Copy and paste the --extern flag patching

cargo-miri/bin.rs

@@ -553,7 +553,7 @@ fn phase_cargo_miri(mut args: env::Args) {
     exec(cmd)
 }
 
-fn phase_cargo_rustc(args: env::Args) {
+fn phase_cargo_rustc(mut args: env::Args) {
     /// Determines if we are being invoked (as rustc) to build a crate for
     /// the "target" architecture, in contrast to the "host" architecture.
     /// Host crates are for build scripts and proc macros and still need to
@@ -646,7 +646,8 @@ fn phase_cargo_rustc(args: env::Args) {
     if !print && target_crate {
         // Forward arguments, but remove "link" from "--emit" to make this a check-only build.
         let emit_flag = "--emit";
-        for arg in args {
+        let extern_flag = "--extern";
+        while let Some(arg) = args.next() {
             if arg.starts_with(emit_flag) {
                 // Patch this argument. First, extract its value.
                 let val = &arg[emit_flag.len()..];
@@ -662,6 +663,17 @@ fn phase_cargo_rustc(args: env::Args) {
                     }
                 }
                 cmd.arg(format!("{}={}", emit_flag, val.join(",")));
+            } else if arg == extern_flag {
+                cmd.arg(extern_flag); // always forward flag, but adjust filename
+                // `--extern` is always passed as a separate argument by cargo.
+                let next_arg = args.next().expect("`--extern` should be followed by a filename");
+                if let Some(next_lib) = next_arg.strip_suffix(".rlib") {
+                    // If this is an rlib, make it an rmeta.
+                    cmd.arg(format!("{}.rmeta", next_lib));
+                } else {
+                    // Some other extern file (e.g., a `.so`). Forward unchanged.
+                    cmd.arg(next_arg);
+                }
             } else {
                 cmd.arg(arg);
             }