enum type instead of variant suggestion unification

Weirdly, we were deciding between a help note and a structured
suggestion based on whether the import candidate span was a dummy—but
we weren't using that span in any case! The dummy-ness of the span
(which appears to be a matter of this-crate vs. other-crate
definition) isn't the right criterion by which we should decide
whether it's germane to mention that "there is an enum variant";
instead, let's use the someness of `def` (which is used as the
`has_unexpected_resolution` argument to `error_code`).

Since `import_candidate_to_paths` has no other callers, we are free to
stop returning the span and rename the function. By using
`span_suggestions_`, we leverage the max-suggestions output limit
already built in to the emitter, thus resolving #56028.

In the matter of message wording, "you can" is redundant (and perhaps
too informal); prefer the imperative.

src/librustc_resolve/lib.rs

@@ -3213,23 +3213,33 @@ impl<'a> Resolver<'a> {
                 let enum_candidates =
                     this.lookup_import_candidates(ident, ns, is_enum_variant);
                 let mut enum_candidates = enum_candidates.iter()
-                    .map(|suggestion| import_candidate_to_paths(&suggestion)).collect::<Vec<_>>();
+                    .map(|suggestion| {
+                        import_candidate_to_enum_paths(&suggestion)
+                    }).collect::<Vec<_>>();
                 enum_candidates.sort();
-                for (sp, variant_path, enum_path) in enum_candidates {
-                    if sp.is_dummy() {
-                        let msg = format!("there is an enum variant `{}`, \
-                                           try using `{}`?",
-                                          variant_path,
-                                          enum_path);
-                        err.help(&msg);
+
+                if !enum_candidates.is_empty() {
+                    // contextualize for E0412 "cannot find type", but don't belabor the point
+                    // (that it's a variant) for E0573 "expected type, found variant"
+                    let preamble = if def.is_none() {
+                        let others = match enum_candidates.len() {
+                            1 => String::new(),
+                            2 => " and 1 other".to_owned(),
+                            n => format!(" and {} others", n)
+                        };
+                        format!("there is an enum variant `{}`{}; ",
+                                enum_candidates[0].0, others)
                     } else {
-                        err.span_suggestion_with_applicability(
-                            span,
-                            "you can try using the variant's enum",
-                            enum_path,
-                            Applicability::MachineApplicable,
-                        );
-                    }
+                        String::new()
+                    };
+                    let msg = format!("{}try using the variant's enum", preamble);
+
+                    err.span_suggestions_with_applicability(
+                        span,
+                        &msg,
+                        enum_candidates.into_iter().map(|(_variant, enum_ty)| enum_ty),
+                        Applicability::MachineApplicable,
+                    );
                 }
             }
             if path.len() == 1 && this.self_type_is_available(span) {
@@ -5128,8 +5138,8 @@ fn path_names_to_string(path: &Path) -> String {
                         .collect::<Vec<_>>())
 }
 
-/// Get the path for an enum and the variant from an `ImportSuggestion` for an enum variant.
-fn import_candidate_to_paths(suggestion: &ImportSuggestion) -> (Span, String, String) {
+/// Get the stringified path for an enum from an `ImportSuggestion` for an enum variant.
+fn import_candidate_to_enum_paths(suggestion: &ImportSuggestion) -> (String, String) {
     let variant_path = &suggestion.path;
     let variant_path_string = path_names_to_string(variant_path);
 
@@ -5140,7 +5150,7 @@ fn import_candidate_to_paths(suggestion: &ImportSuggestion) -> (Span, String, St
     };
     let enum_path_string = path_names_to_string(&enum_path);
 
-    (suggestion.path.span, variant_path_string, enum_path_string)
+    (variant_path_string, enum_path_string)
 }
 
