Suggest the struct variant pattern syntax on usage of unit variant pa…

…ttern for a struct variant

compiler/rustc_hir_typeck/src/lib.rs

@@ -45,7 +45,7 @@ pub use coercion::can_coerce;
 use fn_ctxt::FnCtxt;
 use rustc_data_structures::unord::UnordSet;
 use rustc_errors::codes::*;
-use rustc_errors::{struct_span_code_err, Applicability, ErrorGuaranteed};
+use rustc_errors::{pluralize, struct_span_code_err, Applicability, ErrorGuaranteed};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::intravisit::Visitor;
@@ -421,6 +421,36 @@ fn report_unexpected_variant_res(
             err.multipart_suggestion_verbose(descr, suggestion, Applicability::MaybeIncorrect);
             err
         }
+        Res::Def(DefKind::Variant, _) if expr.is_none() => {
+            let fields = &tcx.expect_variant_res(res).fields.raw;
+
+            let span = qpath.span().shrink_to_hi().to(span.shrink_to_hi());
+            let (msg, sugg) = if fields.is_empty() {
+                ("use the struct variant pattern syntax".to_string(), " {}".to_string())
+            } else {
+                let msg = format!(
+                    "the struct variant's field{s} {are} being ignored",
+                    s = pluralize!(fields.len()),
+                    are = pluralize!("is", fields.len())
+                );
+                let fields = fields
+                    .iter()
+                    .map(|field| format!("{}: _", field.name.to_ident_string()))
+                    .collect::<Vec<_>>()
+                    .join(", ");
+                let sugg = format!(" {{ /* {} */ }}", fields,);
+                (msg, sugg)
+            };
+
+            err.span_label(span, format!("not a {expected}"));
+            err.span_suggestion_verbose(
+                qpath.span().shrink_to_hi().to(span.shrink_to_hi()),
+                msg,
+                sugg,
+                Applicability::HasPlaceholders,
+            );
+            err
+        }
         _ => err.with_span_label(span, format!("not a {expected}")),
     }
     .emit()

tests/ui/empty/empty-struct-braces-pat-1.stderr

@@ -2,13 +2,23 @@ error[E0533]: expected unit struct, unit variant or constant, found struct varia
   --> $DIR/empty-struct-braces-pat-1.rs:24:9
    |
 LL |         E::Empty3 => ()
-   |         ^^^^^^^^^ not a unit struct, unit variant or constant
+   |         ^^^^^^^^^- not a unit struct, unit variant or constant
+   |
+help: use the struct variant pattern syntax
+   |
+LL |         E::Empty3 {} => ()
+   |                   ++
 
 error[E0533]: expected unit struct, unit variant or constant, found struct variant `XE::XEmpty3`
   --> $DIR/empty-struct-braces-pat-1.rs:31:9
    |
 LL |         XE::XEmpty3 => ()
-   |         ^^^^^^^^^^^ not a unit struct, unit variant or constant
+   |         ^^^^^^^^^^^- not a unit struct, unit variant or constant
+   |
+help: use the struct variant pattern syntax
+   |
+LL |         XE::XEmpty3 {} => ()
+   |                     ++
 
 error: aborting due to 2 previous errors
 

tests/ui/empty/empty-struct-braces-pat-3.stderr

@@ -2,25 +2,53 @@ error[E0164]: expected tuple struct or tuple variant, found struct variant `E::E
   --> $DIR/empty-struct-braces-pat-3.rs:17:9
    |
 LL |         E::Empty3() => ()
-   |         ^^^^^^^^^^^ not a tuple struct or tuple variant
+   |         ^^^^^^^^^--
+   |                  |
+   |                  not a tuple struct or tuple variant
+   |
+help: use the struct variant pattern syntax
+   |
+LL |         E::Empty3 {} => ()
+   |                   ~~
 
 error[E0164]: expected tuple struct or tuple variant, found struct variant `XE::XEmpty3`
   --> $DIR/empty-struct-braces-pat-3.rs:21:9
    |
 LL |         XE::XEmpty3() => ()
-   |         ^^^^^^^^^^^^^ not a tuple struct or tuple variant
+   |         ^^^^^^^^^^^--
+   |                    |
+   |                    not a tuple struct or tuple variant
+   |
+help: use the struct variant pattern syntax
+   |
+LL |         XE::XEmpty3 {} => ()
+   |                     ~~
 
 error[E0164]: expected tuple struct or tuple variant, found struct variant `E::Empty3`
   --> $DIR/empty-struct-braces-pat-3.rs:25:9
    |
 LL |         E::Empty3(..) => ()
-   |         ^^^^^^^^^^^^^ not a tuple struct or tuple variant
+   |         ^^^^^^^^^----
+   |                  |
+   |                  not a tuple struct or tuple variant
+   |
+help: use the struct variant pattern syntax
+   |
+LL |         E::Empty3 {} => ()
+   |                   ~~
 
