resolve: Do not finalize shadowed bindings

I.e. do not mark them as used, or non-speculative loaded, or similar.
Previously they were sometimes finalized during early resolution, causing issues like #144793 (comment).

Author: petrochenkov

compiler/rustc_resolve/src/ident.rs

@@ -429,6 +429,10 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         // to detect potential ambiguities.
         let mut innermost_result: Option<(NameBinding<'_>, Flags)> = None;
         let mut determinacy = Determinacy::Determined;
+        // Shadowed bindings don't need to be marked as used or non-speculatively loaded.
+        macro finalize_scope() {
+            if innermost_result.is_none() { finalize } else { None }
+        }
 
         // Go through all the scopes and try to resolve the name.
         let break_result = self.visit_scopes(
@@ -494,13 +498,13 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         _ => Err(Determinacy::Determined),
                     },
                     Scope::Module(module, derive_fallback_lint_id) => {
-                        let (adjusted_parent_scope, finalize) =
+                        let (adjusted_parent_scope, adjusted_finalize) =
                             if matches!(scope_set, ScopeSet::ModuleAndExternPrelude(..)) {
-                                (parent_scope, finalize)
+                                (parent_scope, finalize_scope!())
                             } else {
                                 (
                                     &ParentScope { module, ..*parent_scope },
-                                    finalize.map(|f| Finalize { used: Used::Scope, ..f }),
+                                    finalize_scope!().map(|f| Finalize { used: Used::Scope, ..f }),
                                 )
                             };
                         let binding = this.reborrow().resolve_ident_in_module_unadjusted(
@@ -513,7 +517,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             } else {
                                 Shadowing::Restricted
                             },
-                            finalize,
+                            adjusted_finalize,
                             ignore_binding,
                             ignore_import,
                         );
@@ -562,7 +566,8 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         None => Err(Determinacy::Determined),
                     },
                     Scope::ExternPrelude => {
-                        match this.reborrow().extern_prelude_get(ident, finalize.is_some()) {
+                        match this.reborrow().extern_prelude_get(ident, finalize_scope!().is_some())
+                        {
                             Some(binding) => Ok((binding, Flags::empty())),
                             None => Err(Determinacy::determined(
                                 this.graph_root.unexpanded_invocations.borrow().is_empty(),
@@ -599,8 +604,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             if matches!(ident.name, sym::f16)
                                 && !this.tcx.features().f16()
                                 && !ident.span.allows_unstable(sym::f16)
-                                && finalize.is_some()
-                                && innermost_result.is_none()
+                                && finalize_scope!().is_some()
                             {
                                 feature_err(
                                     this.tcx.sess,
@@ -613,8 +617,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             if matches!(ident.name, sym::f128)
                                 && !this.tcx.features().f128()
                                 && !ident.span.allows_unstable(sym::f128)
-                                && finalize.is_some()
-                                && innermost_result.is_none()
+                                && finalize_scope!().is_some()
                             {
                                 feature_err(
                                     this.tcx.sess,

compiler/rustc_resolve/src/lib.rs

@@ -15,6 +15,7 @@
 #![feature(arbitrary_self_types)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
+#![feature(decl_macro)]
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
 #![feature(rustc_attrs)]

tests/ui/resolve/extern-prelude-speculative.rs

@@ -0,0 +1,10 @@
+// Non-existent path in `--extern` doesn't result in an error if it's shadowed by `extern crate`.
+
+//@ check-pass
+//@ compile-flags: --extern something=/path/to/nowhere
+
+extern crate std as something;
+
+fn main() {
+    something::println!();
+}

tests/ui/rust-2018/uniform-paths/deadlock.rs

@@ -2,7 +2,7 @@
 //@ compile-flags:--extern foo --extern bar
 
 use bar::foo; //~ ERROR can't find crate for `bar`
-use foo::bar; //~ ERROR can't find crate for `foo`
+use foo::bar;
 //~^^ ERROR unresolved imports `bar::foo`, `foo::bar`
 
 fn main() {}

tests/ui/rust-2018/uniform-paths/deadlock.stderr

@@ -4,12 +4,6 @@ error[E0463]: can't find crate for `bar`
 LL | use bar::foo;
    |     ^^^ can't find crate
 
-error[E0463]: can't find crate for `foo`
-  --> $DIR/deadlock.rs:5:5
-   |
-LL | use foo::bar;
-   |     ^^^ can't find crate
-
 error[E0432]: unresolved imports `bar::foo`, `foo::bar`
   --> $DIR/deadlock.rs:4:5
    |
@@ -18,7 +12,7 @@ LL | use bar::foo;
 LL | use foo::bar;
    |     ^^^^^^^^
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors
 
 Some errors have detailed explanations: E0432, E0463.
 For more information about an error, try `rustc --explain E0432`.