Suggest extern crate foo when failing to resolve use foo

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
 

src/test/ui/imports/issue-36881.stderr

@@ -3,6 +3,8 @@ error[E0432]: unresolved import `issue_36881_aux`
    |
 LL |     use issue_36881_aux::Foo;
    |         ^^^^^^^^^^^^^^^ maybe a missing crate `issue_36881_aux`?
+   |
+   = help: consider adding `extern crate issue_36881_aux` to use the `issue_36881_aux` crate
 
 error: aborting due to previous error
 

src/test/ui/imports/issue-37887.stderr

@@ -3,6 +3,8 @@ error[E0432]: unresolved import `libc`
    |
 LL |     use libc::*;
    |         ^^^^ maybe a missing crate `libc`?
+   |
+   = help: consider adding `extern crate libc` to use the `libc` crate
 
 error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
   --> $DIR/issue-37887.rs:2:5

src/test/ui/imports/issue-53269.stderr

@@ -3,6 +3,8 @@ error[E0432]: unresolved import `nonexistent_module`
    |
 LL |     use nonexistent_module::mac;
    |         ^^^^^^^^^^^^^^^^^^ maybe a missing crate `nonexistent_module`?
+   |
+   = help: consider adding `extern crate nonexistent_module` to use the `nonexistent_module` crate
 
 error[E0659]: `mac` is ambiguous
   --> $DIR/issue-53269.rs:8:5

src/test/ui/imports/issue-55457.stderr

@@ -12,6 +12,8 @@ error[E0432]: unresolved import `non_existent`
    |
 LL | use non_existent::non_existent;
    |     ^^^^^^^^^^^^ maybe a missing crate `non_existent`?
+   |
+   = help: consider adding `extern crate non_existent` to use the `non_existent` crate
 
 error: cannot determine resolution for the derive macro `NonExistent`
   --> $DIR/issue-55457.rs:5:10

src/test/ui/imports/tool-mod-child.stderr

@@ -3,24 +3,32 @@ error[E0433]: failed to resolve: maybe a missing crate `clippy`?
    |
 LL | use clippy::a::b;
    |     ^^^^^^ maybe a missing crate `clippy`?
+   |
+   = help: consider adding `extern crate clippy` to use the `clippy` crate
 
 error[E0432]: unresolved import `clippy`
   --> $DIR/tool-mod-child.rs:1:5
    |
 LL | use clippy::a;
    |     ^^^^^^ maybe a missing crate `clippy`?
+   |
+   = help: consider adding `extern crate clippy` to use the `clippy` crate
 
 error[E0433]: failed to resolve: maybe a missing crate `rustdoc`?
   --> $DIR/tool-mod-child.rs:5:5
    |
 LL | use rustdoc::a::b;
    |     ^^^^^^^ maybe a missing crate `rustdoc`?
+   |
+   = help: consider adding `extern crate rustdoc` to use the `rustdoc` crate
 
 error[E0432]: unresolved import `rustdoc`
   --> $DIR/tool-mod-child.rs:4:5
    |
 LL | use rustdoc::a;
    |     ^^^^^^^ maybe a missing crate `rustdoc`?
+   |
+   = help: consider adding `extern crate rustdoc` to use the `rustdoc` crate
 
 error: aborting due to 4 previous errors
 

src/test/ui/imports/unresolved-imports-used.stderr

@@ -15,24 +15,32 @@ error[E0432]: unresolved import `foo`
    |
 LL | use foo::bar;
    |     ^^^ maybe a missing crate `foo`?
+   |
+   = help: consider adding `extern crate foo` to use the `foo` crate
 
 error[E0432]: unresolved import `baz`
   --> $DIR/unresolved-imports-used.rs:12:5
    |
 LL | use baz::*;
    |     ^^^ maybe a missing crate `baz`?
+   |
+   = help: consider adding `extern crate baz` to use the `baz` crate
 
 error[E0432]: unresolved import `foo2`
   --> $DIR/unresolved-imports-used.rs:14:5
    |
 LL | use foo2::bar2;
    |     ^^^^ maybe a missing crate `foo2`?
+   |
+   = help: consider adding `extern crate foo2` to use the `foo2` crate
 
 error[E0432]: unresolved import `baz2`
   --> $DIR/unresolved-imports-used.rs:15:5
    |
 LL | use baz2::*;
    |     ^^^^ maybe a missing crate `baz2`?
+   |
+   = help: consider adding `extern crate baz2` to use the `baz2` crate
 
 error[E0603]: function `quz` is private
   --> $DIR/unresolved-imports-used.rs:9:10

src/test/ui/keyword/extern/keyword-extern-as-identifier-use.stderr

@@ -14,6 +14,8 @@ error[E0432]: unresolved import `r#extern`
    |
 LL | use extern::foo;
    |     ^^^^^^ maybe a missing crate `r#extern`?
+   |
+   = help: consider adding `extern crate r#extern` to use the `r#extern` crate
 
 error: aborting due to 2 previous errors
 

src/test/ui/privacy/restricted/test.stderr

@@ -3,6 +3,8 @@ error[E0433]: failed to resolve: maybe a missing crate `bad`?
    |
 LL |     pub(in bad::path) mod m1 {}
    |            ^^^ maybe a missing crate `bad`?
+   |
+   = help: consider adding `extern crate bad` to use the `bad` crate
 
 error[E0742]: visibilities can only be restricted to ancestor modules
   --> $DIR/test.rs:51:12

src/test/ui/resolve/editions-crate-root-2015.stderr

@@ -3,12 +3,16 @@ error[E0433]: failed to resolve: maybe a missing crate `nonexistant`?
    |
 LL |     fn global_inner(_: ::nonexistant::Foo) {
    |                          ^^^^^^^^^^^ maybe a missing crate `nonexistant`?
+   |
+   = help: consider adding `extern crate nonexistant` to use the `nonexistant` crate
 
