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.

2 files changed, 20 insertions, 3 deletions

diff --git a/lib/compiler/test/match_SUITE.erl b/lib/compiler/test/match_SUITE.erl
index 20969c0b26..7d5669025c 100644
--- a/lib/compiler/test/match_SUITE.erl
+++ b/lib/compiler/test/match_SUITE.erl
@@ -21,14 +21,14 @@
 -export([all/1,
 	 pmatch/1,mixed/1,aliases/1,match_in_call/1,
 	 untuplify/1,shortcut_boolean/1,letify_guard/1,
-	 selectify/1]).
+	 selectify/1,underscore/1]).
 	 
 -include("test_server.hrl").
 
 all(suite) ->
     test_lib:recompile(?MODULE),
     [pmatch,mixed,aliases,match_in_call,untuplify,shortcut_boolean,
-     letify_guard,selectify].
+     letify_guard,selectify,underscore].
 
 pmatch(Config) when is_list(Config) ->
     ?line ok = doit(1),
@@ -352,4 +352,16 @@ sel_same_value2(V) when V =:= 42; V =:= 43 ->
 sel_same_value2(_) ->
     error.
 
+underscore(Config) when is_list(Config) ->
+    case Config of
+	[] ->
+	    %% Assignment to _ at the end of a construct.
+	    _ = length(Config);
+	[_|_] ->
+	    %% Assignment to _ at the end of a construct.
+	    _ = list_to_tuple(Config)
+    end,
+    _ = is_list(Config),
+    ok.
+
 id(I) -> I.
diff --git a/lib/compiler/test/warnings_SUITE.erl b/lib/compiler/test/warnings_SUITE.erl
index 6e60ab88cb..8b98db54f2 100644
--- a/lib/compiler/test/warnings_SUITE.erl
+++ b/lib/compiler/test/warnings_SUITE.erl
@@ -375,7 +375,12 @@ effect(Config) when is_list(Config) ->
                 comp_op ->
                     X =:= 2;
                 cookie ->
-                    erlang:get_cookie()
+                    erlang:get_cookie();
+		result_ignore ->
+                    _ = list_to_integer(X);
+                warn_lc_4 ->
+                    %% No warning because of assignment to _.
+                    [_ = abs(Z) || Z <- [1,2,3]]
                end,
                ok.