recover const Tr bounds (no ~)

compiler/rustc_parse/src/parser/ty.rs

@@ -809,6 +809,20 @@ impl<'a> Parser<'a> {
             self.expect_keyword(kw::Const)?;
             let span = tilde.to(self.prev_token.span);
             self.sess.gated_spans.gate(sym::const_trait_impl, span);
+            Some(span)
+        } else if self.eat_keyword(kw::Const) {
+            let span = self.prev_token.span;
+            self.sess.gated_spans.gate(sym::const_trait_impl, span);
+
+            self.struct_span_err(span, "const bounds must start with `~`")
+                .span_suggestion(
+                    span.shrink_to_lo(),
+                    "add `~`",
+                    "~",
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+
             Some(span)
         } else {
             None

src/test/ui/rfc-2632-const-trait-impl/without-tilde.rs

@@ -3,5 +3,4 @@
 #![feature(const_trait_impl)]
 
 struct S<T: const Tr>;
-//~^ ERROR expected identifier, found keyword `const`
-//~| ERROR expected one of `(`, `+`, `,`, `::`, `<`, `=`, or `>`, found `Tr`
+//~^ ERROR const bounds must start with `~`

src/test/ui/rfc-2632-const-trait-impl/without-tilde.stderr

@@ -1,14 +1,10 @@
-error: expected identifier, found keyword `const`
+error: const bounds must start with `~`
   --> $DIR/without-tilde.rs:5:13
    |
 LL | struct S<T: const Tr>;
-   |             ^^^^^ expected identifier, found keyword
+   |             -^^^^
+   |             |
+   |             help: add `~`: `~`
 
-error: expected one of `(`, `+`, `,`, `::`, `<`, `=`, or `>`, found `Tr`
-  --> $DIR/without-tilde.rs:5:19
-   |
-LL | struct S<T: const Tr>;
-   |                   ^^ expected one of 7 possible tokens
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error