[Issue 5464] New: Attribute to not ignore function result

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=5464

           Summary: Attribute to not ignore function result
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2011-01-20 02:59:14 PST ---
Ignoring the result of some functions like string replace(), or the C
realloc(), or in general the result of strongly pure functions in D, is a
programmer mistake that often enough is a sign of a bug presence.

To face this problem GNU C has the warn_unused_result attribute:
http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
(This is more useful in C than D because in C there are no true built-in
exceptions, so error return values are common, and sometimes ignoring them is a
mistake.)

Some C lints require a void cast where you don't want to use a function result:
cast(void)foo(x);

In a language the default is different and where you don't want to use a
function result you have to add a specific annotation:
unused foo(x);

In D an attribute like @nodiscard (name invented by Andrej Mitrovic) may turn
ignoring return values into errors. To silence this error the programmer uses
something like cast(void).

So the error message may be:
"Error: unused result of @nodiscard function. Use cast(void) to override it."

Strongly pure functions may produce this error even if you don't use
@nodiscard.

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