Re: [PATCH] C/C++: add fix-it hints for various missing symbols

[Jeff Law <law at redhat dot com>]

On 07/03/2017 12:37 PM, David Malcolm wrote:
> This patch improves our C/C++ frontends' handling of missing
> symbols, by making c_parser_require and cp_parser_require use
> "better" locations for the diagnostic, and insert fix-it hints,
> under certain circumstances (see the comments in the patch for
> full details).
> 
> For example, for this code with a missing semicolon:
> 
>   $ cat test.c
>   int missing_semicolon (void)
>   {
>     return 42
>   }
> 
> trunk currently emits:
> 
>   test.c:4:1: error: expected ‘;’ before ‘}’ token
>    }
>    ^
> 
> This patch adds a fix-it hint for the missing semicolon, and puts
> the error at the location of the missing semicolon, printing the
> followup token as a secondary location:
> 
>   test.c:3:12: error: expected ‘;’ before ‘}’ token
>      return 42
>               ^
>               ;
>    }
>    ~
> 
> More examples can be seen in the test cases.
> 
> For reference, clang prints the following:
> 
>   test.c:3:12: error: expected ';' after return statement
>     return 42
>              ^
>              ;
> 
> i.e. describing what syntactic thing came before, which
> I think is likely to be more meaningful to the user.
> 
> clang can also print notes about matching opening symbols
> e.g. the note here:
> 
>   missing-symbol-2.c:25:22: error: expected ']'
>     const char test [42;
>                        ^
>   missing-symbol-2.c:25:19: note: to match this '['
>     const char test [42;
>                     ^
> which, although somewhat redundant for this example, seems much more
> useful if there's non-trivial nesting of constructs, or more than a few
> lines separating the open/close symbols (e.g. showing a stray "namespace {"
> that the user forgot to close).
> 
> I'd like to implement both of these ideas as followups, but in
> the meantime, is the fix-it hint patch OK for trunk?
> (successfully bootstrapped & regrtested on x86_64-pc-linux-gnu)
> 
> gcc/c-family/ChangeLog:
> 	* c-common.c (c_parse_error): Add RICHLOC param, and use it rather
> 	than implicitly using input_location.
> 	(enum missing_token_insertion_kind): New enum.
> 	(get_missing_token_insertion_kind): New function.
> 	(maybe_suggest_missing_token_insertion): New function.
> 	* c-common.h (c_parse_error): Add RICHLOC param.
> 	(maybe_suggest_missing_token_insertion): New decl.
> 
> gcc/c/ChangeLog:
> 	* c-parser.c (struct c_parser): Add "previous_token_loc" field.
> 	(c_parser_consume_token): Set parser->previous_token_loc.
> 	(c_parser_error): Rename to...
> 	(c_parser_error_richloc): ...this, making static, and adding
> 	"richloc" parameter, passing it to the c_parse_error call,
> 	rather than calling c_parser_set_source_position_from_token.
> 	(c_parser_error): Reintroduce, reimplementing in terms of the
> 	above.
> 	(c_parser_require): Add "type_is_unique" param.  Use
> 	c_parser_error_richloc rather than c_parser_error, calling
> 	maybe_suggest_missing_token_insertion.
> 	(c_parser_parms_list_declarator): Override default value of new
> 	"type_is_unique" param to c_parser_require.
> 	(c_parser_asm_statement): Likewise.
> 	* c-parser.h (c_parser_require): Add "type_is_unique" param,
> 	defaulting to true.
> 
> gcc/cp/ChangeLog:
> 	* parser.c (cp_parser_error): Add rich_location to call to
> 	c_parse_error.
> 	(get_required_cpp_ttype): New function.
> 	(cp_parser_required_error): Remove calls to cp_parser_error,
> 	instead setting a non-NULL gmsgid, and handling it if set by
> 	calling c_parse_error, potentially with a fix-it hint.
> 
> gcc/testsuite/ChangeLog:
> 	* c-c++-common/cilk-plus/AN/parser_errors.c: Update expected
> 	output to reflect changes to reported locations of missing
> 	symbols.
> 	* c-c++-common/cilk-plus/AN/parser_errors2.c: Likewise.
> 	* c-c++-common/cilk-plus/AN/parser_errors3.c: Likewise.
> 	* c-c++-common/cilk-plus/AN/pr61191.c: Likewise.
> 	* c-c++-common/gomp/pr63326.c: Likewise.
> 	* c-c++-common/missing-symbol.c: New test case.
> 	* g++.dg/cpp1y/digit-sep-neg.C: Update expected output to reflect
> 	changes to reported locations of missing symbols.
> 	* g++.dg/cpp1y/pr65202.C: Likewise.
> 	* g++.dg/other/do1.C: Likewise.
> 	* g++.dg/missing-symbol-2.C: New test case.
> 	* g++.dg/parse/error11.C: Update expected output to reflect
> 	changes to reported locations of missing symbols.
> 	* g++.dg/parse/pragma2.C: Likewise.
> 	* g++.dg/template/error11.C: Likewise.
> 	* gcc.dg/missing-symbol-2.c: New test case.
> 	* gcc.dg/missing-symbol-3.c: New test case.
> 	* gcc.dg/noncompile/940112-1.c: Update expected output to reflect
> 	changes to reported locations of missing symbols.
> 	* gcc.dg/noncompile/971104-1.c: Likewise.
> 	* obj-c++.dg/exceptions-6.mm: Likewise.
> 	* obj-c++.dg/pr48187.mm: Likewise.
> 	* objc.dg/exceptions-6.m: Likewise.
AFAICT, this never got moved forward after the comments from Richard and
Joseph.




> +}
> +
> +/* Given RICHLOC, a location for a diagnostic describing a missing token
> +   of kind TOKEN_TYPE, potentially add a fix-it hint suggesting the
> +   insertion of the token.
> +
> +   The location of the attemped fix-it hint depends on TOKEN_TYPE:
s/attemped/attempted/


> +
> +  if (gmsgid)
> +    {
> +      /* Emulate rest of cp_parser_error.  */
> +      cp_token *token = cp_lexer_peek_token (parser->lexer);
> +      cp_lexer_set_source_position_from_token (token);
> +
> +      rich_location richloc (line_table, input_location);
So is it worth trying to factor the bits you want to emulate from
cp_parser_error so that they're shared?  Or just a comment in
cp_parser_error in the hopes that if someone changes it in a meaningful
way they'll know to come back here and potentially update this routine?

In general though it looks really good.  I think we just want to make a
decision whether or not there's some way to avoid a long term
maintenance headache noted immediately above.

jeff
Jeff