suggest extern crate foo when failing to resolve use foo

fix ci error

Author: TaKO8Ki

compiler/rustc_resolve/src/diagnostics.rs

@@ -1836,9 +1836,18 @@ impl<'a> Resolver<'a> {
                     )),
                 )
             } else if self.session.edition() == Edition::Edition2015 {
-                (format!("maybe a missing crate `{}`?", ident), None)
+                (
+                    format!("maybe a missing crate `{ident}`?"),
+                    Some((
+                        vec![],
+                        format!(
+                            "consider adding `extern crate {ident}` to use the `{ident}` crate"
+                        ),
+                        Applicability::MaybeIncorrect,
+                    )),
+                )
             } else {
-                (format!("could not find `{}` in the crate root", ident), None)
+                (format!("could not find `{ident}` in the crate root"), None)
             }
         } else if i > 0 {
             let parent = path[i - 1].ident.name;
@@ -1849,7 +1858,7 @@ impl<'a> Resolver<'a> {
                     "the list of imported crates".to_owned()
                 }
                 kw::PathRoot | kw::Crate => "the crate root".to_owned(),
-                _ => format!("`{}`", parent),
+                _ => format!("`{parent}`"),
             };
 
             let mut msg = format!("could not find `{}` in {}", ident, parent);

compiler/rustc_resolve/src/imports.rs

@@ -475,6 +475,10 @@ impl<'a, 'b> ImportResolver<'a, 'b> {
             }
 
             if let Some((suggestions, msg, applicability)) = err.suggestion {
+                if suggestions.is_empty() {
+                    diag.help(&msg);
+                    continue;
+                }
                 diag.multipart_suggestion(&msg, suggestions, applicability);
             }
         }

src/test/rustdoc-ui/intra-doc/unresolved-import-recovery.stderr

@@ -3,6 +3,8 @@ error[E0433]: failed to resolve: maybe a missing crate `unresolved_crate`?
    |
 LL | use unresolved_crate::module::Name;
    |     ^^^^^^^^^^^^^^^^ maybe a missing crate `unresolved_crate`?
+   |
+   = help: consider adding `extern crate unresolved_crate` to use the `unresolved_crate` crate
 
 error: Compilation failed, aborting rustdoc
 

src/test/rustdoc-ui/issue-61732.stderr

@@ -3,6 +3,8 @@ error[E0433]: failed to resolve: maybe a missing crate `r#mod`?
    |
 LL | pub(in crate::r#mod) fn main() {}
    |               ^^^^^ maybe a missing crate `r#mod`?
+   |
+   = help: consider adding `extern crate r#mod` to use the `r#mod` crate
 
 error: Compilation failed, aborting rustdoc
 

src/test/ui/attributes/field-attributes-vis-unresolved.stderr

@@ -3,12 +3,16 @@ error[E0433]: failed to resolve: maybe a missing crate `nonexistent`?
    |
 LL |     pub(in nonexistent) field: u8
    |            ^^^^^^^^^^^ maybe a missing crate `nonexistent`?
+   |
+   = help: consider adding `extern crate nonexistent` to use the `nonexistent` crate
 
 error[E0433]: failed to resolve: maybe a missing crate `nonexistent`?
   --> $DIR/field-attributes-vis-unresolved.rs:22:12
    |
 LL |     pub(in nonexistent) u8
    |            ^^^^^^^^^^^ maybe a missing crate `nonexistent`?
+   |
+   = help: consider adding `extern crate nonexistent` to use the `nonexistent` crate
 
 error: aborting due to 2 previous errors
 

src/test/ui/error-codes/E0432.stderr

@@ -3,6 +3,8 @@ error[E0432]: unresolved import `something`
    |
 LL | use something::Foo;
    |     ^^^^^^^^^ maybe a missing crate `something`?
+   |
+   = help: consider adding `extern crate something` to use the `something` crate
 
 error: aborting due to previous error
 

src/test/ui/feature-gates/feature-gate-extern_absolute_paths.stderr

@@ -3,12 +3,16 @@ error[E0432]: unresolved import `core`
    |
 LL | use core::default;
    |     ^^^^ maybe a missing crate `core`?
+   |
+   = help: consider adding `extern crate core` to use the `core` crate
 
 error[E0433]: failed to resolve: maybe a missing crate `core`?
   --> $DIR/feature-gate-extern_absolute_paths.rs:4:19
    |
 LL |     let _: u8 = ::core::default::Default();
    |                   ^^^^ maybe a missing crate `core`?
+   |
+   = help: consider adding `extern crate core` to use the `core` crate
 
 error: aborting due to 2 previous errors
 

src/test/ui/imports/import3.stderr

@@ -3,6 +3,8 @@ error[E0432]: unresolved import `main`
    |
 LL | use main::bar;
    |     ^^^^ maybe a missing crate `main`?
+   |
+   = help: consider adding `extern crate main` to use the `main` crate
 
 error: aborting due to previous error
 

src/test/ui/imports/issue-1697.stderr

@@ -3,6 +3,8 @@ error[E0432]: unresolved import `unresolved`
    |
 LL | use unresolved::*;
    |     ^^^^^^^^^^ maybe a missing crate `unresolved`?
+   |
+   = help: consider adding `extern crate unresolved` to use the `unresolved` crate
 
 error: aborting due to previous error
 

src/test/ui/imports/issue-33464.stderr

@@ -3,18 +3,24 @@ error[E0432]: unresolved import `abc`
    |
 LL | use abc::one_el;
    |     ^^^ maybe a missing crate `abc`?
+   |
+   = help: consider adding `extern crate abc` to use the `abc` crate
 
 error[E0432]: unresolved import `abc`
   --> $DIR/issue-33464.rs:5:5
    |
 LL | use abc::{a, bbb, cccccc};
    |     ^^^ maybe a missing crate `abc`?
+   |
+   = help: consider adding `extern crate abc` to use the `abc` crate
 
 error[E0432]: unresolved import `a_very_long_name`
   --> $DIR/issue-33464.rs:7:5
    |
 LL | use a_very_long_name::{el, el2};
    |     ^^^^^^^^^^^^^^^^ maybe a missing crate `a_very_long_name`?
+   |
+   = help: consider adding `extern crate a_very_long_name` to use the `a_very_long_name` crate
 
 error: aborting due to 3 previous errors