don't suggest erroneous trailing comma after ..

compiler/rustc_typeck/src/check/pat.rs

@@ -1486,11 +1486,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Returns a diagnostic reporting a struct pattern which does not mention some fields.
     ///
     /// ```text
-    /// error[E0027]: pattern does not mention field `you_cant_use_this_field`
+    /// error[E0027]: pattern does not mention field `bar`
     ///   --> src/main.rs:15:9
     ///    |
     /// LL |     let foo::Foo {} = foo::Foo::new();
-    ///    |         ^^^^^^^^^^^ missing field `you_cant_use_this_field`
+    ///    |         ^^^^^^^^^^^ missing field `bar`
     /// ```
     fn error_unmentioned_fields(
         &self,
@@ -1524,14 +1524,26 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 }
                 _ => return err,
             },
-            [.., field] => (
-                match pat.kind {
-                    PatKind::Struct(_, [_, ..], _) => ", ",
-                    _ => "",
-                },
-                "",
-                field.span.shrink_to_hi(),
-            ),
+            [.., field] => {
+                // if last field has a trailing comma, use the comma
+                // as the span to avoid trailing comma in ultimate
+                // suggestion (Issue #78511)
+                let tail = field.span.shrink_to_hi().until(pat.span.shrink_to_hi());
+                let tail_through_comma = self.tcx.sess.source_map().span_through_char(tail, ',');
+                let sp = if tail_through_comma == tail {
+                    field.span.shrink_to_hi()
+                } else {
+                    tail_through_comma
+                };
+                (
+                    match pat.kind {
+                        PatKind::Struct(_, [_, ..], _) => ", ",
+                        _ => "",
+                    },
+                    "",
+                    sp,
+                )
+            }
         };
         err.span_suggestion(
             sp,

src/test/ui/destructuring-assignment/struct_destructure_fail.stderr

@@ -32,11 +32,11 @@ LL |     Struct { a, _ } = Struct { a: 1, b: 2 };
    |
 help: include the missing field in the pattern
    |
-LL |     Struct { a, b, _ } = Struct { a: 1, b: 2 };
+LL |     Struct { a, b _ } = Struct { a: 1, b: 2 };
    |               ^^^
 help: if you don't care about this missing field, you can explicitly ignore it
    |
-LL |     Struct { a, .., _ } = Struct { a: 1, b: 2 };
+LL |     Struct { a, .. _ } = Struct { a: 1, b: 2 };
    |               ^^^^
 
 error: aborting due to 5 previous errors

src/test/ui/error-codes/E0027.rs

@@ -3,12 +3,21 @@ struct Dog {
     age: u32,
 }
 
+
 fn main() {
     let d = Dog { name: "Rusty".to_string(), age: 8 };
 
     match d {
         Dog { age: x } => {} //~ ERROR pattern does not mention field `name`
     }
+    match d {
+        // trailing comma
+        Dog { name: x, } => {} //~ ERROR pattern does not mention field `age`
+    }
+    match d {
+        // trailing comma with weird whitespace
+        Dog { name: x  , } => {} //~ ERROR pattern does not mention field `age`
+    }
     match d {
         Dog {} => {} //~ ERROR pattern does not mention fields `name`, `age`
     }

src/test/ui/error-codes/E0027.stderr

@@ -1,5 +1,5 @@
 error[E0027]: pattern does not mention field `name`
-  --> $DIR/E0027.rs:10:9
+  --> $DIR/E0027.rs:11:9
    |
 LL |         Dog { age: x } => {}
    |         ^^^^^^^^^^^^^^ missing field `name`
@@ -13,8 +13,38 @@ help: if you don't care about this missing field, you can explicitly ignore it
 LL |         Dog { age: x, .. } => {}
    |                     ^^^^
 
+error[E0027]: pattern does not mention field `age`
+  --> $DIR/E0027.rs:15:9
+   |
+LL |         Dog { name: x, } => {}
+   |         ^^^^^^^^^^^^^^^^ missing field `age`
+   |
+help: include the missing field in the pattern
+   |
+LL |         Dog { name: x, age } => {}
+   |                      ^^^^^
+help: if you don't care about this missing field, you can explicitly ignore it
+   |
+LL |         Dog { name: x, .. } => {}
+   |                      ^^^^
+
+error[E0027]: pattern does not mention field `age`
+  --> $DIR/E0027.rs:19:9
+   |
+LL |         Dog { name: x  , } => {}
+   |         ^^^^^^^^^^^^^^^^^^ missing field `age`
+   |
+help: include the missing field in the pattern
+   |
+LL |         Dog { name: x, age } => {}
+   |                      ^^^^^
+help: if you don't care about this missing field, you can explicitly ignore it
+   |
+LL |         Dog { name: x, .. } => {}
+   |                      ^^^^
+
 error[E0027]: pattern does not mention fields `name`, `age`
-  --> $DIR/E0027.rs:13:9
+  --> $DIR/E0027.rs:22:9
    |
 LL |         Dog {} => {}
    |         ^^^^^^ missing fields `name`, `age`
@@ -28,6 +58,6 @@ help: if you don't care about these missing fields, you can explicitly ignore th
 LL |         Dog { .. } => {}
    |             ^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0027`.