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.