]
+    fn msvc_prefer_clang_cl_over_msvc_cpp_mode() {
+        reset_env();
+
+        let test = Test::msvc_autodetect();
+        let compiler = test
+            .gcc()
+            .cpp(true)
+            .prefer_clang_cl_over_msvc(true)
+            .try_get_compiler()
+            .expect("Failed to get compiler");
+
+        // Verify clang-cl.exe works correctly in C++ mode
+        assert!(
+            compiler.is_like_clang_cl(),
+            "clang-cl.exe should be identified as clang-cl-like in C++ mode"
+        );
+        assert!(
+            compiler.is_like_msvc(),
+            "clang-cl should still be MSVC-like in C++ mode"
+        );
+    }
+}