 

src/test/ui/did_you_mean/issue-56028-there-is-an-enum-variant.rs

@@ -0,0 +1,13 @@
+enum PutDown { Set }
+enum AffixHeart { Set }
+enum CauseToBe { Set }
+enum Determine { Set }
+enum TableDishesAction { Set }
+enum Solidify { Set }
+enum UnorderedCollection { Set }
+
+fn setup() -> Set { Set }
+
+fn main() {
+    setup();
+}

src/test/ui/did_you_mean/issue-56028-there-is-an-enum-variant.stderr

@@ -0,0 +1,38 @@
+error[E0412]: cannot find type `Set` in this scope
+  --> $DIR/issue-56028-there-is-an-enum-variant.rs:9:15
+   |
+LL | fn setup() -> Set { Set }
+   |               ^^^ not found in this scope
+help: there is an enum variant `AffixHeart::Set` and 7 others; try using the variant's enum
+   |
+LL | fn setup() -> AffixHeart { Set }
+   |               ^^^^^^^^^^
+LL | fn setup() -> CauseToBe { Set }
+   |               ^^^^^^^^^
+LL | fn setup() -> Determine { Set }
+   |               ^^^^^^^^^
+LL | fn setup() -> PutDown { Set }
+   |               ^^^^^^^
+and 3 other candidates
+
+error[E0425]: cannot find value `Set` in this scope
+  --> $DIR/issue-56028-there-is-an-enum-variant.rs:9:21
+   |
+LL | fn setup() -> Set { Set }
+   |                     ^^^ not found in this scope
+help: possible candidates are found in other modules, you can import them into scope
+   |
+LL | use AffixHeart::Set;
+   |
+LL | use CauseToBe::Set;
+   |
+LL | use Determine::Set;
+   |
+LL | use PutDown::Set;
+   |
+and 3 other candidates
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0412, E0425.
+For more information about an error, try `rustc --explain E0412`.

src/test/ui/enum/enum-variant-type-2.stderr

@@ -5,7 +5,7 @@ LL | fn foo(x: Foo::Bar) {} //~ ERROR expected type, found variant `Foo::Bar`
    |           ^^^^^^^^
    |           |
    |           not a type
-   |           help: you can try using the variant's enum: `Foo`
+   |           help: try using the variant's enum: `Foo`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-17546.stderr

@@ -5,7 +5,7 @@ LL |     fn new() -> NoResult<MyEnum, String> {
    |                 --------^^^^^^^^^^^^^^^^
    |                 |
    |                 did you mean `Result`?
-   |                 help: you can try using the variant's enum: `foo::MyEnum`
+   |                 help: try using the variant's enum: `foo::MyEnum`
 
 error[E0573]: expected type, found variant `Result`
   --> $DIR/issue-17546.rs:32:17
@@ -48,7 +48,7 @@ LL | fn newer() -> NoResult<foo::MyEnum, String> {
    |               --------^^^^^^^^^^^^^^^^^^^^^
    |               |
    |               did you mean `Result`?
-   |               help: you can try using the variant's enum: `foo::MyEnum`
+   |               help: try using the variant's enum: `foo::MyEnum`
 
 error: aborting due to 4 previous errors
 

src/test/ui/issues/issue-30535.stderr

@@ -2,9 +2,10 @@ error[E0573]: expected type, found variant `foo::Foo::FooV`
   --> $DIR/issue-30535.rs:16:8
    |
 LL |     _: foo::Foo::FooV //~ ERROR expected type, found variant `foo::Foo::FooV`
-   |        ^^^^^^^^^^^^^^ not a type
-   |
-   = help: there is an enum variant `foo::Foo::FooV`, try using `foo::Foo`?
+   |        ^^^^^^^^^^^^^^
+   |        |
+   |        not a type
+   |        help: try using the variant's enum: `foo::Foo`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-35075.stderr

@@ -2,19 +2,21 @@ error[E0412]: cannot find type `Foo` in this scope
   --> $DIR/issue-35075.rs:12:12
    |
 LL |     inner: Foo<T> //~ ERROR cannot find type `Foo` in this scope
-   |            ^^^---
-   |            |
-   |            not found in this scope
-   |            help: you can try using the variant's enum: `Baz`
+   |            ^^^ not found in this scope
+help: there is an enum variant `Baz::Foo`; try using the variant's enum
+   |
+LL |     inner: Baz //~ ERROR cannot find type `Foo` in this scope
+   |            ^^^
 
 error[E0412]: cannot find type `Foo` in this scope
   --> $DIR/issue-35075.rs:16:9
    |
 LL |     Foo(Foo<T>) //~ ERROR cannot find type `Foo` in this scope
-   |         ^^^---
-   |         |
-   |         not found in this scope
-   |         help: you can try using the variant's enum: `Baz`
+   |         ^^^ not found in this scope
+help: there is an enum variant `Baz::Foo`; try using the variant's enum
+   |
+LL |     Foo(Baz) //~ ERROR cannot find type `Foo` in this scope
+   |         ^^^
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-35675.stderr

