Allow true and false in const generic arguments

Author: varkor

src/libsyntax/parse/parser.rs

@@ -5250,9 +5250,13 @@ impl<'a> Parser<'a> {
                     // FIXME(const_generics): to distinguish between idents for types and consts,
                     // we should introduce a GenericArg::Ident in the AST and distinguish when
                     // lowering to the HIR. For now, idents for const args are not permitted.
-                    return Err(
-                        self.fatal("identifiers may currently not be used for const generics")
-                    );
+                    if self.token.is_keyword(kw::True) || self.token.is_keyword(kw::False) {
+                        self.parse_literal_maybe_minus()?
+                    } else {
+                        return Err(
+                            self.fatal("identifiers may currently not be used for const generics")
+                        );
+                    }
                 } else {
                     self.parse_literal_maybe_minus()?
                 };

src/test/ui/const-generics/condition-in-trait-const-arg.rs

@@ -7,4 +7,6 @@ trait IsZeroTrait<const IS_ZERO: bool>{}
 
 impl IsZeroTrait<{0u8 == 0u8}> for () {}
 
+impl IsZeroTrait<true> for ((),) {}
+
 fn main() {}