Rollup merge of #84818 - ABouttefeux:enum-suggest, r=jackh726

suggestion for unit enum variant when matched with a patern

resolve #84700

add suggestion for code like
```rust
enum FarmAnimal {
    Worm,
    Cow,
    Bull,
    Chicken { num_eggs: usize },
    Dog (String),
}

fn what_does_the_animal_say(animal: &FarmAnimal) {

    let noise = match animal {
        FarmAnimal::Cow(_) => "moo".to_string(),
        _ => todo!()
    };

    println!("{:?} says: {:?}", animal, noise);
}
```

```
error[E0532]: expected tuple struct or tuple variant, found unit variant `FarmAnimal::Cow`
  --> $DIR/issue-84700.rs:15:9
   |
LL |     Cow,
   |     --- `FarmAnimal::Cow` defined here
...
LL |         FarmAnimal::Cow(_) => "moo".to_string(),
   |         ^^^^^^^^^^^^^^^^^^ help: use this syntax instead: `FarmAnimal::Cow`
   ```

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -819,6 +819,19 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
             _ => false,
         };
 
+        let find_span = |source: &PathSource<'_>, err: &mut DiagnosticBuilder<'_>| {
+            match source {
+                PathSource::Expr(Some(Expr { span, kind: ExprKind::Call(_, _), .. }))
+                | PathSource::TupleStruct(span, _) => {
+                    // We want the main underline to cover the suggested code as well for
+                    // cleaner output.
+                    err.set_span(*span);
+                    *span
+                }
+                _ => span,
+            }
+        };
+
         let mut bad_struct_syntax_suggestion = |def_id: DefId| {
             let (followed_by_brace, closing_brace) = self.followed_by_brace(span);
 
@@ -862,18 +875,7 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                     }
                 }
                 PathSource::Expr(_) | PathSource::TupleStruct(..) | PathSource::Pat => {
-                    let span = match &source {
+                    let span = find_span(&source, err);
-                        PathSource::Expr(Some(Expr {
-                            span, kind: ExprKind::Call(_, _), ..
-                        }))
-                        | PathSource::TupleStruct(span, _) => {
-                            // We want the main underline to cover the suggested code as well for
-                            // cleaner output.
-                            err.set_span(*span);
-                            *span
-                        }
-                        _ => span,
-                    };
                     if let Some(span) = self.def_span(def_id) {
                         err.span_label(span, &format!("`{}` defined here", path_str));
                     }
@@ -1047,6 +1049,23 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
             ) if ns == ValueNS => {
                 bad_struct_syntax_suggestion(def_id);
             }
+            (Res::Def(DefKind::Ctor(_, CtorKind::Const), def_id), _) if ns == ValueNS => {
+                match source {
+                    PathSource::Expr(_) | PathSource::TupleStruct(..) | PathSource::Pat => {
+                        let span = find_span(&source, err);
+                        if let Some(span) = self.def_span(def_id) {
+                            err.span_label(span, &format!("`{}` defined here", path_str));
+                        }
+                        err.span_suggestion(
+                            span,
+                            &format!("use this syntax instead"),
+                            format!("{path_str}"),
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                    _ => return false,
+                }
+            }
             (Res::Def(DefKind::Ctor(_, CtorKind::Fn), def_id), _) if ns == ValueNS => {
                 if let Some(span) = self.def_span(def_id) {
                     err.span_label(span, &format!("`{}` defined here", path_str));

src/test/ui/empty/empty-struct-unit-pat.stderr

@@ -1,84 +1,154 @@
 error[E0532]: expected tuple struct or tuple variant, found unit struct `Empty2`
   --> $DIR/empty-struct-unit-pat.rs:21:9
    |
+LL | struct Empty2;
+   | -------------- `Empty2` defined here
+...
 LL |         Empty2() => ()
-   |         ^^^^^^ help: a tuple struct with a similar name exists: `XEmpty6`
+   |         ^^^^^^^^
    | 
   ::: $DIR/auxiliary/empty-struct.rs:3:1
    |
 LL | pub struct XEmpty6();
    | --------------------- similarly named tuple struct `XEmpty6` defined here
+   |
+help: use this syntax instead
+   |
+LL |         Empty2 => ()
+   |         ^^^^^^
+help: a tuple struct with a similar name exists
+   |
+LL |         XEmpty6() => ()
+   |         ^^^^^^^
 
 error[E0532]: expected tuple struct or tuple variant, found unit struct `XEmpty2`
   --> $DIR/empty-struct-unit-pat.rs:24:9
    |
 LL |         XEmpty2() => ()
-   |         ^^^^^^^ help: a tuple struct with a similar name exists: `XEmpty6`
+   |         ^^^^^^^^^
    | 
-  ::: $DIR/auxiliary/empty-struct.rs:3:1
+  ::: $DIR/auxiliary/empty-struct.rs:2:1
    |
+LL | pub struct XEmpty2;
+   | ------------------- `XEmpty2` defined here
 LL | pub struct XEmpty6();
    | --------------------- similarly named tuple struct `XEmpty6` defined here
+   |
+help: use this syntax instead
+   |
+LL |         XEmpty2 => ()
+   |         ^^^^^^^
+help: a tuple struct with a similar name exists
+   |
+LL |         XEmpty6() => ()
+   |         ^^^^^^^
 
