Implement ABSL_ATTRIBUTE_PURE_FUNCTION and ABSL_ATTRIBUTE_CONST_FUNCTION

Pure functions have no effects except to return a value, and their
return value depends only on the parameters and global
variables. Functions of this kind can be subject to data flow analysis
and might be eliminated. In practice, this means that repeated calls
to the same function with the same arguments may be optimized away and
only evaluated once.

Const functions are similar to pure functions, but may not depend on
global variables at all.

It is an error not to use the result of a function with one of these
attributes, since these functions have no other visible effects.

Since some Abseil functions are tagged with these attributes, this is
potentially a breaking change for code that doesn't use the result of
these functions.

For example, absl::Minutes() is tagged with
ABSL_ATTRIBUTE_CONST_FUNCTION.  If, for example, the result of
absl::Minutes(n) is unused, some compilers with issue a warning or
error. If this is the case, it is likely that there is a typo in the
code, or the call can be removed.
PiperOrigin-RevId: 560803581
Change-Id: Icd6f218be2cfb7226f8ab6b2d97cd960c0d3d72f

0 files changed, 0 insertions, 0 deletions