From c34ad2d537e41c9b31e240aa1e6fadd994115a16 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Thu, 15 Apr 2010 10:18:53 +0200
Subject: 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.
---
 lib/compiler/test/match_SUITE.erl    | 16 ++++++++++++++--
 lib/compiler/test/warnings_SUITE.erl |  7 ++++++-
 2 files changed, 20 insertions(+), 3 deletions(-)

(limited to 'lib/compiler/test')

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.
 
-- 
cgit v1.2.3


From feed8f6669ee2ca84139d297056857c6b05fa351 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Thu, 15 Apr 2010 10:19:02 +0200
Subject: compiler tests: Eliminate "result of expression is ignored" warnings

---
 lib/compiler/test/float_SUITE.erl   | 2 +-
 lib/compiler/test/num_bif_SUITE.erl | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

(limited to 'lib/compiler/test')

diff --git a/lib/compiler/test/float_SUITE.erl b/lib/compiler/test/float_SUITE.erl
index 3d2dbf47e9..0d05aa70c2 100644
--- a/lib/compiler/test/float_SUITE.erl
+++ b/lib/compiler/test/float_SUITE.erl
@@ -41,7 +41,7 @@ float_sub(A)->
 float_mul(0, _, _)->
     ok;
 float_mul(Iter, A, B) when is_float(A), is_float(B) ->
-    A*B,
+    _ = A*B,
     float_mul(Iter-1, A, B).
 
 %% Thanks to Mikael Pettersson and Tobias Lindahl (HiPE).
diff --git a/lib/compiler/test/num_bif_SUITE.erl b/lib/compiler/test/num_bif_SUITE.erl
index c246f56611..1be6839972 100644
--- a/lib/compiler/test/num_bif_SUITE.erl
+++ b/lib/compiler/test/num_bif_SUITE.erl
@@ -166,7 +166,7 @@ t_list_to_float_safe(Config) when is_list(Config) ->
 
 t_list_to_float_risky(Config) when is_list(Config) ->
     ?line Many_Ones = lists:duplicate(25000, $1),
-    ?line list_to_float("2."++Many_Ones),
+    ?line _ = list_to_float("2."++Many_Ones),
     ?line {'EXIT', {badarg, _}} = (catch list_to_float("2"++Many_Ones)),
     ok.
 
@@ -186,7 +186,7 @@ t_list_to_integer(Config) when is_list(Config) ->
 
     %% Bignums.
     ?line 123456932798748738738 = list_to_integer("123456932798748738738"),
-    ?line list_to_integer(lists:duplicate(2000, $1)),
+    ?line _ = list_to_integer(lists:duplicate(2000, $1)),
     ok.
 
 %% Tests round/1.
-- 
cgit v1.2.3