diff options
author | Björn Gustavsson <[email protected]> | 2010-04-15 10:18:53 +0200 |
---|---|---|
committer | Björn Gustavsson <[email protected]> | 2010-04-15 16:41:50 +0200 |
commit | c34ad2d537e41c9b31e240aa1e6fadd994115a16 (patch) | |
tree | 2cbe3141d13a1ac40fdfa7486eef89be053f3100 /lib/compiler/src/sys_core_fold.erl | |
parent | ab47252a5f7d540d4119d38dffe69acca86d2a41 (diff) | |
download | otp-c34ad2d537e41c9b31e240aa1e6fadd994115a16.tar.gz otp-c34ad2d537e41c9b31e240aa1e6fadd994115a16.tar.bz2 otp-c34ad2d537e41c9b31e240aa1e6fadd994115a16.zip |
Silence warnings for expressions that are assigned to "_"
There is currently no zero-cost way to silence the warning
"the result of the expression is ignored", which is issued
for code such as:
list_to_integer(S),
ok
Such code can be useful for assertions or input validation.
Teach the compiler to silence the warning for expressions
that are explicitly assigned to to the "_" variable,
such as:
_ = list_to_integer(S),
ok
Implement it by having the v3_core pass annotate calls in
Core Erlang like this:
let <_> = ( call 'erlang':'list_to_integer'(S) -| ['result_not_wanted'] )
in 'ok'
and modifiy sys_core_fold to suppress the warning for any call
having the annotation.
We deliberately do not make it possible to silence the warnings
for expressions like:
{build,an,unnecessary,term}, ok
or
is_list(L), ok
because we don't know of any real-world scenarios in which that would
be useful.
Diffstat (limited to 'lib/compiler/src/sys_core_fold.erl')
-rw-r--r-- | lib/compiler/src/sys_core_fold.erl | 17 |
1 files changed, 13 insertions, 4 deletions
diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl index 068478496b..3b374be015 100644 --- a/lib/compiler/src/sys_core_fold.erl +++ b/lib/compiler/src/sys_core_fold.erl @@ -602,15 +602,23 @@ count_bits_1(Int, Bits) -> count_bits_1(Int bsr 64, Bits+64). %% a rewritten expression consisting of a sequence of %% the arguments only is returned. -useless_call(effect, #c_call{module=#c_literal{val=Mod}, +useless_call(effect, #c_call{anno=Anno, + module=#c_literal{val=Mod}, name=#c_literal{val=Name}, args=Args}=Call) -> A = length(Args), case erl_bifs:is_safe(Mod, Name, A) of false -> case erl_bifs:is_pure(Mod, Name, A) of - true -> add_warning(Call, result_ignored); - false -> ok + true -> + case member(result_not_wanted, Anno) of + false -> + add_warning(Call, result_ignored); + true -> + ok + end; + false -> + ok end, no; true -> @@ -2806,7 +2814,8 @@ format_error({no_effect,{erlang,F,A}}) -> end, flatten(io_lib:format(Fmt, Args)); format_error(result_ignored) -> - "the result of the expression is ignored"; + "the result of the expression is ignored " + "(suppress the warning by assigning the expression to the _ variable)"; format_error(useless_building) -> "a term is constructed, but never used"; format_error(bin_opt_alias) -> |