Add support for _ placeholder. (#661)

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/syntax/lexer.lpp

@@ -56,6 +56,7 @@ CONTINUATION_TYPE "__Continuation"
 CONTINUATION      "__continuation"
 RUN               "__run"
 AWAIT             "__await"
+UNDERSCORE        "_"
 
 identifier    [A-Za-z_][A-Za-z0-9_]*
 integer_literal   [0-9]+
@@ -116,6 +117,7 @@ operand_start [(A-Za-z0-9_"]
 {CONTINUATION}    { return yy::parser::make_CONTINUATION(context.current_token_position); }
 {RUN}      { return yy::parser::make_RUN(context.current_token_position); }
 {AWAIT}    { return yy::parser::make_AWAIT(context.current_token_position); }
+{UNDERSCORE}      { return yy::parser::make_UNDERSCORE(context.current_token_position); }
 
 "=" return yy::parser::make_EQUAL(context.current_token_position);
 "-" return yy::parser::make_MINUS(context.current_token_position);

executable_semantics/syntax/parser.ypp

@@ -104,6 +104,7 @@ void yy::parser::error(
 %type <const Carbon::Expression*> return_type
 %type <const Carbon::Expression*> paren_expression
 %type <const Carbon::Expression*> tuple
+%type <std::optional<std::string>> binding_lhs
 %type <Carbon::Member*> variable_declaration
 %type <Carbon::Member*> member
 %type <std::list<Carbon::Member*>> member_list
@@ -146,6 +147,7 @@ void yy::parser::error(
 %token DBLARROW "=>"
 %token DEFAULT
 %token AUTO
+%token UNDERSCORE
 %token
   EQUAL  "="
   MINUS  "-"
@@ -199,6 +201,10 @@ pattern:
   expression
     { $$ = $1; }
 ;
+binding_lhs:
+  identifier { $$ = $1; }
+| UNDERSCORE { $$ = std::nullopt; }
+;
 expression:
   identifier
     { $$ = Carbon::Expression::MakeIdentifierExpression(yylineno, $1); }
@@ -206,7 +212,7 @@ expression:
     { $$ = Carbon::Expression::MakeFieldAccessExpression(yylineno, $1, $2); }
 | expression "[" expression "]"
     { $$ = Carbon::Expression::MakeIndexExpression(yylineno, $1, $3); }
-| identifier ":" expression
+| binding_lhs ":" expression
     {
       $$ = Carbon::Expression::MakeBindingExpression(yylineno, $1, $3);
     }