@@ -2,10 +2,11 @@ error[E0412]: cannot find type `Apple` in this scope
   --> $DIR/issue-35675.rs:17:29
    |
 LL | fn should_return_fruit() -> Apple {
+   |                             ^^^^^ not found in this scope
+help: there is an enum variant `Fruit::Apple`; try using the variant's enum
+   |
+LL | fn should_return_fruit() -> Fruit {
    |                             ^^^^^
-   |                             |
-   |                             not found in this scope
-   |                             help: you can try using the variant's enum: `Fruit`
 
 error[E0425]: cannot find function `Apple` in this scope
   --> $DIR/issue-35675.rs:19:5
@@ -24,7 +25,7 @@ LL | fn should_return_fruit_too() -> Fruit::Apple {
    |                                 ^^^^^^^^^^^^
    |                                 |
    |                                 not a type
-   |                                 help: you can try using the variant's enum: `Fruit`
+   |                                 help: try using the variant's enum: `Fruit`
 
 error[E0425]: cannot find function `Apple` in this scope
   --> $DIR/issue-35675.rs:25:5
@@ -41,27 +42,34 @@ error[E0573]: expected type, found variant `Ok`
    |
 LL | fn foo() -> Ok {
    |             ^^ not a type
+help: try using the variant's enum
    |
-   = help: there is an enum variant `std::prelude::v1::Ok`, try using `std::prelude::v1`?
-   = help: there is an enum variant `std::result::Result::Ok`, try using `std::result::Result`?
+LL | fn foo() -> std::prelude::v1 {
+   |             ^^^^^^^^^^^^^^^^
+LL | fn foo() -> std::result::Result {
+   |             ^^^^^^^^^^^^^^^^^^^
 
 error[E0412]: cannot find type `Variant3` in this scope
   --> $DIR/issue-35675.rs:34:13
    |
 LL | fn bar() -> Variant3 {
-   |             ^^^^^^^^
-   |             |
-   |             not found in this scope
-   |             help: you can try using the variant's enum: `x::Enum`
+   |             ^^^^^^^^ not found in this scope
+help: there is an enum variant `x::Enum::Variant3`; try using the variant's enum
+   |
+LL | fn bar() -> x::Enum {
+   |             ^^^^^^^
 
 error[E0573]: expected type, found variant `Some`
   --> $DIR/issue-35675.rs:38:13
    |
 LL | fn qux() -> Some {
    |             ^^^^ not a type
+help: try using the variant's enum
    |
-   = help: there is an enum variant `std::prelude::v1::Option::Some`, try using `std::prelude::v1::Option`?
-   = help: there is an enum variant `std::prelude::v1::Some`, try using `std::prelude::v1`?
+LL | fn qux() -> std::prelude::v1::Option {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^
+LL | fn qux() -> std::prelude::v1 {
+   |             ^^^^^^^^^^^^^^^^
 
 error: aborting due to 7 previous errors
 

src/test/ui/variants/variant-used-as-type.stderr

@@ -3,28 +3,24 @@ error[E0573]: expected type, found variant `Ty::A`
    |
 LL |     B(Ty::A),
    |       ^^^^^ not a type
-help: you can try using the variant's enum
-   |
-LL |     B(Ty),
-   |       ^^
-help: you can try using the variant's enum
+help: try using the variant's enum
    |
 LL |     B(E),
    |       ^
+LL |     B(Ty),
+   |       ^^
 
 error[E0573]: expected type, found variant `E::A`
   --> $DIR/variant-used-as-type.rs:27:6
    |
 LL | impl E::A {}
    |      ^^^^ not a type
-help: you can try using the variant's enum
-   |
-LL | impl Ty {}
-   |      ^^
-help: you can try using the variant's enum
+help: try using the variant's enum
    |
 LL | impl E {}
    |      ^
+LL | impl Ty {}
+   |      ^^
 
 error: aborting due to 2 previous errors