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/libsyntax/parse/parser.rs

@@ -2263,8 +2263,24 @@ impl<'a> Parser<'a> {
         let lo = self.span;
 
         // Check if a colon exists one ahead. This means we're parsing a fieldname.
-        let (fieldname, expr, is_shorthand) = if self.look_ahead(1, |t| t == &token::Colon) {
+        let (fieldname, expr, is_shorthand) = if self.look_ahead(1, |t| {
+            t == &token::Colon || t == &token::Eq
+        }) {
             let fieldname = self.parse_field_name()?;
+
+            // Check for an equals token. This means the source incorrectly attempts to
+            // initialize a field with an eq rather than a colon.
+            if self.token == token::Eq {
+                self.diagnostic()
+                    .struct_span_err(self.span, "expected `:`, found `=`")
+                    .span_suggestion_with_applicability(
+                        fieldname.span.shrink_to_hi().to(self.span),
+                        "replace equals symbol with a colon",
+                        ":".to_string(),
+                        Applicability::MachineApplicable,
+                    )
+                    .emit();
+            }
             self.bump(); // `:`
             (fieldname, self.parse_expr()?, false)
         } else {

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
+