Emit a single error when importing a path with _

When encountering `use _;`, `use _::*'` or similar, do not emit two errors for that single mistake. This also side-steps the issue of resolve errors suggesting adding a crate named `_` to `Cargo.toml`.

Author: estebank

compiler/rustc_resolve/src/imports.rs

@@ -608,7 +608,9 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             }
         }
 
-        self.throw_unresolved_import_error(errors, glob_error);
+        if !errors.is_empty() {
+            self.throw_unresolved_import_error(errors, glob_error);
+        }
     }
 
     pub(crate) fn check_hidden_glob_reexports(
@@ -686,16 +688,19 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         errors.retain(|(_import, err)| match err.module {
             // Skip `use` errors for `use foo::Bar;` if `foo.rs` has unrecovered parse errors.
             Some(def_id) if self.mods_with_parse_errors.contains(&def_id) => false,
+            // If we've encountered something like `use _;`, we've already emitted an error stating
+            // that `_` is not a valid identifier, so we silence the resolve error.
+            _ if err.segment == Some(kw::Underscore) => false,
             _ => true,
         });
+
         if errors.is_empty() {
+            self.tcx.dcx().delayed_bug("expected a parse or \"`_` can't be an identifier\" error");
             return;
         }
 
-        /// Upper limit on the number of `span_label` messages.
-        const MAX_LABEL_COUNT: usize = 10;
-
         let span = MultiSpan::from_spans(errors.iter().map(|(_, err)| err.span).collect());
+
         let paths = errors
             .iter()
             .map(|(import, err)| {
@@ -715,6 +720,9 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             diag.note(note.clone());
         }
 
+        /// Upper limit on the number of `span_label` messages.
+        const MAX_LABEL_COUNT: usize = 10;
+
         for (import, err) in errors.into_iter().take(MAX_LABEL_COUNT) {
             if let Some(label) = err.label {
                 diag.span_label(err.span, label);

tests/ui/imports/multiple-extern-by-macro-for-underscore.ed2015.stderr

@@ -4,12 +4,5 @@ error: expected identifier, found reserved identifier `_`
 LL |     use ::_;
    |           ^ expected identifier, found reserved identifier
 
-error[E0432]: unresolved import `_`
-  --> $DIR/multiple-extern-by-macro-for-underscore.rs:18:9
-   |
-LL |     use ::_;
-   |         ^^^ no `_` in the root
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0432`.

tests/ui/imports/multiple-extern-by-macro-for-underscore.rs

@@ -15,6 +15,6 @@ macro_rules! m {
 m!();
 
 fn main() {
-    use ::_; //[ed2015]~ ERROR: unresolved import `_`
+    use ::_;
     //~^ ERROR: expected identifier, found reserved identifier `_`
 }

tests/ui/underscore-imports/issue-110164.ed2015.stderr

@@ -5,19 +5,19 @@ LL | use _::a;
    |     ^ expected identifier, found reserved identifier
 
 error: expected identifier, found reserved identifier `_`
-  --> $DIR/issue-110164.rs:11:5
+  --> $DIR/issue-110164.rs:10:5
    |
 LL | use _::*;
    |     ^ expected identifier, found reserved identifier
 
 error: expected identifier, found reserved identifier `_`
-  --> $DIR/issue-110164.rs:16:9
+  --> $DIR/issue-110164.rs:14:9
    |
 LL |     use _::a;
    |         ^ expected identifier, found reserved identifier
 
 error: expected identifier, found reserved identifier `_`
-  --> $DIR/issue-110164.rs:19:9
+  --> $DIR/issue-110164.rs:16:9
    |
 LL |     use _::*;
    |         ^ expected identifier, found reserved identifier
@@ -34,30 +34,6 @@ error[E0432]: unresolved import `crate::*`
 LL | use crate::*;
    |     ^^^^^^^^ cannot glob-import a module into itself
 
-error[E0432]: unresolved import `_`
-  --> $DIR/issue-110164.rs:11:5
-   |
-LL | use _::*;
-   |     ^ `_` is not a valid crate or module name
-
-error[E0432]: unresolved import `_`
-  --> $DIR/issue-110164.rs:8:5
-   |
-LL | use _::a;
-   |     ^ `_` is not a valid crate or module name
-
-error[E0432]: unresolved import `_`
-  --> $DIR/issue-110164.rs:16:9
-   |
-LL |     use _::a;
-   |         ^ `_` is not a valid crate or module name
-
-error[E0432]: unresolved import `_`
-  --> $DIR/issue-110164.rs:19:9
-   |
-LL |     use _::*;
-   |         ^ `_` is not a valid crate or module name
-
-error: aborting due to 10 previous errors
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0432`.

tests/ui/underscore-imports/issue-110164.ed2021.stderr

@@ -5,19 +5,19 @@ LL | use _::a;
    |     ^ expected identifier, found reserved identifier
 
 error: expected identifier, found reserved identifier `_`
-  --> $DIR/issue-110164.rs:11:5
+  --> $DIR/issue-110164.rs:10:5
    |
 LL | use _::*;
    |     ^ expected identifier, found reserved identifier
 
 error: expected identifier, found reserved identifier `_`
-  --> $DIR/issue-110164.rs:16:9
+  --> $DIR/issue-110164.rs:14:9
    |
 LL |     use _::a;
    |         ^ expected identifier, found reserved identifier
 
 error: expected identifier, found reserved identifier `_`
-  --> $DIR/issue-110164.rs:19:9
+  --> $DIR/issue-110164.rs:16:9
    |
 LL |     use _::*;
    |         ^ expected identifier, found reserved identifier
@@ -34,38 +34,6 @@ error[E0432]: unresolved import `crate::*`
 LL | use crate::*;
    |     ^^^^^^^^ cannot glob-import a module into itself
 
-error[E0432]: unresolved import `_`
-  --> $DIR/issue-110164.rs:11:5
-   |
-LL | use _::*;
-   |     ^ use of unresolved module or unlinked crate `_`
-   |
-   = help: you might be missing a crate named `_`
-
-error[E0432]: unresolved import `_`
-  --> $DIR/issue-110164.rs:8:5
-   |
-LL | use _::a;
-   |     ^ use of unresolved module or unlinked crate `_`
-   |
-   = help: you might be missing a crate named `_`
-
-error[E0432]: unresolved import `_`
-  --> $DIR/issue-110164.rs:16:9
-   |
-LL |     use _::a;
-   |         ^ use of unresolved module or unlinked crate `_`
-   |
-   = help: you might be missing a crate named `_`
-
-error[E0432]: unresolved import `_`
-  --> $DIR/issue-110164.rs:19:9
-   |
-LL |     use _::*;
-   |         ^ use of unresolved module or unlinked crate `_`
-   |
-   = help: you might be missing a crate named `_`
-
-error: aborting due to 10 previous errors
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0432`.

tests/ui/underscore-imports/issue-110164.rs

@@ -7,16 +7,12 @@ use crate::*;
 //~^ ERROR unresolved import `crate::*`
 use _::a;
 //~^ ERROR expected identifier, found reserved identifier `_`
-//~| ERROR unresolved import `_`
 use _::*;
 //~^ ERROR expected identifier, found reserved identifier `_`
-//~| ERROR unresolved import `_`
 
 fn main() {
     use _::a;
     //~^ ERROR expected identifier, found reserved identifier `_`
-    //~| ERROR unresolved import `_`
     use _::*;
     //~^ ERROR expected identifier, found reserved identifier `_`
-    //~| ERROR unresolved import `_`
 }