Auto merge of #2511 - RalfJung:extern-so, r=RalfJung

some extern-so cleanup and fixes

build.rs

@@ -2,5 +2,5 @@ fn main() {
     // Re-export the TARGET environment variable so it can
     // be accessed by miri.
     let target = std::env::var("TARGET").unwrap();
-    println!("cargo:rustc-env=TARGET={:?}", target);
+    println!("cargo:rustc-env=TARGET={}", target);
 }

src/bin/miri.rs

@@ -534,14 +534,14 @@ fn main() {
             let filename = param.to_string();
             if std::path::Path::new(&filename).exists() {
                 if let Some(other_filename) = miri_config.external_so_file {
-                    panic!(
-                        "-Zmiri-extern-so-file external SO file is already set to {}",
+                    show_error!(
+                        "-Zmiri-extern-so-file is already set to {}",
                         other_filename.display()
                     );
                 }
                 miri_config.external_so_file = Some(filename.into());
             } else {
-                panic!("-Zmiri-extern-so-file path {} does not exist", filename);
+                show_error!("-Zmiri-extern-so-file `{}` does not exist", filename);
             }
         } else {
             // Forward to rustc.

src/machine.rs

@@ -418,9 +418,11 @@ impl<'mir, 'tcx> Evaluator<'mir, 'tcx> {
             since_progress_report: 0,
             external_so_lib: config.external_so_file.as_ref().map(|lib_file_path| {
                 // Check if host target == the session target.
-                if option_env!("TARGET") == Some(target_triple) {
+                if env!("TARGET") != target_triple {
                     panic!(
-                        "calling external C functions in linked .so file requires target and host to be the same"
+                        "calling external C functions in linked .so file requires host and target to be the same: host={}, target={}",
+                        env!("TARGET"),
+                        target_triple,
                     );
                 }
                 // Note: it is the user's responsibility to provide a correct SO file.
@@ -429,7 +431,7 @@ impl<'mir, 'tcx> Evaluator<'mir, 'tcx> {
                 (
                     unsafe {
                         libloading::Library::new(lib_file_path)
-                            .expect("Failed to read specified shared object file")
+                            .expect("failed to read specified extern shared object file")
                     },
                     lib_file_path.clone(),
                 )

src/shims/foreign_items.rs

@@ -370,11 +370,10 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
     ) -> InterpResult<'tcx, EmulateByNameResult<'mir, 'tcx>> {
         let this = self.eval_context_mut();
 
-        // First deal with any external C functions in linked .so file
-        // (if any SO file is specified, and if the host target == the session target)
+        // First deal with any external C functions in linked .so file.
         if this.machine.external_so_lib.as_ref().is_some() {
             // An Ok(false) here means that the function being called was not exported
-            // by the specified SO file; we should continue and check if it corresponds to
+            // by the specified `.so` file; we should continue and check if it corresponds to
             // a provided shim.
             if this.call_external_c_fct(link_name, dest, args)? {
                 return Ok(EmulateByNameResult::NeedsJumping);