Gracefully handle mistyping -> as => in function return type

src/test/ui/fn/fn-fat-arrow-return.fixed

@@ -0,0 +1,18 @@
+// run-rustfix
+#![allow(unused)]
+fn a() -> usize { 0 }
+//~^ ERROR return types are denoted using `->`
+
+fn bar(_: u32) {}
+
+fn baz() -> *const dyn Fn(u32) { unimplemented!() }
+
+fn foo() {
+    match () {
+        _ if baz() == &bar as &dyn Fn(u32) => (),
+        () => (),
+    }
+}
+
+fn main() {
+}

src/test/ui/fn/fn-fat-arrow-return.rs

@@ -0,0 +1,18 @@
+// run-rustfix
+#![allow(unused)]
+fn a() => usize { 0 }
+//~^ ERROR return types are denoted using `->`
+
+fn bar(_: u32) {}
+
+fn baz() -> *const dyn Fn(u32) { unimplemented!() }
+
+fn foo() {
+    match () {
+        _ if baz() == &bar as &dyn Fn(u32) => (),
+        () => (),
+    }
+}
+
+fn main() {
+}

src/test/ui/fn/fn-fat-arrow-return.stderr

@@ -0,0 +1,8 @@
+error: return types are denoted using `->`
+  --> $DIR/fn-fat-arrow-return.rs:3:8
+   |
+LL | fn a() => usize { 0 }
+   |        ^^ help: use `->` instead
+
+error: aborting due to previous error
+

src/test/ui/fn/fn-fat-arrow-return2.rs

@@ -0,0 +1,10 @@
+fn a() => impl Fn() => bool {
+    //~^ ERROR return types are denoted using `->`
+    //~| ERROR expected `;` or `{`, found `=>`
+    unimplemented!()
+}
+
+fn main() {
+    let foo = |a: bool| => bool { a };
+    dbg!(foo(false));
+}

src/test/ui/fn/fn-fat-arrow-return2.stderr

@@ -0,0 +1,14 @@
+error: return types are denoted using `->`
+  --> $DIR/fn-fat-arrow-return2.rs:1:8
+   |
+LL | fn a() => impl Fn() => bool {
+   |        ^^ help: use `->` instead
+
+error: expected `;` or `{`, found `=>`
+  --> $DIR/fn-fat-arrow-return2.rs:1:21
+   |
+LL | fn a() => impl Fn() => bool {
+   |                     ^^ expected `;` or `{`
+
+error: aborting due to 2 previous errors
+