Add CC_PREFER_CLANG_CL_OVER_MSVC environment variable to override prefer_clang_cl_over_msvc

Fixes #1673

Author: ArchangelX360

src/lib.rs

@@ -552,7 +552,7 @@ impl Build {
             shell_escaped_flags: None,
             build_cache: Arc::default(),
             inherit_rustflags: true,
-            prefer_clang_cl_over_msvc: false,
+            prefer_clang_cl_over_msvc: get_prefer_clang_cl_over_msvc_from_env().unwrap_or(false),
         }
     }
 
@@ -1386,8 +1386,17 @@ impl Build {
     /// Prefer to use clang-cl over msvc.
     ///
     /// This option defaults to `false`.
+    ///
+    /// This option is always overridden by the environment variable `CC_PREFER_CLANG_CL_OVER_MSVC`, if set
     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;
+        match get_prefer_clang_cl_over_msvc_from_env() {
+            Some(b) => {
+                self.cargo_output.print_warning(&format_args!("`prefer_clang_cl_over_msvc` is overridden by {:?} to {:?}, the value set by this call is ignored", CC_PREFER_CLANG_CL_OVER_MSVC_ENV_VAR, b));
+            },
+            None => {
+                self.prefer_clang_cl_over_msvc = prefer_clang_cl_over_msvc;
+            }
+        }
         self
     }
 
@@ -4475,6 +4484,19 @@ fn check_exe(mut exe: PathBuf) -> Option<PathBuf> {
     check.then_some(exe)
 }
 
+const CC_PREFER_CLANG_CL_OVER_MSVC_ENV_VAR: &str = "CC_PREFER_CLANG_CL_OVER_MSVC";
+fn get_prefer_clang_cl_over_msvc_from_env() -> Option<bool> {
+    #[allow(clippy::disallowed_methods)]
+    match env::var_os(CC_PREFER_CLANG_CL_OVER_MSVC_ENV_VAR) {
+        None => None,
+        Some(v) => match v.to_str() {
+            Some("1") | Some("true") | Some("yes") => Some(true),
+            Some("0") | Some("false") | Some("no") => Some(false),
+            _ => None,
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

tests/test.rs

@@ -1081,4 +1081,88 @@ mod msvc_clang_cl_tests {
             "clang-cl should still be MSVC-like in C++ mode"
         );
     }
+
+    #[test]
+    fn msvc_prefer_clang_cl_over_msvc_enabled_by_env() {
+        reset_env();
+        std::env::set_var("CC_PREFER_CLANG_CL_OVER_MSVC", "1");
+
+        let test = Test::msvc_autodetect();
+
+        let compiler = test
+            .gcc()
+            .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"
+        );
+        std::env::remove_var("CC_PREFER_CLANG_CL_OVER_MSVC"); // Clean up after test
+    }
+
+    #[test]
+    fn msvc_prefer_clang_cl_over_msvc_disabled_by_env() {
+        reset_env();
+        std::env::set_var("CC_PREFER_CLANG_CL_OVER_MSVC", "0");
+
+        let test = Test::msvc_autodetect();
+
+        let compiler = test
+            .gcc()
+            .try_get_compiler()
+            .expect("Failed to get compiler");
+
+        assert!(compiler.is_like_msvc(), "Should still be MSVC-like");
+        assert!(!compiler.is_like_clang_cl(), "Should not use clang-cl");
+        std::env::remove_var("CC_PREFER_CLANG_CL_OVER_MSVC"); // Clean up after test
+    }
+
+    #[test]
+    fn msvc_prefer_clang_cl_over_msvc_disabled_by_env_precedence() {
+        reset_env();
+        std::env::set_var("CC_PREFER_CLANG_CL_OVER_MSVC", "0");
+
+        let test = Test::msvc_autodetect();
+
+        let compiler = test
+            .gcc()
+            .prefer_clang_cl_over_msvc(true)
+            .try_get_compiler()
+            .expect("Failed to get compiler");
+
+        assert!(compiler.is_like_msvc(), "Should still be MSVC-like");
+        assert!(!compiler.is_like_clang_cl(), "Should not use clang-cl");
+        std::env::remove_var("CC_PREFER_CLANG_CL_OVER_MSVC"); // Clean up after test
+    }
+
+    #[test]
+    fn msvc_prefer_clang_cl_over_msvc_enabled_by_env_precedence() {
+        reset_env();
+        std::env::set_var("CC_PREFER_CLANG_CL_OVER_MSVC", "1");
+
+        let test = Test::msvc_autodetect();
+
+        let compiler = test
+            .gcc()
+            .prefer_clang_cl_over_msvc(false)
+            .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"
+        );
+        std::env::remove_var("CC_PREFER_CLANG_CL_OVER_MSVC"); // Clean up after test
+    }
 }