[DO NOT MERGE] Remove most of existing_match

Author: bjorn3

compiler/rustc_metadata/src/creader.rs

@@ -516,7 +516,7 @@ impl CStore {
 
     fn existing_match(
         &self,
-        externs: &Externs,
+        _externs: &Externs,
         name: Symbol,
         hash: Option<Svh>,
     ) -> Option<CrateNum> {
@@ -535,6 +535,13 @@ impl CStore {
                 None => {}
             }
 
+            /*
+            if !data.used() {
+                // Ignore speculatively loaded crates. Using them can hide ambiguity errors:
+                // https://github.com/rust-lang/rust/issues/147966#issuecomment-3444198198
+                continue;
+            }
+
             // When the hash is None we're dealing with a top-level dependency
             // in which case we may have a specification on the command line for
             // this library. Even though an upstream library may have loaded
@@ -563,6 +570,7 @@ impl CStore {
             // 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