Silence some resolve errors when there have been glob import errors

When encountering `use foo::*;` where `foo` fails to be found, and we later
encounter resolution errors, we silence those later errors.

A single case of the above, for an *existing* import on a big codebase would
otherwise have a huge number of knock-down spurious errors.

Ideally, instead of a global flag to silence all subsequent resolve errors,
we'd want to introduce an unameable binding in the appropriate rib as a
sentinel when there's a failed glob import, so when we encounter a resolve
error we can search for that sentinel and if found, and only then, silence
that error. The current approach is just a quick proof of concept to
iterate over.

Partially address #96799.

compiler/rustc_resolve/src/imports.rs

@@ -536,6 +536,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         let determined_imports = mem::take(&mut self.determined_imports);
         let indeterminate_imports = mem::take(&mut self.indeterminate_imports);
 
+        let mut glob_error = false;
         for (is_indeterminate, import) in determined_imports
             .iter()
             .map(|i| (false, i))
@@ -547,6 +548,8 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             self.import_dummy_binding(*import, is_indeterminate);
 
             if let Some(err) = unresolved_import_error {
+                glob_error |= import.is_glob();
+
                 if let ImportKind::Single { source, ref source_bindings, .. } = import.kind {
                     if source.name == kw::SelfLower {
                         // Silence `unresolved import` error if E0429 is already emitted
@@ -562,7 +565,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 {
                     // In the case of a new import line, throw a diagnostic message
                     // for the previous line.
-                    self.throw_unresolved_import_error(errors);
+                    self.throw_unresolved_import_error(errors, glob_error);
                     errors = vec![];
                 }
                 if seen_spans.insert(err.span) {
@@ -573,7 +576,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         }
 
         if !errors.is_empty() {
-            self.throw_unresolved_import_error(errors);
+            self.throw_unresolved_import_error(errors, glob_error);
             return;
         }
 
@@ -599,9 +602,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             }
         }
 
-        if !errors.is_empty() {
-            self.throw_unresolved_import_error(errors);
-        }
+        self.throw_unresolved_import_error(errors, glob_error);
     }
 
     pub(crate) fn check_hidden_glob_reexports(
@@ -673,7 +674,11 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         }
     }
 
-    fn throw_unresolved_import_error(&mut self, errors: Vec<(Import<'_>, UnresolvedImportError)>) {
+    fn throw_unresolved_import_error(
+        &mut self,
+        errors: Vec<(Import<'_>, UnresolvedImportError)>,
+        glob_error: bool,
+    ) {
         if errors.is_empty() {
             return;
         }
@@ -752,7 +757,10 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             }
         }
 
-        diag.emit();
+        let guar = diag.emit();
+        if glob_error {
+            self.glob_error = Some(guar);
+        }
     }
 
     /// Attempts to resolve the given import, returning:

compiler/rustc_resolve/src/late.rs

@@ -4039,9 +4039,12 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
     }
 
     #[inline]
-    /// If we're actually rustdoc then avoid giving a name resolution error for `cfg()` items.
+    /// If we're actually rustdoc then avoid giving a name resolution error for `cfg()` items or
+    // an invalid `use foo::*;` was found, which can cause unbounded ammounts of "item not found"
+    // errors. We silence them all.
     fn should_report_errs(&self) -> bool {
         !(self.r.tcx.sess.opts.actually_rustdoc && self.in_func_body)
+            && !self.r.glob_error.is_some()
     }
 
     // Resolve in alternative namespaces if resolution in the primary namespace fails.

compiler/rustc_resolve/src/lib.rs

@@ -37,7 +37,7 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, FxIndexSet};
 use rustc_data_structures::intern::Interned;
 use rustc_data_structures::steal::Steal;
 use rustc_data_structures::sync::{FreezeReadGuard, Lrc};
-use rustc_errors::{Applicability, Diag, ErrCode};
+use rustc_errors::{Applicability, Diag, ErrCode, ErrorGuaranteed};
 use rustc_expand::base::{DeriveResolution, SyntaxExtension, SyntaxExtensionKind};
 use rustc_feature::BUILTIN_ATTRIBUTES;
 use rustc_hir::def::Namespace::{self, *};
@@ -1052,6 +1052,7 @@ pub struct Resolver<'a, 'tcx> {
 
     /// Maps glob imports to the names of items actually imported.
     glob_map: FxHashMap<LocalDefId, FxHashSet<Symbol>>,
+    glob_error: Option<ErrorGuaranteed>,
     visibilities_for_hashing: Vec<(LocalDefId, ty::Visibility)>,
     used_imports: FxHashSet<NodeId>,
     maybe_unused_trait_imports: FxIndexSet<LocalDefId>,
@@ -1421,6 +1422,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             ast_transform_scopes: FxHashMap::default(),
 
             glob_map: Default::default(),
+            glob_error: None,
             visibilities_for_hashing: Default::default(),
             used_imports: FxHashSet::default(),
             maybe_unused_trait_imports: Default::default(),