1 files changed, 41 insertions, 1 deletions

diff --git a/src/parser/grammar/items/traits.rs b/src/parser/grammar/items/traits.rs
index 7fd011ffd..c7450b761 100644
--- a/src/parser/grammar/items/traits.rs
+++ b/src/parser/grammar/items/traits.rs
@@ -14,7 +14,47 @@ pub(super) fn trait_item(p: &mut Parser) {
 pub(super) fn impl_item(p: &mut Parser) {
     assert!(p.at(IMPL_KW));
     p.bump();
-    p.expect(IDENT);
+    if choose_type_params_over_qpath(p) {
+        type_params::list(p);
+    }
+
+    // TODO: never type
+    // impl ! {}
+
+    // test impl_item_neg
+    // impl !Send for X {}
+    p.eat(EXCL);
+    types::type_(p);
+    if p.eat(FOR_KW) {
+        types::type_(p);
+    }
+    type_params::where_clause(p);
     p.expect(L_CURLY);
     p.expect(R_CURLY);
 }
+
+fn choose_type_params_over_qpath(p: &Parser) -> bool {
+    // There's an ambiguity between generic parameters and qualified paths in impls.
+    // If we see `<` it may start both, so we have to inspect some following tokens.
+    // The following combinations can only start generics,
+    // but not qualified paths (with one exception):
+    //     `<` `>` - empty generic parameters
+    //     `<` `#` - generic parameters with attributes
+    //     `<` (LIFETIME|IDENT) `>` - single generic parameter
+    //     `<` (LIFETIME|IDENT) `,` - first generic parameter in a list
+    //     `<` (LIFETIME|IDENT) `:` - generic parameter with bounds
+    //     `<` (LIFETIME|IDENT) `=` - generic parameter with a default
+    // The only truly ambiguous case is
+    //     `<` IDENT `>` `::` IDENT ...
+    // we disambiguate it in favor of generics (`impl<T> ::absolute::Path<T> { ... }`)
+    // because this is what almost always expected in practice, qualified paths in impls
+    // (`impl <Type>::AssocTy { ... }`) aren't even allowed by type checker at the moment.
+    if !p.at(L_ANGLE) {
+        return false;
+    }
+    if p.nth(1) == POUND || p.nth(1) == R_ANGLE {
+        return true;
+    }
+    (p.nth(1) == LIFETIME || p.nth(1) == IDENT)
+        && (p.nth(2) == R_ANGLE || p.nth(2) == COMMA || p.nth(2) == COLON || p.nth(2) == EQ)
+}