Format sources into the error message when loading codegen backends

cc rust-lang/rustc_codegen_cranelift#1447

Author: nagisa

compiler/rustc_codegen_cranelift/Cargo.lock

@@ -246,12 +246,12 @@ checksum = "9cdc71e17332e86d2e1d38c1f99edcb6288ee11b815fb1a4b049eaa2114d369b"
 
 [[package]]
 name = "libloading"
-version = "0.7.4"
+version = "0.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b67380fd3b2fbe7527a606e18729d21c6f3951633d0500574c4dc22d2d638b9f"
+checksum = "c571b676ddfc9a8c12f1f3d3085a7b163966a8fd8098a90640953ce5f6170161"
 dependencies = [
  "cfg-if",
- "winapi",
+ "windows-sys",
 ]
 
 [[package]]

compiler/rustc_interface/src/lib.rs

@@ -1,9 +1,10 @@
 #![feature(box_patterns)]
 #![feature(decl_macro)]
+#![feature(error_iter)]
 #![feature(internal_output_capture)]
-#![feature(thread_spawn_unchecked)]
 #![feature(lazy_cell)]
 #![feature(let_chains)]
+#![feature(thread_spawn_unchecked)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]
 #![deny(rustc::untranslatable_diagnostic)]

compiler/rustc_interface/src/util.rs

@@ -162,15 +162,21 @@ 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 {
+    fn format_err(e: &(dyn std::error::Error + 'static)) -> String {
+        e.sources().map(|e| format!(": {e}")).collect()
+    }
     let lib = unsafe { Library::new(path) }.unwrap_or_else(|err| {
-        let err = format!("couldn't load codegen backend {path:?}: {err}");
+        let err = format!("couldn't load codegen backend {path:?}{}", format_err(&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}");
-            early_dcx.early_fatal(err);
+            let e = format!(
+                "`__rustc_codegen_backend` symbol lookup in the codegen backend failed{}",
+                format_err(&e)
+            );
+            early_dcx.early_fatal(e);
         });
 
     // Intentionally leak the dynamic library. We can't ever unload it