Structured suggestion for extern crate foo when foo isn't resolved in import

When encountering a name in an import that could have come from a crate that wasn't imported, use a structured suggestion to suggest `extern crate foo;` pointing at the right place in the crate.

When encountering `_` in an import, do not suggest `extern crate _;`.

```
error[E0432]: unresolved import `spam`
  --> $DIR/import-from-missing-star-3.rs:2:9
   |
LL |     use spam::*;
   |         ^^^^ maybe a missing crate `spam`?
   |
help: consider importing the `spam` crate
   |
LL + extern crate spam;
   |
```

Author: estebank

compiler/rustc_interface/src/passes.rs

@@ -544,7 +544,13 @@ fn resolver_for_lowering_raw<'tcx>(
     let arenas = Resolver::arenas();
     let _ = tcx.registered_tools(()); // Uses `crate_for_resolver`.
     let (krate, pre_configured_attrs) = tcx.crate_for_resolver(()).steal();
-    let mut resolver = Resolver::new(tcx, &pre_configured_attrs, krate.spans.inner_span, &arenas);
+    let mut resolver = Resolver::new(
+        tcx,
+        &pre_configured_attrs,
+        krate.spans.inner_span,
+        krate.spans.inject_use_span,
+        &arenas,
+    );
     let krate = configure_and_expand(krate, &pre_configured_attrs, &mut resolver);
 
     // Make sure we don't mutate the cstore from here on.

compiler/rustc_resolve/src/diagnostics.rs

@@ -2026,14 +2026,17 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                         Applicability::MaybeIncorrect,
                     )),
                 )
+            } else if ident.name == kw::Underscore {
+                (format!("`_` is not a valid crate or module name"), None)
             } else if self.tcx.sess.is_rust_2015() {
                 (
                     format!("you might be missing crate `{ident}`"),
                     Some((
-                        vec![],
-                        format!(
-                            "consider adding `extern crate {ident}` to use the `{ident}` crate"
-                        ),
+                        vec![(
+                            self.current_crate_outer_attr_insert_span,
+                            format!("extern crate {ident};\n"),
+                        )],
+                        format!("consider importing the `{ident}` crate"),
                         Applicability::MaybeIncorrect,
                     )),
                 )

compiler/rustc_resolve/src/lib.rs

@@ -1180,6 +1180,10 @@ pub struct Resolver<'a, 'tcx> {
     /// Simplified analogue of module `resolutions` but in trait impls, excluding glob delegations.
     /// Needed because glob delegations exclude explicitly defined names.
     impl_binding_keys: FxHashMap<LocalDefId, FxHashSet<BindingKey>>,
+
+    /// This is the `Span` where an `extern crate foo;` suggestion would be inserted, if `foo`
+    /// could be a crate that wasn't imported. For diagnostics use only.
+    current_crate_outer_attr_insert_span: Span,
 }
 
 /// Nothing really interesting here; it just provides memory for the rest of the crate.
@@ -1342,6 +1346,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         tcx: TyCtxt<'tcx>,
         attrs: &[ast::Attribute],
         crate_span: Span,
+        current_crate_outer_attr_insert_span: Span,
         arenas: &'a ResolverArenas<'a>,
     ) -> Resolver<'a, 'tcx> {
         let root_def_id = CRATE_DEF_ID.to_def_id();
@@ -1525,6 +1530,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
             glob_delegation_invoc_ids: Default::default(),
             impl_unexpanded_invocations: Default::default(),
             impl_binding_keys: Default::default(),
+            current_crate_outer_attr_insert_span,
         };
 
         let root_parent_scope = ParentScope::module(graph_root, &resolver);

tests/rustdoc-ui/ice-unresolved-import-100241.stderr

@@ -4,7 +4,10 @@ error[E0432]: unresolved import `inner`
 LL |     pub use inner::S;
    |             ^^^^^ you might be missing crate `inner`
    |
-   = help: consider adding `extern crate inner` to use the `inner` crate
+help: consider importing the `inner` crate
+   |
+LL + extern crate inner;
+   |
 
