[PATCH] attribs: Allow optional second arg for attr deprecated [PR102049]

Martin Sebor msebor@gmail.com
Mon Nov 1 23:15:03 GMT 2021


On 10/11/21 9:17 AM, Marek Polacek via Gcc-patches wrote:
> Any thoughts?

I'm a little unsure.  Clang just uses the replacement string
as the text of the fix-it note as is, so it does nothing to
help programmers make sure the replacement is in sync with
what it's supposed to replace.  E.g., for this Clang output
is below:

__attribute__ ((deprecated ("foo is bad", "use bar instead")))
void foo (void);
void baz (void) { foo (); }

int bar;

<source>:2:19: warning: 'foo' is deprecated: foo is bad 
[-Wdeprecated-declarations]
void baz (void) { foo (); }
                   ^~~
                   use bar instead

Since bar is a variable it's hard to see how it might be used
instead of the function foo().  Fix-its, as I understand them,
are meant not just as a visual clue but also to let IDEs and
other tools automatically apply the fixes.  With buggy fix-its
this obviously wouldn't work.

I think the replacement would be useful if it had to reference
an existing symbol of the same kind, and if the compiler helped
enforce it.  Otherwise it seems like a recipe for bit rot and
for tings/tools not working well together.

Martin

> 
> On Thu, Sep 23, 2021 at 12:16:36PM -0400, Marek Polacek via Gcc-patches wrote:
>> Clang implements something we don't have:
>>
>> __attribute__((deprecated("message", "replacement")));
>>
>> which seems pretty neat so I wrote this patch to add it to gcc.
>>
>> It doesn't allow the optional second argument in the standard [[]]
>> form so as not to clash with possible future standard additions.
>>
>> I had hoped we could print a nice fix-it replacement hint, but that
>> won't be possible until warn_deprecated_use gets something better than
>> input_location.
>>
>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>
>> 	PR c++/102049
>>
>> gcc/c-family/ChangeLog:
>>
>> 	* c-attribs.c (c_common_attribute_table): Increase max_len for
>> 	deprecated.
>> 	(handle_deprecated_attribute): Allow an optional second argument
>> 	in the GNU form of attribute deprecated.
>>
>> gcc/c/ChangeLog:
>>
>> 	* c-parser.c (c_parser_std_attribute): Give a diagnostic when
>> 	the standard form of an attribute deprecated has a second argument.
>>
>> gcc/ChangeLog:
>>
>> 	* doc/extend.texi: Document attribute deprecated with an
>> 	optional second argument.
>> 	* tree.c (warn_deprecated_use): Print the replacement argument,
>> 	if any.
>>
>> gcc/testsuite/ChangeLog:
>>
>> 	* gcc.dg/c2x-attr-deprecated-3.c: Adjust dg-error.
>> 	* c-c++-common/Wdeprecated-arg-1.c: New test.
>> ---
>>   gcc/c-family/c-attribs.c                      | 17 ++++++++++++-
>>   gcc/c/c-parser.c                              |  8 ++++++
>>   gcc/doc/extend.texi                           | 24 ++++++++++++++++++
>>   .../c-c++-common/Wdeprecated-arg-1.c          | 21 ++++++++++++++++
>>   gcc/testsuite/gcc.dg/c2x-attr-deprecated-3.c  |  2 +-
>>   gcc/tree.c                                    | 25 +++++++++++++++----
>>   6 files changed, 90 insertions(+), 7 deletions(-)
>>   create mode 100644 gcc/testsuite/c-c++-common/Wdeprecated-arg-1.c
>>
>> diff --git a/gcc/c-family/c-attribs.c b/gcc/c-family/c-attribs.c
>> index 007b928c54b..ef857a9ae2c 100644
>> --- a/gcc/c-family/c-attribs.c
>> +++ b/gcc/c-family/c-attribs.c
>> @@ -409,7 +409,7 @@ const struct attribute_spec c_common_attribute_table[] =
>>        to prevent its usage in source code.  */
>>     { "no vops",                0, 0, true,  false, false, false,
>>   			      handle_novops_attribute, NULL },
>> -  { "deprecated",             0, 1, false, false, false, false,
>> +  { "deprecated",             0, 2, false, false, false, false,
>>   			      handle_deprecated_attribute, NULL },
>>     { "unavailable",            0, 1, false, false, false, false,
>>   			      handle_unavailable_attribute, NULL },
>> @@ -4107,6 +4107,21 @@ handle_deprecated_attribute (tree *node, tree name,
>>         error ("deprecated message is not a string");
>>         *no_add_attrs = true;
>>       }
>> +  else if (TREE_CHAIN (args) != NULL_TREE)
>> +    {
