Rollup merge of #22939 - bleibig:grammar-updates, r=sanxiyn

 Updates to the bison grammar to account for recent grammar additions and new tests. In particular:

* Support parsing `impl MyTrait for .. { }`
* Support parsing ExprQualifiedPaths without \"as TRAIT_REF\" such as `<Foo>::bar(&Foo)`
* Support parsing \"for\" clauses at the beginning of where clauses such as `where for<'a, 'b> &'a T: Bar<'b>`

Author: Manishearth

src/grammar/parser-lalr.y

@@ -152,6 +152,12 @@ extern char *yytext;
 %precedence MOD_SEP
 %precedence RARROW ':'
 
+// In where clauses, "for" should have greater precedence when used as
+// a higher ranked constraint than when used as the beginning of a
+// for_in_type (which is a ty)
+%precedence FORTYPE
+%precedence FOR
+
 // Binops & unops, and their precedences
 %precedence BOX
 %precedence BOXPLACE
@@ -582,6 +588,14 @@ item_impl
 {
   $$ = mk_node("ItemImplNeg", 7, $1, $3, $5, $7, $8, $10, $11);
 }
+| maybe_unsafe IMPL generic_params trait_ref FOR DOTDOT '{' '}'
+{
+  $$ = mk_node("ItemImplDefault", 3, $1, $3, $4);
+}
+| maybe_unsafe IMPL generic_params '!' trait_ref FOR DOTDOT '{' '}'
+{
+  $$ = mk_node("ItemImplDefaultNeg", 3, $1, $3, $4);
+}
 ;
 
 maybe_impl_items
@@ -769,10 +783,14 @@ where_predicates
 ;
 
 where_predicate
-: lifetime ':' bounds    { $$ = mk_node("WherePredicate", 2, $1, $3); }
-| ty ':' ty_param_bounds { $$ = mk_node("WherePredicate", 2, $1, $3); }
+: maybe_for_lifetimes lifetime ':' bounds    { $$ = mk_node("WherePredicate", 3, $1, $2, $4); }
+| maybe_for_lifetimes ty ':' ty_param_bounds { $$ = mk_node("WherePredicate", 3, $1, $2, $4); }
 ;
 
+maybe_for_lifetimes
+: FOR '<' lifetimes '>' { $$ = mk_none(); }
+| %prec FORTYPE %empty  { $$ = mk_none(); }
+
 ty_params
 : ty_param               { $$ = mk_node("TyParams", 1, $1); }
 | ty_params ',' ty_param { $$ = ext_node($1, 1, $3); }
@@ -1024,7 +1042,8 @@ ty_qualified_path_and_generic_values
 }
 | ty_qualified_path ',' ty_sums maybe_bindings
 {
-  $$ = mk_node("GenericValues", 3, mk_none(), ext_node(mk_node("TySums", 1, $1), 1, $3), $4); }
+  $$ = mk_node("GenericValues", 3, mk_none(), mk_node("TySums", 2, $1, $3), $4);
+}
 ;
 
 ty_qualified_path
@@ -1513,31 +1532,35 @@ nonblock_prefix_expr
 ;
 
 expr_qualified_path
-: '<' ty_sum AS trait_ref '>' MOD_SEP ident
+: '<' ty_sum maybe_as_trait_ref '>' MOD_SEP ident
 {
-  $$ = mk_node("ExprQualifiedPath", 3, $2, $4, $7);
+  $$ = mk_node("ExprQualifiedPath", 3, $2, $3, $6);
 }
-| '<' ty_sum AS trait_ref '>' MOD_SEP ident generic_args
+| '<' ty_sum maybe_as_trait_ref '>' MOD_SEP ident generic_args
 {
-  $$ = mk_node("ExprQualifiedPath", 4, $2, $4, $7, $8);
+  $$ = mk_node("ExprQualifiedPath", 4, $2, $3, $6, $7);
 }
-| SHL ty_sum AS trait_ref '>' MOD_SEP ident AS trait_ref '>' MOD_SEP ident
+| SHL ty_sum maybe_as_trait_ref '>' MOD_SEP ident maybe_as_trait_ref '>' MOD_SEP ident
 {
-  $$ = mk_node("ExprQualifiedPath", 3, mk_node("ExprQualifiedPath", 3, $2, $4, $7), $9, $12);
+  $$ = mk_node("ExprQualifiedPath", 3, mk_node("ExprQualifiedPath", 3, $2, $3, $6), $7, $10);
 }
-| SHL ty_sum AS trait_ref '>' MOD_SEP ident generic_args AS trait_ref '>' MOD_SEP ident
+| SHL ty_sum maybe_as_trait_ref '>' MOD_SEP ident generic_args maybe_as_trait_ref '>' MOD_SEP ident
 {
-  $$ = mk_node("ExprQualifiedPath", 3, mk_node("ExprQualifiedPath", 4, $2, $4, $7, $8), $10, $13);
+  $$ = mk_node("ExprQualifiedPath", 3, mk_node("ExprQualifiedPath", 4, $2, $3, $6, $7), $8, $11);
 }
-| SHL ty_sum AS trait_ref '>' MOD_SEP ident AS trait_ref '>' MOD_SEP ident generic_args
+| SHL ty_sum maybe_as_trait_ref '>' MOD_SEP ident maybe_as_trait_ref '>' MOD_SEP ident generic_args
 {
-  $$ = mk_node("ExprQualifiedPath", 4, mk_node("ExprQualifiedPath", 3, $2, $4, $7), $9, $12, $13);
+  $$ = mk_node("ExprQualifiedPath", 4, mk_node("ExprQualifiedPath", 3, $2, $3, $6), $7, $10, $11);
 }
-| SHL ty_sum AS trait_ref '>' MOD_SEP ident generic_args AS trait_ref '>' MOD_SEP ident generic_args
+| SHL ty_sum maybe_as_trait_ref '>' MOD_SEP ident generic_args maybe_as_trait_ref '>' MOD_SEP ident generic_args
 {
-  $$ = mk_node("ExprQualifiedPath", 4, mk_node("ExprQualifiedPath", 4, $2, $4, $7, $8), $10, $13, $14);
+  $$ = mk_node("ExprQualifiedPath", 4, mk_node("ExprQualifiedPath", 4, $2, $3, $6, $7), $8, $11, $12);
 }
 
+maybe_as_trait_ref
+: AS trait_ref { $$ = $2; }
+| %empty       { $$ = mk_none(); }
+;
 
 lambda_expr
 : %prec LAMBDA