resolve: improve "try using the enum's variant"

This commit improves the "try using the enum's variant" suggestion:

- Variants in suggestions would not result in more errors (e.g. use
  of a struct variant is only suggested if the suggestion can
  trivially construct that variant). Therefore, suggestions are only
  emitted for variants that have no fields (since the suggestion
  can't know what value fields would have).
- Suggestions include the syntax for constructing the variant. If a
  struct or tuple variant is suggested, then it is constructed in the
  suggestion - unless in pattern-matching or when arguments are already
  provided.
- A help message is added which mentions the variants which are no
  longer suggested.

Signed-off-by: David Wood <david@davidtw.co>

src/test/ui/did_you_mean/issue-43871-enum-instead-of-variant.stderr

@@ -2,46 +2,33 @@ error[E0423]: expected function, tuple struct or tuple variant, found enum `Opti
   --> $DIR/issue-43871-enum-instead-of-variant.rs:19:13
    |
 LL |     let x = Option(1);
-   |             ^^^^^^
+   |             ^^^^^^ help: try using one of the enum's variants: `std::option::Option::Some`
    |
-help: try using one of the enum's variants
-   |
-LL |     let x = std::option::Option::None(1);
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^
-LL |     let x = std::option::Option::Some(1);
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: you might have meant to construct the enum's non-tuple variant
 
 error[E0532]: expected tuple struct or tuple variant, found enum `Option`
   --> $DIR/issue-43871-enum-instead-of-variant.rs:21:12
    |
 LL |     if let Option(_) = x {
-   |            ^^^^^^
+   |            ^^^^^^ help: try using one of the enum's variants: `std::option::Option::Some`
    |
-help: try using one of the enum's variants
-   |
-LL |     if let std::option::Option::None(_) = x {
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^
-LL |     if let std::option::Option::Some(_) = x {
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^
+   = help: you might have meant to match against the enum's non-tuple variant
 
 error[E0532]: expected tuple struct or tuple variant, found enum `Example`
   --> $DIR/issue-43871-enum-instead-of-variant.rs:27:12
    |
 LL |     if let Example(_) = y {
-   |            ^^^^^^^
+   |            ^^^^^^^ help: try using one of the enum's variants: `Example::Ex`
    |
-help: try using one of the enum's variants
-   |
-LL |     if let Example::Ex(_) = y {
-   |            ^^^^^^^^^^^
-LL |     if let Example::NotEx(_) = y {
-   |            ^^^^^^^^^^^^^^
+   = help: you might have meant to match against the enum's non-tuple variant
 
 error[E0423]: expected function, tuple struct or tuple variant, found enum `Void`
   --> $DIR/issue-43871-enum-instead-of-variant.rs:31:13
    |
 LL |     let y = Void();
    |             ^^^^
+   |
+   = help: the enum has no tuple variants to construct
 
 error[E0423]: expected function, tuple struct or tuple variant, found enum `ManyVariants`
   --> $DIR/issue-43871-enum-instead-of-variant.rs:33:13
@@ -49,17 +36,8 @@ error[E0423]: expected function, tuple struct or tuple variant, found enum `Many
 LL |     let z = ManyVariants();
    |             ^^^^^^^^^^^^
    |
-help: try using one of the enum's variants
-   |
-LL |     let z = ManyVariants::One();
-   |             ^^^^^^^^^^^^^^^^^
-LL |     let z = ManyVariants::Two();
-   |             ^^^^^^^^^^^^^^^^^
-LL |     let z = ManyVariants::Three();
-   |             ^^^^^^^^^^^^^^^^^^^
-LL |     let z = ManyVariants::Four();
-   |             ^^^^^^^^^^^^^^^^^^
-     and 6 other candidates
+   = help: the enum has no tuple variants to construct
+   = help: you might have meant to construct one of the enum's non-tuple variants
 
 error: aborting due to 5 previous errors
 

src/test/ui/issues/issue-73427.rs

@@ -0,0 +1,44 @@
+enum A {
+    StructWithFields { x: () },
+    TupleWithFields(()),
+    Struct {},
+    Tuple(),
+    Unit,
+}
+
+enum B {
+    StructWithFields { x: () },
+    TupleWithFields(()),
+}
+
+enum C {
+    StructWithFields { x: () },
+    TupleWithFields(()),
+    Unit,
+}
+
+enum D {
+    TupleWithFields(()),
+    Unit,
+}
+
+fn main() {
+    // Only variants without fields are suggested (and others mentioned in a note) where an enum
+    // is used rather than a variant.
+
+    A.foo();
+    //~^ ERROR expected value, found enum `A`
+    B.foo();
+    //~^ ERROR expected value, found enum `B`
+    C.foo();
+    //~^ ERROR expected value, found enum `C`
+    D.foo();
+    //~^ ERROR expected value, found enum `D`
+
+    // Only tuple variants are suggested in calls or tuple struct pattern matching.
+
+    let x = A(3);
+    //~^ ERROR expected function, tuple struct or tuple variant, found enum `A`
+    if let A(3) = x { }
+    //~^ ERROR expected tuple struct or tuple variant, found enum `A`
+}