>> +      /* We allow an optional second argument in the GNU form of
>> +	 attribute deprecated, which specifies the replacement.  */
>> +      if (flags & ATTR_FLAG_CXX11)
>> +	{
>> +	  error ("replacement argument only allowed in GNU attributes");
>> +	  *no_add_attrs = true;
>> +	}
>> +      else if (TREE_CODE (TREE_VALUE (TREE_CHAIN (args))) != STRING_CST)
>> +	{
>> +	  error ("replacement argument is not a string");
>> +	  *no_add_attrs = true;
>> +	}
>> +    }
>>   
>>     if (DECL_P (*node))
>>       {
>> diff --git a/gcc/c/c-parser.c b/gcc/c/c-parser.c
>> index fa29d2c15fc..2b47f01d166 100644
>> --- a/gcc/c/c-parser.c
>> +++ b/gcc/c/c-parser.c
>> @@ -4952,6 +4952,14 @@ c_parser_std_attribute (c_parser *parser, bool for_tm)
>>   	TREE_VALUE (attribute)
>>   	  = c_parser_attribute_arguments (parser, takes_identifier,
>>   					  require_string, false);
>> +	if (c_parser_next_token_is (parser, CPP_COMMA)
>> +	    && strcmp (IDENTIFIER_POINTER (name), "deprecated") == 0)
>> +	  {
>> +	    error_at (open_loc, "replacement argument only allowed in "
>> +		      "GNU attributes");
>> +	    c_parser_skip_until_found (parser, CPP_CLOSE_PAREN, NULL);
>> +	    return error_mark_node;
>> +	  }
>>         }
>>       else
>>         c_parser_balanced_token_sequence (parser);
>> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
>> index 9501a60f20e..7d399f4b2bc 100644
>> --- a/gcc/doc/extend.texi
>> +++ b/gcc/doc/extend.texi
>> @@ -2860,6 +2860,7 @@ StrongAlias (allocate, alloc);
>>   
>>   @item deprecated
>>   @itemx deprecated (@var{msg})
>> +@itemx deprecated (@var{msg}, @var{replacement})
>>   @cindex @code{deprecated} function attribute
>>   The @code{deprecated} attribute results in a warning if the function
>>   is used anywhere in the source file.  This is useful when identifying
>> @@ -2880,6 +2881,13 @@ results in a warning on line 3 but not line 2.  The optional @var{msg}
>>   argument, which must be a string, is printed in the warning if
>>   present.
>>   
>> +This attribute accepts an optional second argument which specifies
>> +the replacement that ought to be used instead:
>> +
>> +@smallexample
>> +int old_fn () __attribute__ ((deprecated("unsafe", "new_fn")));
>> +@end smallexample
>> +
>>   The @code{deprecated} attribute can also be used for variables and
>>   types (@pxref{Variable Attributes}, @pxref{Type Attributes}.)
>>   
>> @@ -7409,6 +7417,7 @@ attribute is also not copied.  @xref{Common Function Attributes}.
>>   
>>   @item deprecated
>>   @itemx deprecated (@var{msg})
>> +@itemx deprecated (@var{msg}, @var{replacement})
>>   @cindex @code{deprecated} variable attribute
>>   The @code{deprecated} attribute results in a warning if the variable
>>   is used anywhere in the source file.  This is useful when identifying
>> @@ -7429,6 +7438,13 @@ results in a warning on line 3 but not line 2.  The optional @var{msg}
>>   argument, which must be a string, is printed in the warning if
>>   present.
>>   
>> +This attribute accepts an optional second argument which specifies
>> +the replacement that ought to be used instead:
>> +
>> +@smallexample
>> +extern int old_var __attribute__ ((deprecated("unsafe", "new_var")));
>> +@end smallexample
>> +
>>   The @code{deprecated} attribute can also be used for functions and
>>   types (@pxref{Common Function Attributes},
>>   @pxref{Common Type Attributes}).
>> @@ -8492,6 +8508,7 @@ struct __attribute__ ((copy ( (struct A *)0)) B @{ /* @r{@dots{}} */ @};
>>   
>>   @item deprecated
>>   @itemx deprecated (@var{msg})
>> +@itemx deprecated (@var{msg}, @var{replacement})
>>   @cindex @code{deprecated} type attribute
>>   The @code{deprecated} attribute results in a warning if the type
>>   is used anywhere in the source file.  This is useful when identifying
>> @@ -8522,6 +8539,13 @@ present.  Control characters in the string will be replaced with
>>   escape sequences, and if the @option{-fmessage-length} option is set
>>   to 0 (its default value) then any newline characters will be ignored.
>>   
>> +This attribute accepts an optional second argument which specifies
>> +the replacement that ought to be used instead:
>> +
>> +@smallexample
