Auto merge of #37278 - matklad:lone-lifetime, r=jseyfried

Fix syntax error in the compiler

Currently `rustc` accepts the following code: `fn f<'a>() where 'a {}`. This should be a syntax error, shouldn't it?

Not sure if my changes actually compile, waiting for the LLVM to build.

src/libsyntax/parse/parser.rs

@@ -4409,7 +4409,7 @@ impl<'a> Parser<'a> {
                     let bounded_lifetime =
                         self.parse_lifetime()?;
 
-                    self.eat(&token::Colon);
+                    self.expect(&token::Colon)?;
 
                     let bounds =
                         self.parse_lifetimes(token::BinOp(token::Plus))?;