fix ice in suggesting

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -405,13 +405,20 @@ impl<'a> Parser<'a> {
             let prev_span = self.prev_token.span.shrink_to_lo();
             let snapshot = self.create_snapshot_for_diagnostic();
             self.bump();
-            if self.parse_ty().is_ok() && self.token == token::Eq {
-                err.span_suggestion_verbose(
-                    prev_span,
-                    "you might have meant to introduce a new binding",
-                    "let ".to_string(),
-                    Applicability::MaybeIncorrect,
-                );
+            match self.parse_ty() {
+                Ok(_) => {
+                    if self.token == token::Eq {
+                        err.span_suggestion_verbose(
+                            prev_span,
+                            "you might have meant to introduce a new binding",
+                            "let ".to_string(),
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
+                }
+                Err(err) => {
+                    err.cancel();
+                }
             }
             self.restore_snapshot(snapshot);
         }

tests/ui/type/missing-let-in-binding-3.rs

@@ -0,0 +1,5 @@
+struct A {
+    : :u8, //~ ERROR expected identifier, found `:`
+}
+
+fn main() {}

tests/ui/type/missing-let-in-binding-3.stderr

@@ -0,0 +1,10 @@
+error: expected identifier, found `:`
+  --> $DIR/missing-let-in-binding-3.rs:2:5
+   |
+LL | struct A {
+   |        - while parsing this struct
+LL |     : :u8,
+   |     ^ expected identifier
+
+error: aborting due to previous error
+