Suggest accessing field when appropriate

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

@@ -1838,20 +1838,24 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 .iter()
                 .filter(|field| field.vis.is_accessible_from(field.did, self.tcx))
                 .map(|field| (field.ident.name, field.ty(self.tcx, expected_substs)))
-                .inspect(|(name, ty)| {
-                    debug!("suggest_field_where_appropriate: name={:?}, ty={:?}", name, ty)
-                })
                 .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
                             ),
-                            format!("{}.{}", snippet, name),
+                            suggestion,
                             Applicability::MaybeIncorrect,
                         );
                     }

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

@@ -10,17 +10,26 @@ enum B {
     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]
-    // note: you might have meant to use field `b` of type `B`
+    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 {
-        B::Fst => (),
-        B::Snd => (),
+        //~^ 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]
+        _ => (),
     }
-    //~^^^ ERROR mismatched types [E0308]
-    // note: you might have meant to use field `b` of type `B`
-    //~^^^^ ERROR mismatched types [E0308]
-    // note: you might have meant to use field `b` of type `B`
 }

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

@@ -10,17 +10,26 @@ enum B {
     Snd,
 }
 
+union Foo {
+    bar: u32,
+    qux: f32,
+}
+
 fn main() {
     let a = A { b: B::Fst };
-    if let B::Fst = a {};
-    //~^ ERROR mismatched types [E0308]
-    // note: you might have meant to use field `b` of type `B`
+    if let B::Fst = a {}; //~ ERROR mismatched types [E0308]
+    //~^ HELP you might have meant to use field `b` of type `B`
     match a {
-        B::Fst => (),
-        B::Snd => (),
+        //~^ 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]
+        _ => (),
     }
-    //~^^^ ERROR mismatched types [E0308]
-    // note: you might have meant to use field `b` of type `B`
-    //~^^^^ ERROR mismatched types [E0308]
-    // note: you might have meant to use field `b` of type `B`
 }

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

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/field-access.rs:15:12
+  --> $DIR/field-access.rs:20:12
    |
 LL |     Fst,
    |     --- unit variant defined here
@@ -15,13 +15,14 @@ LL |     if let B::Fst = a.b {};
    |                     ^^^
 
 error[E0308]: mismatched types
-  --> $DIR/field-access.rs:19:9
+  --> $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`
    |
@@ -31,14 +32,14 @@ LL |     match a.b {
    |           ^^^
 
 error[E0308]: mismatched types
-  --> $DIR/field-access.rs:20:9
+  --> $DIR/field-access.rs:26:9
    |
 LL |     Snd,
    |     --- unit variant defined here
 ...
 LL |     match a {
    |           - this expression has type `A`
-LL |         B::Fst => (),
+...
 LL |         B::Snd => (),
    |         ^^^^^^ expected struct `A`, found enum `B`
    |
@@ -47,6 +48,20 @@ help: you might have meant to use field `b` of type `B`
 LL |     match a.b {
    |           ^^^
 
-error: aborting due to 3 previous errors
+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`.