Rollup merge of rust-lang#121978 - GuillaumeGomez:dylib-duplicated-path, r=bjorn3

Fix duplicated path in the "not found dylib" error

While working on the gcc backend, I couldn't figure out why I had this error:

```
error: couldn't load codegen backend /checkout/compiler/rustc_codegen_gcc/target/release/librustc_codegen_gcc.so/checkout/compiler/rustc_codegen_gcc/target/release/librustc_codegen_gcc.so: cannot open shared object file: No such file or directory
```

As you can see, the path is duplicated for some reason. After investigating a bit more, I realized that `libloading::Error::LoadLibraryExW` starts with the path of the not found dylib, making it appear twice in our error afterward (because we do render it like this: `{path}{err}`, and since the `err` starts with the path...).

Thanks to ```@bjorn3``` for linking me to rust-lang#121392. :)

Author: matthiaskrgr

compiler/rustc_metadata/src/creader.rs

@@ -1132,7 +1132,13 @@ fn load_dylib(path: &Path, max_attempts: usize) -> Result<libloading::Library, S
             Err(err) => {
                 // Only try to recover from this specific error.
                 if !matches!(err, libloading::Error::LoadLibraryExW { .. }) {
-                    return Err(err.to_string());
+                    let err = format_dlopen_err(&err);
+                    // We include the path of the dylib in the error ourselves, so
+                    // if it's in the error, we strip it.
+                    if let Some(err) = err.strip_prefix(&format!(": {}", path.display())) {
+                        return Err(err.to_string());
+                    }
+                    return Err(err);
                 }
 
                 last_error = Some(err);

tests/ui/codegen/duplicated-path-in-error.rs

@@ -0,0 +1,7 @@
+//@ only-linux
+//@ compile-flags: -Zcodegen-backend=/non-existing-one.so
+
+// This test ensures that the error of the "not found dylib" doesn't duplicate
+// the path of the dylib.
+
+fn main() {}

tests/ui/codegen/duplicated-path-in-error.stderr

@@ -0,0 +1,2 @@
+error: couldn't load codegen backend /non-existing-one.so: cannot open shared object file: No such file or directory
+