suggest wrapping patterns with compatible enum variants

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -65,6 +65,7 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::{Item, ItemKind, Node};
 use rustc_middle::dep_graph::DepContext;
+use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::{
     self,
     error::TypeError,
@@ -1736,6 +1737,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             };
 
             if should_suggest_fixes {
+                self.suggest_tuple_pattern(cause, &exp_found, diag);
                 self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
                 self.suggest_accessing_field_where_appropriate(cause, &exp_found, diag);
                 self.suggest_await_on_expect_found(cause, span, &exp_found, diag);
@@ -1766,6 +1768,73 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         self.note_error_origin(diag, cause, exp_found, terr);
     }
 
+    fn suggest_tuple_pattern(
+        &self,
+        cause: &ObligationCause<'tcx>,
+        exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
+        diag: &mut Diagnostic,
+    ) {
+        // Heavily inspired by `FnCtxt::suggest_compatible_variants`, with
+        // some modifications due to that being in typeck and this being in infer.
+        if let ObligationCauseCode::Pattern { .. } = cause.code() {
+            if let ty::Adt(expected_adt, substs) = exp_found.expected.kind() {
+                let compatible_variants: Vec<_> = expected_adt
+                    .variants()
+                    .iter()
+                    .filter(|variant| {
+                        variant.fields.len() == 1 && variant.ctor_kind == hir::def::CtorKind::Fn
+                    })
+                    .filter_map(|variant| {
+                        let sole_field = &variant.fields[0];
+                        let sole_field_ty = sole_field.ty(self.tcx, substs);
+                        if same_type_modulo_infer(sole_field_ty, exp_found.found) {
+                            let variant_path =
+                                with_no_trimmed_paths!(self.tcx.def_path_str(variant.def_id));
+                            // FIXME #56861: DRYer prelude filtering
+                            if let Some(path) = variant_path.strip_prefix("std::prelude::") {
+                                if let Some((_, path)) = path.split_once("::") {
+                                    return Some(path.to_string());
+                                }
+                            }
+                            Some(variant_path)
+                        } else {
+                            None
+                        }
+                    })
+                    .collect();
+                match &compatible_variants[..] {
+                    [] => {}
+                    [variant] => {
+                        diag.multipart_suggestion_verbose(
+                            &format!("try wrapping the pattern in `{}`", variant),
+                            vec![
+                                (cause.span.shrink_to_lo(), format!("{}(", variant)),
+                                (cause.span.shrink_to_hi(), ")".to_string()),
+                            ],
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                    _ => {
+                        // More than one matching variant.
+                        diag.multipart_suggestions(
+                            &format!(
+                                "try wrapping the pattern in a variant of `{}`",
+                                self.tcx.def_path_str(expected_adt.did())
+                            ),
+                            compatible_variants.into_iter().map(|variant| {
+                                vec![
+                                    (cause.span.shrink_to_lo(), format!("{}(", variant)),
+                                    (cause.span.shrink_to_hi(), ")".to_string()),
+                                ]
+                            }),
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                }
+            }
+        }
+    }
+
     pub fn get_impl_future_output_ty(&self, ty: Ty<'tcx>) -> Option<Binder<'tcx, Ty<'tcx>>> {
         if let ty::Opaque(def_id, substs) = ty.kind() {
             let future_trait = self.tcx.require_lang_item(LangItem::Future, None);

src/test/ui/did_you_mean/compatible-variants-in-pat.rs

@@ -0,0 +1,41 @@
+enum Foo {
+    Bar(Bar),
+}
+struct Bar {
+    x: i32,
+}
+
+fn a(f: Foo) {
+    match f {
+        Bar { x } => {
+            //~^ ERROR mismatched types
+            //~| HELP try wrapping
+        }
+    }
+}
+
+struct S;
+
+fn b(s: Option<S>) {
+    match s {
+        S => {
+            //~^ ERROR mismatched types
+            //~| HELP try wrapping
+            //~| HELP introduce a new binding instead
+        }
+        _ => {}
+    }
+}
+
+fn c(s: Result<S, S>) {
+    match s {
+        S => {
+            //~^ ERROR mismatched types
+            //~| HELP try wrapping
+            //~| HELP introduce a new binding instead
+        }
+        _ => {}
+    }
+}
+
+fn main() {}

