Better recovery

compiler/rustc_parse/src/parser/generics.rs

@@ -353,6 +353,20 @@ impl<'a> Parser<'a> {
         if !self.eat_keyword(exp!(Where)) {
             return Ok((where_clause, None));
         }
+
+        if self.eat_noexpect(&token::Colon) {
+            let colon_span = self.prev_token.span;
+            self.dcx()
+                .struct_span_err(colon_span, "unexpected colon after `where`")
+                .with_span_suggestion_short(
+                    colon_span,
+                    "remove the colon",
+                    "",
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+        }
+
         where_clause.has_where_token = true;
         let where_lo = self.prev_token.span;
 

tests/ui/structs-enums/recover-enum-with-bad-where.rs

@@ -1,6 +1,8 @@
 pub enum Foo<T>
 where:
-//~^ ERROR expected one of `#`, `{`, lifetime, or type, found `:`
+//~^ ERROR unexpected colon after `where`
     T: Missing, {}
+//~^ ERROR cannot find trait `Missing` in this scope
+// (evidence that we continue parsing after the erroneous colon)
 
 fn main() {}

tests/ui/structs-enums/recover-enum-with-bad-where.stderr

@@ -1,10 +1,15 @@
-error: expected one of `#`, `{`, lifetime, or type, found `:`
+error: unexpected colon after `where`
   --> $DIR/recover-enum-with-bad-where.rs:2:6
    |
-LL | pub enum Foo<T>
-   |          --- while parsing this enum
 LL | where:
-   |      ^ expected one of `#`, `{`, lifetime, or type
+   |      ^ help: remove the colon
 
-error: aborting due to 1 previous error
+error[E0405]: cannot find trait `Missing` in this scope
+  --> $DIR/recover-enum-with-bad-where.rs:4:8
+   |
+LL |     T: Missing, {}
+   |        ^^^^^^^ not found in this scope
 
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0405`.