 error: aborting due to 1 previous error
 

tests/rustdoc-ui/intra-doc/unresolved-import-recovery.stderr

@@ -4,7 +4,10 @@ error[E0433]: failed to resolve: you might be missing crate `unresolved_crate`
 LL | use unresolved_crate::module::Name;
    |     ^^^^^^^^^^^^^^^^ you might be missing crate `unresolved_crate`
    |
-   = help: consider adding `extern crate unresolved_crate` to use the `unresolved_crate` crate
+help: consider importing the `unresolved_crate` crate
+   |
+LL + extern crate unresolved_crate;
+   |
 
 error: aborting due to 1 previous error
 

tests/rustdoc-ui/issues/issue-61732.stderr

@@ -4,7 +4,10 @@ error[E0433]: failed to resolve: you might be missing crate `r#mod`
 LL | pub(in crate::r#mod) fn main() {}
    |               ^^^^^ you might be missing crate `r#mod`
    |
-   = help: consider adding `extern crate r#mod` to use the `r#mod` crate
+help: consider importing the `r#mod` crate
+   |
+LL + extern crate r#mod;
+   |
 
 error: aborting due to 1 previous error
 

tests/ui/attributes/field-attributes-vis-unresolved.stderr

@@ -4,15 +4,21 @@ error[E0433]: failed to resolve: you might be missing crate `nonexistent`
 LL |     pub(in nonexistent) field: u8
    |            ^^^^^^^^^^^ you might be missing crate `nonexistent`
    |
-   = help: consider adding `extern crate nonexistent` to use the `nonexistent` crate
+help: consider importing the `nonexistent` crate
+   |
+LL + extern crate nonexistent;
+   |
 
 error[E0433]: failed to resolve: you might be missing crate `nonexistent`
   --> $DIR/field-attributes-vis-unresolved.rs:22:12
    |
 LL |     pub(in nonexistent) u8
    |            ^^^^^^^^^^^ you might be missing crate `nonexistent`
    |
-   = help: consider adding `extern crate nonexistent` to use the `nonexistent` crate
+help: consider importing the `nonexistent` crate
+   |
+LL + extern crate nonexistent;
+   |
 
 error: aborting due to 2 previous errors
 

tests/ui/error-codes/E0432.stderr

@@ -4,7 +4,10 @@ error[E0432]: unresolved import `something`
 LL | use something::Foo;
    |     ^^^^^^^^^ you might be missing crate `something`
    |
-   = help: consider adding `extern crate something` to use the `something` crate
+help: consider importing the `something` crate
+   |
+LL + extern crate something;
+   |
 
 error: aborting due to 1 previous error
 

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

@@ -4,7 +4,10 @@ error[E0432]: unresolved import `spam`
 LL |     use spam::*;
    |         ^^^^ you might be missing crate `spam`
    |
-   = help: consider adding `extern crate spam` to use the `spam` crate
+help: consider importing the `spam` crate
+   |
+LL + extern crate spam;
+   |
 
 error: aborting due to 1 previous error
 

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

@@ -4,15 +4,21 @@ error[E0432]: unresolved import `spam`
 LL |     use spam::*;
    |         ^^^^ you might be missing crate `spam`
    |
-   = help: consider adding `extern crate spam` to use the `spam` crate
+help: consider importing the `spam` crate
+   |
+LL + extern crate spam;
+   |
 
 error[E0432]: unresolved import `spam`
   --> $DIR/import-from-missing-star-3.rs:27:13
    |
 LL |         use spam::*;
    |             ^^^^ you might be missing crate `spam`
    |
-   = help: consider adding `extern crate spam` to use the `spam` crate
+help: consider importing the `spam` crate
+   |
+LL + extern crate spam;
+   |
 
 error: aborting due to 2 previous errors