src/test/ui/issues/issue-73427.stderr

@@ -0,0 +1,72 @@
+error[E0423]: expected value, found enum `A`
+  --> $DIR/issue-73427.rs:29:5
+   |
+LL |     A.foo();
+   |     ^
+   |
+   = help: you might have meant to use one of the enum's other variants that have fields
+help: try using one of the enum's variants
+   |
+LL |     (A::Struct {}).foo();
+   |     ^^^^^^^^^^^^^^
+LL |     (A::Tuple()).foo();
+   |     ^^^^^^^^^^^^
+LL |     A::Unit.foo();
+   |     ^^^^^^^
+
+error[E0423]: expected value, found enum `B`
+  --> $DIR/issue-73427.rs:31:5
+   |
+LL |     B.foo();
+   |     ^
+   |
+   = help: you might have meant to use one of the enum's variants
+
+error[E0423]: expected value, found enum `C`
+  --> $DIR/issue-73427.rs:33:5
+   |
+LL |     C.foo();
+   |     ^ help: try using one of the enum's variants: `C::Unit`
+   |
+   = help: you might have meant to use one of the enum's other variants that have fields
+
+error[E0423]: expected value, found enum `D`
+  --> $DIR/issue-73427.rs:35:5
+   |
+LL |     D.foo();
+   |     ^ help: try using one of the enum's variants: `D::Unit`
+   |
+   = help: you might have meant to use the enum's other variant that has fields
+
+error[E0423]: expected function, tuple struct or tuple variant, found enum `A`
+  --> $DIR/issue-73427.rs:40:13
+   |
+LL |     let x = A(3);
+   |             ^
+   |
+   = help: you might have meant to construct one of the enum's non-tuple variants
+help: try using one of the enum's variants
+   |
+LL |     let x = A::TupleWithFields(3);
+   |             ^^^^^^^^^^^^^^^^^^
+LL |     let x = A::Tuple(3);
+   |             ^^^^^^^^
+
+error[E0532]: expected tuple struct or tuple variant, found enum `A`
+  --> $DIR/issue-73427.rs:42:12
+   |
+LL |     if let A(3) = x { }
+   |            ^
+   |
+   = help: you might have meant to match against one of the enum's non-tuple variants
+help: try using one of the enum's variants
+   |
+LL |     if let A::TupleWithFields(3) = x { }
+   |            ^^^^^^^^^^^^^^^^^^
+LL |     if let A::Tuple(3) = x { }
+   |            ^^^^^^^^
+
+error: aborting due to 6 previous errors
+
+Some errors have detailed explanations: E0423, E0532.
+For more information about an error, try `rustc --explain E0423`.

src/test/ui/resolve/privacy-enum-ctor.stderr

@@ -2,31 +2,17 @@ error[E0423]: expected value, found enum `n::Z`
   --> $DIR/privacy-enum-ctor.rs:23:9
    |
 LL |         n::Z;
-   |         ^^^^
+   |         ^^^^ help: try using one of the enum's variants: `m::Z::Unit`
    |
-help: try using one of the enum's variants
-   |
-LL |         m::Z::Fn;
-   |         ^^^^^^^^
-LL |         m::Z::Struct;
-   |         ^^^^^^^^^^^^
-LL |         m::Z::Unit;
-   |         ^^^^^^^^^^
+   = help: you might have meant to use one of the enum's other variants that have fields
 
 error[E0423]: expected value, found enum `Z`
   --> $DIR/privacy-enum-ctor.rs:25:9
    |
 LL |         Z;
-   |         ^
+   |         ^ help: try using one of the enum's variants: `m::Z::Unit`
    |
-help: try using one of the enum's variants
-   |
-LL |         m::Z::Fn;
-   |         ^^^^^^^^
-LL |         m::Z::Struct;
-   |         ^^^^^^^^^^^^
-LL |         m::Z::Unit;
-   |         ^^^^^^^^^^
+   = help: you might have meant to use one of the enum's other variants that have fields
 
 error[E0423]: expected value, found struct variant `Z::Struct`
   --> $DIR/privacy-enum-ctor.rs:29:20
@@ -48,12 +34,9 @@ LL |     fn f() {
 LL |     let _: E = m::E;
    |                ^^^^
    |
+   = help: you might have meant to use one of the enum's other variants that have fields
 help: try using one of the enum's variants
    |
-LL |     let _: E = E::Fn;
-   |                ^^^^^
-LL |     let _: E = E::Struct;
-   |                ^^^^^^^^^
 LL |     let _: E = E::Unit;
    |                ^^^^^^^
 help: a function with a similar name exists
@@ -84,12 +67,9 @@ error[E0423]: expected value, found enum `E`
 LL |     let _: E = E;
    |                ^
    |
+   = help: you might have meant to use one of the enum's other variants that have fields
 help: try using one of the enum's variants
    |
-LL |     let _: E = E::Fn;
-   |                ^^^^^
-LL |     let _: E = E::Struct;
-   |                ^^^^^^^^^
 LL |     let _: E = E::Unit;
    |                ^^^^^^^
 help: consider importing one of these items instead
@@ -132,16 +112,9 @@ error[E0423]: expected value, found enum `m::n::Z`
   --> $DIR/privacy-enum-ctor.rs:57:16
    |
 LL |     let _: Z = m::n::Z;
-   |                ^^^^^^^
+   |                ^^^^^^^ help: try using one of the enum's variants: `m::Z::Unit`
    |
-help: try using one of the enum's variants
-   |
-LL |     let _: Z = m::Z::Fn;
-   |                ^^^^^^^^
-LL |     let _: Z = m::Z::Struct;
-   |                ^^^^^^^^^^^^
-LL |     let _: Z = m::Z::Unit;
-   |                ^^^^^^^^^^
+   = help: you might have meant to use one of the enum's other variants that have fields
 
 error[E0412]: cannot find type `Z` in this scope
   --> $DIR/privacy-enum-ctor.rs:61:12