 error[E0433]: failed to resolve: maybe a missing crate `nonexistant`?
   --> $DIR/editions-crate-root-2015.rs:7:30
    |
 LL |     fn crate_inner(_: crate::nonexistant::Foo) {
    |                              ^^^^^^^^^^^ maybe a missing crate `nonexistant`?
+   |
+   = help: consider adding `extern crate nonexistant` to use the `nonexistant` crate
 
 error[E0412]: cannot find type `nonexistant` in the crate root
   --> $DIR/editions-crate-root-2015.rs:11:25

src/test/ui/resolve/extern-prelude-fail.stderr

@@ -3,12 +3,16 @@ error[E0432]: unresolved import `extern_prelude`
    |
 LL |     use extern_prelude::S;
    |         ^^^^^^^^^^^^^^ maybe a missing crate `extern_prelude`?
+   |
+   = help: consider adding `extern crate extern_prelude` to use the `extern_prelude` crate
 
 error[E0433]: failed to resolve: maybe a missing crate `extern_prelude`?
   --> $DIR/extern-prelude-fail.rs:8:15
    |
 LL |     let s = ::extern_prelude::S;
    |               ^^^^^^^^^^^^^^ maybe a missing crate `extern_prelude`?
+   |
+   = help: consider adding `extern crate extern_prelude` to use the `extern_prelude` crate
 
 error: aborting due to 2 previous errors
 

src/test/ui/resolve/issue-82865.stderr

@@ -3,6 +3,8 @@ error[E0433]: failed to resolve: maybe a missing crate `x`?
    |
 LL | use x::y::z;
    |     ^ maybe a missing crate `x`?
+   |
+   = help: consider adding `extern crate x` to use the `x` crate
 
 error[E0599]: no function or associated item named `z` found for struct `Box<_, _>` in the current scope
   --> $DIR/issue-82865.rs:8:10

src/test/ui/resolve/resolve-bad-visibility.stderr

@@ -21,12 +21,16 @@ error[E0433]: failed to resolve: maybe a missing crate `nonexistent`?
    |
 LL | pub(in nonexistent) struct G;
    |        ^^^^^^^^^^^ 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 `too_soon`?
   --> $DIR/resolve-bad-visibility.rs:8:8
    |
 LL | pub(in too_soon) struct H;
    |        ^^^^^^^^ maybe a missing crate `too_soon`?
+   |
+   = help: consider adding `extern crate too_soon` to use the `too_soon` crate
 
 error: aborting due to 5 previous errors
 

src/test/ui/simd/portable-intrinsics-arent-exposed.stderr

@@ -3,6 +3,8 @@ error[E0433]: failed to resolve: maybe a missing crate `core`?
    |
 LL | use core::simd::intrinsics;
    |     ^^^^ maybe a missing crate `core`?
+   |
+   = help: consider adding `extern crate core` to use the `core` crate
 
 error[E0432]: unresolved import `std::simd::intrinsics`
   --> $DIR/portable-intrinsics-arent-exposed.rs:5:5

src/test/ui/unresolved/unresolved-asterisk-imports.stderr

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

src/test/ui/unresolved/unresolved-import.rs

@@ -1,5 +1,6 @@
 use foo::bar; //~ ERROR unresolved import `foo` [E0432]
               //~^ maybe a missing crate `foo`?
+              //~| HELP consider adding `extern crate foo` to use the `foo` crate
 
 use bar::Baz as x; //~ ERROR unresolved import `bar::Baz` [E0432]
                    //~| no `Baz` in `bar`

src/test/ui/unresolved/unresolved-import.stderr

@@ -3,9 +3,11 @@ error[E0432]: unresolved import `foo`
    |
 LL | use foo::bar;
    |     ^^^ maybe a missing crate `foo`?
+   |
+   = help: consider adding `extern crate foo` to use the `foo` crate
 
 error[E0432]: unresolved import `bar::Baz`
-  --> $DIR/unresolved-import.rs:4:5
+  --> $DIR/unresolved-import.rs:5:5
    |
 LL | use bar::Baz as x;
    |     ^^^^^---^^^^^
@@ -14,7 +16,7 @@ LL | use bar::Baz as x;
    |     no `Baz` in `bar`
 
 error[E0432]: unresolved import `food::baz`
-  --> $DIR/unresolved-import.rs:9:5
+  --> $DIR/unresolved-import.rs:10:5
    |
 LL | use food::baz;
    |     ^^^^^^---
@@ -23,7 +25,7 @@ LL | use food::baz;
    |     no `baz` in `food`
 
 error[E0432]: unresolved import `food::beens`
-  --> $DIR/unresolved-import.rs:14:12
+  --> $DIR/unresolved-import.rs:15:12
    |
 LL | use food::{beens as Foo};
    |            -----^^^^^^^
@@ -32,13 +34,13 @@ LL | use food::{beens as Foo};
    |            help: a similar name exists in the module: `beans`
 
 error[E0432]: unresolved import `MyEnum`
-  --> $DIR/unresolved-import.rs:38:9
+  --> $DIR/unresolved-import.rs:39:9
    |
 LL |     use MyEnum::*;
    |         ^^^^^^ help: a similar path exists: `self::MyEnum`
 
 error[E0432]: unresolved import `Enum`
-  --> $DIR/unresolved-import.rs:48:9
+  --> $DIR/unresolved-import.rs:49:9
    |
 LL |     use Enum::*;
    |         ^^^^ help: a similar path exists: `self::Enum`