ignore compiler injected crate loading failure

Author: chenyukang

compiler/rustc_metadata/src/creader.rs

@@ -731,7 +731,11 @@ impl CStore {
             }
             Err(err) => {
                 debug!("failed to resolve crate {} {:?}", name, dep_kind);
-                if !tcx.sess.dcx().has_errors().is_some() {
+                // crate maybe injrected with `standard_library_imports::inject`, their span is dummy.
+                // we ignore compiler-injected prelude/sysroot loads here so they don't suppress
+                // unrelated diagnostics, such as `unsupported targets for std library` etc,
+                // these maybe helpful for users to resolve crate loading failure.
+                if !tcx.sess.dcx().has_errors().is_some() && !span.is_dummy() {
                     self.has_crate_resolve_with_fail = true;
                 }
                 let missing_core = self

tests/ui/extern-flag/empty-extern-arg.rs

@@ -5,3 +5,6 @@
 //@ ignore-emscripten missing eh_catch_typeinfo lang item
 
 fn main() {}
+
+//~? ERROR `#[panic_handler]` function required, but not found
+//~? ERROR unwinding panics are not supported without std

tests/ui/extern-flag/empty-extern-arg.stderr

@@ -2,5 +2,12 @@ error: extern location for std does not exist:
 
 error: cannot resolve a prelude import
 
-error: aborting due to 2 previous errors
+error: `#[panic_handler]` function required, but not found
+
+error: unwinding panics are not supported without std
+   |
+   = help: using nightly cargo, use -Zbuild-std with panic="abort" to avoid unwinding
+   = note: since the core library is usually precompiled with panic="unwind", rebuilding your crate with panic="abort" may not be enough to fix the problem
+
+error: aborting due to 4 previous errors