Disable -Zdual-proc-macros if the target doesn't support proc-macros

Author: bjorn3

src/bootstrap/src/core/builder.rs

@@ -1683,10 +1683,24 @@ impl<'a> Builder<'a> {
         match mode {
             Mode::Std | Mode::ToolBootstrap | Mode::ToolStd => {}
             Mode::Rustc | Mode::Codegen | Mode::ToolRustc => {
-                // Build proc macros both for the host and the target
+                // Build proc macros both for the host and the target unless proc-macros are not
+                // supported by the target.
                 if target != compiler.host && cmd_kind != Kind::Check {
-                    cargo.arg("-Zdual-proc-macros");
-                    rustflags.arg("-Zdual-proc-macros");
+                    let error = command(&self.rustc(compiler))
+                        .arg("--target")
+                        .arg(target.rustc_target_arg())
+                        .arg("--print=file-names")
+                        .arg("--crate-type=proc-macro")
+                        .arg("-")
+                        .run_capture(self)
+                        .stderr();
+                    let not_supported = error.lines().any(|line| {
+                        line.contains("unsupported crate type `proc-macro`")
+                    });
+                    if !not_supported {
+                        cargo.arg("-Zdual-proc-macros");
+                        rustflags.arg("-Zdual-proc-macros");
+                    }
                 }
             }
         }