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 @@ pub(crate) fn finalize_imports(&mut self) {
         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 @@ pub(crate) fn finalize_imports(&mut self) {
             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 @@ pub(crate) fn finalize_imports(&mut self) {
                 {
                     // 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 @@ pub(crate) fn finalize_imports(&mut self) {
         }
 
         if !errors.is_empty() {
-            self.throw_unresolved_import_error(errors);
+            self.throw_unresolved_import_error(errors, glob_error);
             return;
         }
 
@@ -599,9 +602,7 @@ pub(crate) fn finalize_imports(&mut self) {
             }
         }
 
-        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 @@ pub(crate) fn check_hidden_glob_reexports(
         }
     }
 
-    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 @@ fn throw_unresolved_import_error(&mut self, errors: Vec<(Import<'_>, UnresolvedI
             }
         }
 
-        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 @@ fn report_error(&mut self, span: Span, resolution_error: ResolutionError<'a>) {
     }
 
     #[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::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 @@ pub fn new(
             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(),

tests/ui/imports/import-from-missing-star-2.rs

@@ -0,0 +1,12 @@
+mod foo {
+    use spam::*; //~ ERROR unresolved import `spam` [E0432]
+}
+
+fn main() {
+    // Expect this to pass because the compiler knows there's a failed `*` import in `foo` that
+    // might have caused it.
+    foo::bar();
+    // FIXME: these two should *fail* because they can't be fixed by fixing the glob import in `foo`
+    ham(); // should error but doesn't
+    eggs(); // should error but doesn't
+}

tests/ui/imports/import-from-missing-star-2.stderr

@@ -0,0 +1,11 @@
+error[E0432]: unresolved import `spam`
+  --> $DIR/import-from-missing-star-2.rs:2:9
+   |
+LL |     use spam::*;
+   |         ^^^^ maybe a missing crate `spam`?
+   |
+   = help: consider adding `extern crate spam` to use the `spam` crate
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0432`.

tests/ui/imports/import-from-missing-star-3.rs

@@ -0,0 +1,43 @@
+mod foo {
+    use spam::*; //~ ERROR unresolved import `spam` [E0432]
+
+    fn x() {
+        // Expect these to pass because the compiler knows there's a failed `*` import that might
+        // fix it.
+        eggs();
+        foo::bar();
+    }
+}
+
+mod bar {
+    fn z() {}
+    fn x() {
+        // Expect these to pass because the compiler knows there's a failed `*` import that might
+        // fix it.
+        foo::bar();
+        z();
+        // FIXME: should error but doesn't because as soon as there's a single glob import error, we
+        // silence all resolve errors.
+        eggs();
+    }
+}
+
+mod baz {
+    fn x() {
+        use spam::*; //~ ERROR unresolved import `spam` [E0432]
+        fn qux() {}
+        qux();
+        // Expect this to pass because the compiler knows there's a local failed `*` import that
+        // might have caused it.
+        eggs();
+        // Expect this to pass because the compiler knows there's a failed `*` import in `foo` that
+        // might have caused it.
+        foo::bar();
+    }
+}
+
+fn main() {
+    // FIXME: should error but doesn't because as soon as there's a single glob import error, we
+    // silence all resolve errors.
+    ham();
+}

tests/ui/imports/import-from-missing-star-3.stderr

@@ -0,0 +1,19 @@
+error[E0432]: unresolved import `spam`
+  --> $DIR/import-from-missing-star-3.rs:2:9
+   |
+LL |     use spam::*;
+   |         ^^^^ maybe a missing crate `spam`?
+   |
+   = help: consider adding `extern crate spam` to use the `spam` crate
+
+error[E0432]: unresolved import `spam`
+  --> $DIR/import-from-missing-star-3.rs:27:13
+   |
+LL |         use spam::*;
+   |             ^^^^ maybe a missing crate `spam`?
+   |
+   = help: consider adding `extern crate spam` to use the `spam` crate
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0432`.

tests/ui/imports/import-from-missing-star.rs

@@ -0,0 +1,10 @@
+use spam::*; //~ ERROR unresolved import `spam` [E0432]
+
+fn main() {
+    // Expect these to pass because the compiler knows there's a failed `*` import that might have
+    // caused it.
+    ham();
+    eggs();
+    // Even this case, as we might have expected `spam::foo` to exist.
+    foo::bar();
+}

tests/ui/imports/import-from-missing-star.stderr

@@ -0,0 +1,11 @@
+error[E0432]: unresolved import `spam`
+  --> $DIR/import-from-missing-star.rs:1:5
+   |
+LL | use spam::*;
+   |     ^^^^ maybe a missing crate `spam`?
+   |
+   = help: consider adding `extern crate spam` to use the `spam` crate
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0432`.

tests/ui/imports/issue-31212.rs

@@ -6,5 +6,5 @@ mod foo {
 }
 
 fn main() {
-    foo::f(); //~ ERROR cannot find function `f` in module `foo`
+    foo::f(); // cannot find function `f` in module `foo`, but silenced
 }

tests/ui/imports/issue-31212.stderr

@@ -4,13 +4,6 @@ error[E0432]: unresolved import `self::*`
 LL |     pub use self::*;
    |             ^^^^^^^ cannot glob-import a module into itself
 
-error[E0425]: cannot find function `f` in module `foo`
-  --> $DIR/issue-31212.rs:9:10
-   |
-LL |     foo::f();
-   |          ^ not found in `foo`
+error: aborting due to 1 previous error
 
-error: aborting due to 2 previous errors
-
-Some errors have detailed explanations: E0425, E0432.
-For more information about an error, try `rustc --explain E0425`.
+For more information about this error, try `rustc --explain E0432`.