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 rust-lang#96799.

Author: estebank

compiler/rustc_resolve/src/imports.rs

@@ -547,6 +547,10 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             self.import_dummy_binding(*import, is_indeterminate);
 
             if let Some(err) = unresolved_import_error {
+                if let ImportKind::Glob { .. } = import.kind {
+                    self.glob_error = true;
+                }
+
                 if let ImportKind::Single { source, ref source_bindings, .. } = import.kind {
                     if source.name == kw::SelfLower {
                         // Silence `unresolved import` error if E0429 is already emitted

compiler/rustc_resolve/src/late.rs

@@ -3784,7 +3784,15 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
 
         let Finalize { node_id, path_span, .. } = finalize;
         let report_errors = |this: &mut Self, res: Option<Res>| {
-            if this.should_report_errs() {
+            if this.r.glob_error && !path.is_empty() {
+                // An invalid `use foo::*;` was found, which can cause unbounded ammounts of "item
+                // not found" errors. We silence them until all cases of these bad glob imports are
+                // resolved.
+                this.r.dcx().span_delayed_bug(
+                    path[0].ident.span,
+                    "silenced resolve error due to earlier failed glob import",
+                );
+            } else if this.should_report_errs() {
                 let (err, candidates) =
                     this.smart_resolve_report_errors(path, None, path_span, source, res);
 
@@ -3845,6 +3853,17 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                 _ => return Some(parent_err),
             };
 
+            if this.r.glob_error && !path.is_empty() {
+                // An invalid `use foo::*;` was found, which can cause unbounded ammounts of "item
+                // not found" errors. We silence them until all cases of these bad glob imports are
+                // resolved.
+                this.r.dcx().span_delayed_bug(
+                    path[0].ident.span,
+                    "silenced resolve error due to earlier failed glob import",
+                );
+                return None;
+            }
+
             let (mut err, candidates) = this.smart_resolve_report_errors(
                 prefix_path,
                 following_seg,

compiler/rustc_resolve/src/lib.rs

@@ -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: bool,
     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: false,
             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 2 previous errors
+error: aborting due to 1 previous error
 
-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`.