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.

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) ->