Format sources into the error message when loading codegen backends

cc rust-lang/rustc_codegen_cranelift#1447

Author: nagisa

compiler/rustc_interface/src/util.rs

@@ -163,13 +163,17 @@ pub(crate) fn run_in_thread_pool_with_globals<F: FnOnce() -> R + Send, R: Send>(
 
 fn load_backend_from_dylib(early_dcx: &EarlyDiagCtxt, path: &Path) -> MakeBackendFn {
     let lib = unsafe { Library::new(path) }.unwrap_or_else(|err| {
-        let err = format!("couldn't load codegen backend {path:?}: {err}");
+        let err = err.sources().fold(String::new(), |s, c| write!(s, ": {}", c).unwrap());
+        let err = format!("couldn't load codegen backend {path:?}{err}");
         early_dcx.early_fatal(err);
     });
 
     let backend_sym = unsafe { lib.get::<MakeBackendFn>(b"__rustc_codegen_backend") }
         .unwrap_or_else(|e| {
-            let err = format!("couldn't load codegen backend: {e}");
+            let err = err.sources().fold(String::new(), |s, c| write!(s, ": {}", c).unwrap());
+            let err = format!(
+                "`__rustc_codegen_backend` symbol lookup in the codegen backend failed: {err}"
+            );
             early_dcx.early_fatal(err);
         });