src/test/ui/did_you_mean/compatible-variants-in-pat.stderr

@@ -0,0 +1,68 @@
+error[E0308]: mismatched types
+  --> $DIR/compatible-variants-in-pat.rs:10:9
+   |
+LL |     match f {
+   |           - this expression has type `Foo`
+LL |         Bar { x } => {
+   |         ^^^^^^^^^ expected enum `Foo`, found struct `Bar`
+   |
+help: try wrapping the pattern in `Foo::Bar`
+   |
+LL |         Foo::Bar(Bar { x }) => {
+   |         +++++++++         +
+
+error[E0308]: mismatched types
+  --> $DIR/compatible-variants-in-pat.rs:21:9
+   |
+LL | struct S;
+   | --------- unit struct defined here
+...
+LL |     match s {
+   |           - this expression has type `Option<S>`
+LL |         S => {
+   |         ^
+   |         |
+   |         expected enum `Option`, found struct `S`
+   |         `S` is interpreted as a unit struct, not a new binding
+   |
+   = note: expected enum `Option<S>`
+            found struct `S`
+help: try wrapping the pattern in `Some`
+   |
+LL |         Some(S) => {
+   |         +++++ +
+help: introduce a new binding instead
+   |
+LL |         other_s => {
+   |         ~~~~~~~
+
+error[E0308]: mismatched types
+  --> $DIR/compatible-variants-in-pat.rs:32:9
+   |
+LL | struct S;
+   | --------- unit struct defined here
+...
+LL |     match s {
+   |           - this expression has type `Result<S, S>`
+LL |         S => {
+   |         ^
+   |         |
+   |         expected enum `Result`, found struct `S`
+   |         `S` is interpreted as a unit struct, not a new binding
+   |
+   = note: expected enum `Result<S, S>`
+            found struct `S`
+help: try wrapping the pattern in a variant of `Result`
+   |
+LL |         Ok(S) => {
+   |         +++ +
+LL |         Err(S) => {
+   |         ++++ +
+help: introduce a new binding instead
+   |
+LL |         other_s => {
+   |         ~~~~~~~
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/issues/issue-12552.stderr

@@ -8,6 +8,10 @@ LL |     Some(k) => match k {
    |
    = note: expected enum `Result<_, {integer}>`
               found enum `Option<_>`
+help: try wrapping the pattern in `Ok`
+   |
+LL |     Ok(Some(k)) => match k {
+   |     +++       +
 
 error[E0308]: mismatched types
   --> $DIR/issue-12552.rs:9:5
@@ -20,6 +24,10 @@ LL |     None => ()
    |
    = note: expected enum `Result<_, {integer}>`
               found enum `Option<_>`
+help: try wrapping the pattern in `Ok`
+   |
+LL |     Ok(None) => ()
+   |     +++    +
 
 error: aborting due to 2 previous errors
 

src/test/ui/issues/issue-3680.stderr

@@ -8,6 +8,10 @@ LL |         Err(_) => ()
    |
    = note: expected enum `Option<_>`
               found enum `Result<_, _>`
+help: try wrapping the pattern in `Some`
+   |
+LL |         Some(Err(_)) => ()
+   |         +++++      +
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-5358-1.stderr

@@ -8,6 +8,10 @@ LL |         Either::Right(_) => {}
    |
    = note: expected struct `S`
                 found enum `Either<_, _>`
+help: try wrapping the pattern in `S`
+   |
+LL |         S(Either::Right(_)) => {}
+   |         ++                +
 help: you might have meant to use field `0` whose type is `Either<usize, usize>`
    |
 LL |     match S(Either::Left(5)).0 {