 error[E0164]: expected tuple struct or tuple variant, found struct variant `XE::XEmpty3`
   --> $DIR/empty-struct-braces-pat-3.rs:29:9
    |
 LL |         XE::XEmpty3(..) => ()
-   |         ^^^^^^^^^^^^^^^ not a tuple struct or tuple variant
+   |         ^^^^^^^^^^^----
+   |                    |
+   |                    not a tuple struct or tuple variant
+   |
+help: use the struct variant pattern syntax
+   |
+LL |         XE::XEmpty3 {} => ()
+   |                     ~~
 
 error: aborting due to 4 previous errors
 

tests/ui/issues/issue-63983.stderr

@@ -11,7 +11,12 @@ error[E0533]: expected unit struct, unit variant or constant, found struct varia
   --> $DIR/issue-63983.rs:10:9
    |
 LL |         MyEnum::Struct => "",
-   |         ^^^^^^^^^^^^^^ not a unit struct, unit variant or constant
+   |         ^^^^^^^^^^^^^^- not a unit struct, unit variant or constant
+   |
+help: the struct variant's field is being ignored
+   |
+LL |         MyEnum::Struct { /* s: _ */ } => "",
+   |                        ++++++++++++++
 
 error: aborting due to 2 previous errors
 

tests/ui/parser/recover/recover-from-bad-variant.stderr

@@ -18,7 +18,14 @@ error[E0164]: expected tuple struct or tuple variant, found struct variant `Enum
   --> $DIR/recover-from-bad-variant.rs:10:9
    |
 LL |         Enum::Foo(a, b) => {}
-   |         ^^^^^^^^^^^^^^^ not a tuple struct or tuple variant
+   |         ^^^^^^^^^------
+   |                  |
+   |                  not a tuple struct or tuple variant
+   |
+help: the struct variant's fields are being ignored
+   |
+LL |         Enum::Foo { /* a: _, b: _ */ } => {}
+   |                   ~~~~~~~~~~~~~~~~~~~~
 
 error[E0769]: tuple variant `Enum::Bar` written as struct variant
   --> $DIR/recover-from-bad-variant.rs:12:9

tests/ui/suggestions/issue-84700.stderr

@@ -11,7 +11,14 @@ error[E0164]: expected tuple struct or tuple variant, found struct variant `Farm
   --> $DIR/issue-84700.rs:17:9
    |
 LL |         FarmAnimal::Chicken(_) => "cluck, cluck!".to_string(),
-   |         ^^^^^^^^^^^^^^^^^^^^^^ not a tuple struct or tuple variant
+   |         ^^^^^^^^^^^^^^^^^^^---
+   |                            |
+   |                            not a tuple struct or tuple variant
+   |
+help: the struct variant's field is being ignored
+   |
+LL |         FarmAnimal::Chicken { /* num_eggs: _ */ } => "cluck, cluck!".to_string(),
+   |                             ~~~~~~~~~~~~~~~~~~~~~
 
 error: aborting due to 2 previous errors
 

tests/ui/type-alias-enum-variants/incorrect-variant-form-through-alias-caught.stderr

@@ -13,13 +13,25 @@ error[E0533]: expected unit struct, unit variant or constant, found struct varia
   --> $DIR/incorrect-variant-form-through-alias-caught.rs:10:9
    |
 LL |     let Alias::Braced = panic!();
-   |         ^^^^^^^^^^^^^ not a unit struct, unit variant or constant
+   |         ^^^^^^^^^^^^^- not a unit struct, unit variant or constant
+   |
+help: use the struct variant pattern syntax
+   |
+LL |     let Alias::Braced {} = panic!();
+   |                       ++
 
 error[E0164]: expected tuple struct or tuple variant, found struct variant `Alias::Braced`
   --> $DIR/incorrect-variant-form-through-alias-caught.rs:12:9
    |
 LL |     let Alias::Braced(..) = panic!();
-   |         ^^^^^^^^^^^^^^^^^ not a tuple struct or tuple variant
+   |         ^^^^^^^^^^^^^----
+   |                      |
+   |                      not a tuple struct or tuple variant
+   |
+help: use the struct variant pattern syntax
+   |
+LL |     let Alias::Braced {} = panic!();
+   |                       ~~
 
 error[E0618]: expected function, found enum variant `Alias::Unit`
   --> $DIR/incorrect-variant-form-through-alias-caught.rs:15:5