 error[E0532]: expected tuple struct or tuple variant, found unit struct `Empty2`
   --> $DIR/empty-struct-unit-pat.rs:28:9
    |
+LL | struct Empty2;
+   | -------------- `Empty2` defined here
+...
 LL |         Empty2(..) => ()
-   |         ^^^^^^ help: a tuple struct with a similar name exists: `XEmpty6`
+   |         ^^^^^^^^^^
    | 
   ::: $DIR/auxiliary/empty-struct.rs:3:1
    |
 LL | pub struct XEmpty6();
    | --------------------- similarly named tuple struct `XEmpty6` defined here
+   |
+help: use this syntax instead
+   |
+LL |         Empty2 => ()
+   |         ^^^^^^
+help: a tuple struct with a similar name exists
+   |
+LL |         XEmpty6(..) => ()
+   |         ^^^^^^^
 
 error[E0532]: expected tuple struct or tuple variant, found unit struct `XEmpty2`
   --> $DIR/empty-struct-unit-pat.rs:32:9
    |
 LL |         XEmpty2(..) => ()
-   |         ^^^^^^^ help: a tuple struct with a similar name exists: `XEmpty6`
+   |         ^^^^^^^^^^^
    | 
-  ::: $DIR/auxiliary/empty-struct.rs:3:1
+  ::: $DIR/auxiliary/empty-struct.rs:2:1
    |
+LL | pub struct XEmpty2;
+   | ------------------- `XEmpty2` defined here
 LL | pub struct XEmpty6();
    | --------------------- similarly named tuple struct `XEmpty6` defined here
+   |
+help: use this syntax instead
+   |
+LL |         XEmpty2 => ()
+   |         ^^^^^^^
+help: a tuple struct with a similar name exists
+   |
+LL |         XEmpty6(..) => ()
+   |         ^^^^^^^
 
 error[E0532]: expected tuple struct or tuple variant, found unit variant `E::Empty4`
   --> $DIR/empty-struct-unit-pat.rs:37:9
    |
+LL |     Empty4
+   |     ------ `E::Empty4` defined here
+...
 LL |         E::Empty4() => ()
-   |         ^^^^^^^^^ not a tuple struct or tuple variant
+   |         ^^^^^^^^^^^ help: use this syntax instead: `E::Empty4`
 
 error[E0532]: expected tuple struct or tuple variant, found unit variant `XE::XEmpty4`
   --> $DIR/empty-struct-unit-pat.rs:41:9
    |
 LL |         XE::XEmpty4() => (),
-   |         ^^^^-------
+   |         ^^^^^^^^^^^^^
-   |             |
-   |             help: a tuple variant with a similar name exists: `XEmpty5`
    | 
-  ::: $DIR/auxiliary/empty-struct.rs:8:5
+  ::: $DIR/auxiliary/empty-struct.rs:7:5
    |
+LL |     XEmpty4,
+   |     ------- `XE::XEmpty4` defined here
 LL |     XEmpty5(),
    |     --------- similarly named tuple variant `XEmpty5` defined here
+   |
+help: use this syntax instead
+   |
+LL |         XE::XEmpty4 => (),
+   |         ^^^^^^^^^^^
+help: a tuple variant with a similar name exists
+   |
+LL |         XE::XEmpty5() => (),
+   |             ^^^^^^^
 
 error[E0532]: expected tuple struct or tuple variant, found unit variant `E::Empty4`
   --> $DIR/empty-struct-unit-pat.rs:46:9
    |
+LL |     Empty4
+   |     ------ `E::Empty4` defined here
+...
 LL |         E::Empty4(..) => ()
-   |         ^^^^^^^^^ not a tuple struct or tuple variant
+   |         ^^^^^^^^^^^^^ help: use this syntax instead: `E::Empty4`
 
 error[E0532]: expected tuple struct or tuple variant, found unit variant `XE::XEmpty4`
   --> $DIR/empty-struct-unit-pat.rs:50:9
    |
 LL |         XE::XEmpty4(..) => (),
-   |         ^^^^-------
+   |         ^^^^^^^^^^^^^^^
-   |             |
-   |             help: a tuple variant with a similar name exists: `XEmpty5`
    | 
-  ::: $DIR/auxiliary/empty-struct.rs:8:5
+  ::: $DIR/auxiliary/empty-struct.rs:7:5
    |
+LL |     XEmpty4,
+   |     ------- `XE::XEmpty4` defined here
 LL |     XEmpty5(),
    |     --------- similarly named tuple variant `XEmpty5` defined here
+   |
+help: use this syntax instead
+   |
+LL |         XE::XEmpty4 => (),
+   |         ^^^^^^^^^^^
+help: a tuple variant with a similar name exists
+   |
+LL |         XE::XEmpty5(..) => (),
+   |             ^^^^^^^
 
 error: aborting due to 8 previous errors
 

src/test/ui/issues/issue-32004.stderr

@@ -21,8 +21,11 @@ LL |         Foo::Baz => {}
 error[E0532]: expected tuple struct or tuple variant, found unit struct `S`
   --> $DIR/issue-32004.rs:16:9
    |
+LL | struct S;
+   | --------- `S` defined here
+...
 LL |         S(()) => {}
-   |         ^ not a tuple struct or tuple variant
+   |         ^^^^^ help: use this syntax instead: `S`
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-pr29383.stderr

