prevent using the default cc when that'd result in a broken build

Author: pietroalbini

src/bootstrap/cc_detect.rs

@@ -88,6 +88,9 @@ fn new_cc_build(build: &Build, target: TargetSelection) -> cc::Build {
 }
 
 pub fn find(build: &mut Build) {
+    let host_target = build.build;
+    let default_cc = new_cc_build(build, host_target).get_compiler();
+
     // For all targets we're going to need a C compiler for building some shims
     // and such as well as for being a linker for Rust code.
     let targets = build
@@ -107,6 +110,34 @@ pub fn find(build: &mut Build) {
         }
 
         let compiler = cfg.get_compiler();
+        if target != host_target
+            && !compiler.is_like_clang()
+            && compiler.path() == default_cc.path()
+        {
+            // See https://github.com/rust-lang/rust/issues/111142 for why this was added.
+
+            let underscore_target = target.triple.replace('-', "_");
+            eprintln!(
+                "Error: the automatic detection of the C compiler for target {target} \
+                 returned the same compiler as the currnet host target:"
+            );
+            eprintln!(
+                "This is likely wrong. Please specify the correct compiler for that target, \
+                either with environment variables:"
+            );
+            eprintln!();
+            eprintln!("    CC_{underscore_target}=path/to/target");
+            eprintln!("    CXX_{underscore_target}=path/to/target");
+            eprintln!();
+            eprintln!("...or in config.toml:");
+            eprintln!();
+            eprintln!("    [target.\"{target}\"]");
+            eprintln!("    cc = \"path/to/target\"");
+            eprintln!("    cxx = \"path/to/target\"");
+            eprintln!();
+            crate::detail_exit(1);
+        }
+
         let ar = if let ar @ Some(..) = config.and_then(|c| c.ar.clone()) {
             ar
         } else {