Add suggestion for incorrect field syntax.

This commit adds a suggestion when a `=` character is used when
specifying the value of a field in a struct constructor incorrectly
instead of a `:` character.

src/test/ui/issues/issue-57684.fixed

@@ -0,0 +1,37 @@
+// run-rustfix
+
+#![allow(warnings)]
+
+// This test checks that the following error is emitted when a `=` character is used to initialize
+// a struct field when a `:` is expected.
+//
+// ```
+// error: struct fields are initialized with a colon
+//   --> $DIR/issue-57684.rs:12:20
+//    |
+// LL |     let _ = X { f1 = 5 };
+//    |                    ^ help: replace equals symbol with a colon: `:`
+// ```
+
+struct X {
+    f1: i32,
+}
+
+struct Y {
+    f1: i32,
+    f2: i32,
+    f3: i32,
+}
+
+fn main() {
+    let _ = X { f1: 5 };
+    //~^ ERROR expected `:`, found `=`
+
+    let f3 = 3;
+    let _ = Y {
+        f1: 5,
+        //~^ ERROR expected `:`, found `=`
+        f2: 4,
+        f3,
+    };
+}

src/test/ui/issues/issue-57684.rs

@@ -0,0 +1,37 @@
+// run-rustfix
+
+#![allow(warnings)]
+
+// This test checks that the following error is emitted when a `=` character is used to initialize
+// a struct field when a `:` is expected.
+//
+// ```
+// error: struct fields are initialized with a colon
+//   --> $DIR/issue-57684.rs:12:20
+//    |
+// LL |     let _ = X { f1 = 5 };
+//    |                    ^ help: replace equals symbol with a colon: `:`
+// ```
+
+struct X {
+    f1: i32,
+}
+
+struct Y {
+    f1: i32,
+    f2: i32,
+    f3: i32,
+}
+
+fn main() {
+    let _ = X { f1 = 5 };
+    //~^ ERROR expected `:`, found `=`
+
+    let f3 = 3;
+    let _ = Y {
+        f1 = 5,
+        //~^ ERROR expected `:`, found `=`
+        f2: 4,
+        f3,
+    };
+}

src/test/ui/issues/issue-57684.stderr

@@ -0,0 +1,18 @@
+error: expected `:`, found `=`
+  --> $DIR/issue-57684.rs:27:20
+   |
+LL |     let _ = X { f1 = 5 };
+   |                   -^
+   |                   |
+   |                   help: replace equals symbol with a colon: `:`
+
+error: expected `:`, found `=`
+  --> $DIR/issue-57684.rs:32:12
+   |
+LL |         f1 = 5,
+   |           -^
+   |           |
+   |           help: replace equals symbol with a colon: `:`
+
+error: aborting due to 2 previous errors
+