@@ -1,14 +1,20 @@
 error[E0532]: expected tuple struct or tuple variant, found unit variant `E::A`
   --> $DIR/issue-pr29383.rs:9:14
    |
+LL |     A,
+   |     - `E::A` defined here
+...
 LL |         Some(E::A(..)) => {}
-   |              ^^^^ not a tuple struct or tuple variant
+   |              ^^^^^^^^ help: use this syntax instead: `E::A`
 
 error[E0532]: expected tuple struct or tuple variant, found unit variant `E::B`
   --> $DIR/issue-pr29383.rs:11:14
    |
+LL |     B,
+   |     - `E::B` defined here
+...
 LL |         Some(E::B(..)) => {}
-   |              ^^^^ not a tuple struct or tuple variant
+   |              ^^^^^^^^ help: use this syntax instead: `E::B`
 
 error: aborting due to 2 previous errors
 

src/test/ui/match/match-pattern-field-mismatch-2.stderr

@@ -1,8 +1,11 @@
 error[E0532]: expected tuple struct or tuple variant, found unit variant `Color::NoColor`
   --> $DIR/match-pattern-field-mismatch-2.rs:12:11
    |
+LL |         NoColor,
+   |         ------- `Color::NoColor` defined here
+...
 LL |           Color::NoColor(_) => { }
-   |           ^^^^^^^^^^^^^^ not a tuple struct or tuple variant
+   |           ^^^^^^^^^^^^^^^^^ help: use this syntax instead: `Color::NoColor`
 
 error: aborting due to previous error
 

src/test/ui/pattern/pattern-error-continue.stderr

@@ -9,11 +9,21 @@ error[E0532]: expected tuple struct or tuple variant, found unit variant `A::D`
    |
 LL |     B(isize, isize),
    |     --------------- similarly named tuple variant `B` defined here
+LL |     C(isize, isize, isize),
+LL |     D
+   |     - `A::D` defined here
 ...
 LL |         A::D(_) => (),
-   |         ^^^-
+   |         ^^^^^^^
-   |            |
+   |
-   |            help: a tuple variant with a similar name exists: `B`
+help: use this syntax instead
+   |
+LL |         A::D => (),
+   |         ^^^^
+help: a tuple variant with a similar name exists
+   |
+LL |         A::B(_) => (),
+   |            ^
 
 error[E0023]: this pattern has 3 fields, but the corresponding tuple variant has 2 fields
   --> $DIR/pattern-error-continue.rs:17:9

src/test/ui/suggestions/issue-84700.rs

@@ -0,0 +1,26 @@
+// test for suggestion on fieldless enum variant
+
+#[derive(PartialEq, Debug)]
+enum FarmAnimal {
+    Worm,
+    Cow,
+    Bull,
+    Chicken { num_eggs: usize },
+    Dog (String),
+}
+
+fn what_does_the_animal_say(animal: &FarmAnimal) {
+
+    let noise = match animal {
+        FarmAnimal::Cow(_) => "moo".to_string(),
+        //~^ ERROR expected tuple struct or tuple variant, found unit variant `FarmAnimal::Cow`
+        FarmAnimal::Chicken(_) => "cluck, cluck!".to_string(),
+        //~^ ERROR expected tuple struct or tuple variant, found struct variant `FarmAnimal::Chicken`
+        FarmAnimal::Dog{..} => "woof!".to_string(),
+        _ => todo!()
+    };
+
+    println!("{:?} says: {:?}", animal, noise);
+}
+
+fn main() {}

src/test/ui/suggestions/issue-84700.stderr

@@ -0,0 +1,21 @@
+error[E0532]: expected tuple struct or tuple variant, found unit variant `FarmAnimal::Cow`
+  --> $DIR/issue-84700.rs:15:9
+   |
+LL |     Cow,
+   |     --- `FarmAnimal::Cow` defined here
+...
+LL |         FarmAnimal::Cow(_) => "moo".to_string(),
+   |         ^^^^^^^^^^^^^^^^^^ help: use this syntax instead: `FarmAnimal::Cow`
+
+error[E0532]: expected tuple struct or tuple variant, found struct variant `FarmAnimal::Chicken`
+  --> $DIR/issue-84700.rs:17:9
+   |
+LL |     Chicken { num_eggs: usize },
+   |     --------------------------- `FarmAnimal::Chicken` defined here
+...
+LL |         FarmAnimal::Chicken(_) => "cluck, cluck!".to_string(),
+   |         ^^^^^^^^^^^^^^^^^^^^^^ help: use struct pattern syntax instead: `FarmAnimal::Chicken { num_eggs }`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0532`.