Suggest accessing field when appropriate

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

@@ -1661,6 +1661,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         debug!("exp_found {:?} terr {:?}", exp_found, terr);
         if let Some(exp_found) = exp_found {
             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);
         }
 
@@ -1819,6 +1820,53 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         }
     }
 
+    fn suggest_accessing_field_where_appropriate(
+        &self,
+        cause: &ObligationCause<'tcx>,
+        exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
+        diag: &mut DiagnosticBuilder<'tcx>,
+    ) {
+        debug!(
+            "suggest_accessing_field_where_appropriate(cause={:?}, exp_found={:?})",
+            cause, exp_found
+        );
+        if let ty::Adt(expected_def, expected_substs) = exp_found.expected.kind() {
+            if expected_def.is_enum() {
+                return;
+            }
+
+            if let Some((name, ty)) = expected_def
+                .non_enum_variant()
+                .fields
+                .iter()
+                .filter(|field| field.vis.is_accessible_from(field.did, self.tcx))
+                .map(|field| (field.ident.name, field.ty(self.tcx, expected_substs)))
+                .find(|(_, ty)| ty::TyS::same_type(ty, exp_found.found))
+            {
+                if let ObligationCauseCode::Pattern { span: Some(span), .. } = cause.code {
+                    if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
+                        let suggestion = if expected_def.is_struct() {
+                            format!("{}.{}", snippet, name)
+                        } else if expected_def.is_union() {
+                            format!("unsafe {{ {}.{} }}", snippet, name)
+                        } else {
+                            return;
+                        };
+                        diag.span_suggestion(
+                            span,
+                            &format!(
+                                "you might have meant to use field `{}` of type `{}`",
+                                name, ty
+                            ),
+                            suggestion,
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                }
+            }
+        }
+    }
+
     /// When encountering a case where `.as_ref()` on a `Result` or `Option` would be appropriate,
     /// suggests it.
     fn suggest_as_ref_where_appropriate(

src/test/ui/suggestions/field-access.fixed

@@ -0,0 +1,35 @@
+// run-rustfix
+#![allow(dead_code)]
+
+struct A {
+    b: B,
+}
+
+enum B {
+    Fst,
+    Snd,
+}
+
+union Foo {
+    bar: u32,
+    qux: f32,
+}
+
+fn main() {
+    let a = A { b: B::Fst };
+    if let B::Fst = a.b {}; //~ ERROR mismatched types [E0308]
+    //~^ HELP you might have meant to use field `b` of type `B`
+    match a.b {
+        //~^ HELP you might have meant to use field `b` of type `B`
+        //~| HELP you might have meant to use field `b` of type `B`
+        B::Fst => (), //~ ERROR mismatched types [E0308]
+        B::Snd => (), //~ ERROR mismatched types [E0308]
+    }
+
+    let foo = Foo { bar: 42 };
+    match unsafe { foo.bar } {
+        //~^ HELP you might have meant to use field `bar` of type `u32`
+        1u32 => (), //~ ERROR mismatched types [E0308]
+        _ => (),
+    }
+}

src/test/ui/suggestions/field-access.rs

@@ -0,0 +1,35 @@
+// run-rustfix
+#![allow(dead_code)]
+
+struct A {
+    b: B,
+}
+
+enum B {
+    Fst,
+    Snd,
+}
+
+union Foo {
+    bar: u32,
+    qux: f32,
+}
+
+fn main() {
+    let a = A { b: B::Fst };
+    if let B::Fst = a {}; //~ ERROR mismatched types [E0308]
+    //~^ HELP you might have meant to use field `b` of type `B`
+    match a {
+        //~^ HELP you might have meant to use field `b` of type `B`
+        //~| HELP you might have meant to use field `b` of type `B`
+        B::Fst => (), //~ ERROR mismatched types [E0308]
+        B::Snd => (), //~ ERROR mismatched types [E0308]
+    }
+
+    let foo = Foo { bar: 42 };
+    match foo {
+        //~^ HELP you might have meant to use field `bar` of type `u32`
+        1u32 => (), //~ ERROR mismatched types [E0308]
+        _ => (),
+    }
+}

src/test/ui/suggestions/field-access.stderr

@@ -0,0 +1,67 @@
+error[E0308]: mismatched types
+  --> $DIR/field-access.rs:20:12
+   |
+LL |     Fst,
+   |     --- unit variant defined here
+...
+LL |     if let B::Fst = a {};
+   |            ^^^^^^   - this expression has type `A`
+   |            |
+   |            expected struct `A`, found enum `B`
+   |
+help: you might have meant to use field `b` of type `B`
+   |
+LL |     if let B::Fst = a.b {};
+   |                     ^^^
+
+error[E0308]: mismatched types
+  --> $DIR/field-access.rs:25:9
+   |
+LL |     Fst,
+   |     --- unit variant defined here
+...
+LL |     match a {
+   |           - this expression has type `A`
+...
+LL |         B::Fst => (),
+   |         ^^^^^^ expected struct `A`, found enum `B`
+   |
+help: you might have meant to use field `b` of type `B`
+   |
+LL |     match a.b {
+   |           ^^^
+
+error[E0308]: mismatched types
+  --> $DIR/field-access.rs:26:9
+   |
+LL |     Snd,
+   |     --- unit variant defined here
+...
+LL |     match a {
+   |           - this expression has type `A`
+...
+LL |         B::Snd => (),
+   |         ^^^^^^ expected struct `A`, found enum `B`
+   |
+help: you might have meant to use field `b` of type `B`
+   |
+LL |     match a.b {
+   |           ^^^
+
+error[E0308]: mismatched types
+  --> $DIR/field-access.rs:32:9
+   |
+LL |     match foo {
+   |           --- this expression has type `Foo`
+LL |
+LL |         1u32 => (),
+   |         ^^^^ expected union `Foo`, found `u32`
+   |
+help: you might have meant to use field `bar` of type `u32`
+   |
+LL |     match unsafe { foo.bar } {
+   |           ^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0308`.