Don't leak sysroot crates through dependencies

Previously if a dependency of the current crate depended on a sysroot
crate, then extern crate would in the current crate would pick the first
loaded version of said sysroot crate even in case of an ambiguity. This
is surprising and brittle. For -Ldependency= we already blocked this,
but the fix didn't account for sysroot crates.

Author: bjorn3

compiler/rustc_metadata/src/creader.rs

@@ -519,7 +519,6 @@ impl CStore {
         externs: &Externs,
         name: Symbol,
         hash: Option<Svh>,
-        kind: PathKind,
     ) -> Option<CrateNum> {
         for (cnum, data) in self.iter_crate_data() {
             if data.name() != name {
@@ -561,27 +560,9 @@ impl CStore {
                 continue;
             }
 
-            // Alright, so we've gotten this far which means that `data` has the
-            // right name, we don't have a hash, and we don't have a --extern
-            // pointing for ourselves. We're still not quite yet done because we
-            // have to make sure that this crate was found in the crate lookup
-            // path (this is a top-level dependency) as we don't want to
-            // implicitly load anything inside the dependency lookup path.
-            let prev_kind = source
-                .dylib
-                .as_ref()
-                .or(source.rlib.as_ref())
-                .or(source.rmeta.as_ref())
-                .expect("No sources for crate")
-                .1;
-            if kind.matches(prev_kind) {
-                return Some(cnum);
-            } else {
-                debug!(
-                    "failed to load existing crate {}; kind {:?} did not match prev_kind {:?}",
-                    name, kind, prev_kind
-                );
-            }
+            // While the crate name matched, no --extern crate_name=path matched. It is possible
+            // that we have already loaded the target crate, but if that happens CStore::load will
+            // indicate so and we gracefully handle this, just potentially wasting a bit of time.
         }
 
         None
@@ -818,9 +799,7 @@ impl CStore {
         let path_kind = if dep.is_some() { PathKind::Dependency } else { PathKind::Crate };
         let private_dep = origin.private_dep();
 
-        let result = if let Some(cnum) =
-            self.existing_match(&tcx.sess.opts.externs, name, hash, path_kind)
-        {
+        let result = if let Some(cnum) = self.existing_match(&tcx.sess.opts.externs, name, hash) {
             (LoadResult::Previous(cnum), None)
         } else {
             info!("falling back to a load");