>> +typedef int oldtype __attribute__ ((deprecated("unsafe", "newtype")));
>> +@end smallexample
>> +
>>   The @code{deprecated} attribute can also be used for functions and
>>   variables (@pxref{Function Attributes}, @pxref{Variable Attributes}.)
>>   
>> diff --git a/gcc/testsuite/c-c++-common/Wdeprecated-arg-1.c b/gcc/testsuite/c-c++-common/Wdeprecated-arg-1.c
>> new file mode 100644
>> index 00000000000..b03e0e42ad6
>> --- /dev/null
>> +++ b/gcc/testsuite/c-c++-common/Wdeprecated-arg-1.c
>> @@ -0,0 +1,21 @@
>> +/* PR c++/102049 */
>> +/* { dg-do compile } */
>> +/* { dg-additional-options "-std=c++11" { target c++ } } */
>> +
>> +typedef int __attribute__((deprecated("obsolete", "newtype"))) oldtype;
>> +__attribute__((deprecated("unsafe", "get_it_s"))) void get_it (int); /* { dg-message "declared here" } */
>> +[[gnu::deprecated("g", "")]] void g(); /* { dg-error "only allowed in GNU attributes" } */
>> +[[deprecated("h", "")]] void h(); /* { dg-error "only allowed in GNU attributes" } */
>> +__attribute__((deprecated("unsafe", 1))) void f2(); /* { dg-error "argument is not a string" } */
>> +__attribute__((deprecated("unsafe", "newvar"))) int oldvar; /* { dg-message "declared here" } */
>> +
>> +int
>> +main ()
>> +{
>> +  get_it (42); /* { dg-warning "is deprecated: unsafe" } */
>> +  /* { dg-message "use .get_it_s. instead" "" { target *-*-* } .-1 } */
>> +  oldtype t; /* { dg-warning "is deprecated: obsolete" } */
>> +  /* { dg-message "use .newtype. instead" "" { target *-*-* } .-1 } */
>> +  int i = oldvar; /* { dg-warning "is deprecated: unsafe" } */
>> +  /* { dg-message "use .newvar. instead" "" { target *-*-* } .-1 } */
>> +}
>> diff --git a/gcc/testsuite/gcc.dg/c2x-attr-deprecated-3.c b/gcc/testsuite/gcc.dg/c2x-attr-deprecated-3.c
>> index 044725e5123..6cb70705bbf 100644
>> --- a/gcc/testsuite/gcc.dg/c2x-attr-deprecated-3.c
>> +++ b/gcc/testsuite/gcc.dg/c2x-attr-deprecated-3.c
>> @@ -6,6 +6,6 @@
>>   
>>   [[deprecated(0)]] int b; /* { dg-error "expected" } */
>>   
>> -[[deprecated("", 123)]] int c; /* { dg-error "expected" } */
>> +[[deprecated("", 123)]] int c; /* { dg-error "only allowed in GNU attributes" } */
>>   
>>   [[deprecated((""))]] int d; /* { dg-error "expected" } */
>> diff --git a/gcc/tree.c b/gcc/tree.c
>> index 561b9cd56bd..5cb2d57a960 100644
>> --- a/gcc/tree.c
>> +++ b/gcc/tree.c
>> @@ -12002,8 +12002,9 @@ bool
>>   warn_deprecated_use (tree node, tree attr)
>>   {
>>     escaped_string msg;
>> +  escaped_string rep;
>>   
>> -  if (node == 0 || !warn_deprecated_decl)
>> +  if (node == NULL_TREE || !warn_deprecated_decl)
>>       return false;
>>   
>>     if (!attr)
>> @@ -12023,7 +12024,12 @@ warn_deprecated_use (tree node, tree attr)
>>       attr = lookup_attribute ("deprecated", attr);
>>   
>>     if (attr)
>> -    msg.escape (TREE_STRING_POINTER (TREE_VALUE (TREE_VALUE (attr))));
>> +    {
>> +      tree val = TREE_VALUE (attr);
>> +      msg.escape (TREE_STRING_POINTER (TREE_VALUE (val)));
>> +      if (TREE_CHAIN (val))
>> +	rep.escape (TREE_STRING_POINTER (TREE_VALUE (TREE_CHAIN (val))));
>> +    }
>>   
>>     bool w = false;
>>     if (DECL_P (node))
>> @@ -12036,7 +12042,11 @@ warn_deprecated_use (tree node, tree attr)
>>   	w = warning (OPT_Wdeprecated_declarations,
>>   		     "%qD is deprecated", node);
>>         if (w)
>> -	inform (DECL_SOURCE_LOCATION (node), "declared here");
>> +	{
>> +	  if (rep)
>> +	    inform (input_location, "use %qs instead", (const char *) rep);
>> +	  inform (DECL_SOURCE_LOCATION (node), "declared here");
>> +	}
>>       }
>>     else if (TYPE_P (node))
>>       {
>> @@ -12072,8 +12082,13 @@ warn_deprecated_use (tree node, tree attr)
>>   			 "type is deprecated");
>>   	}
>>   
>> -      if (w && decl)
>> -	inform (DECL_SOURCE_LOCATION (decl), "declared here");
>> +      if (w)
>> +	{
>> +	  if (rep)
>> +	    inform (input_location, "use %qs instead", (const char *) rep);
>> +	  if (decl)
>> +	    inform (DECL_SOURCE_LOCATION (decl), "declared here");
>> +	}
>>       }
>>   
>>     return w;
>>
>> base-commit: f6a05b23cc2e53c38e8321ddb5d2cbe40737e506
>> -- 
>> 2.31.1
>>
> 
> Marek
> 



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