Introduce is_map_key/2 guard BIF

This complements the `map_get/2` guard BIF introduced in #1784.

Rationale.

`map_get/2` allows accessing map fields in guards, but it might be
problematic in more complex guard expressions, for example:

foo(X) when map_get(a, X) =:= 1 or is_list(X) -> ...

The `is_list/1` part of the guard could never succeed since the
`map_get/2` guard would fail the whole guard expression. In this
situation, this could be solved by using `;` instead of `or` to separate
the guards, but it is not possible in every case.

To solve this situation, this PR proposes a `is_map_key/2` guard that
allows to check if a map has key inside a guard before trying to access
that key. When combined with `is_map/1` this allows to construct a
purely boolean guard expression testing a value of a key in a map.

Implementation.

Given the use case motivating the introduction of this function, the PR
contains compiler optimisations that produce optimial code for the
following guard expression:

foo(X) when is_map(X) and is_map_key(a, X) and map_get(a, X) =:= 1 -> ok;
foo(_) -> error.

Given all three tests share the failure label, the `is_map_key/2` and
`is_map/2` tests are optimised away.

As with `map_get/2` the `is_map_key/2` BIF is allowed in match specs.

1 files changed, 20 insertions, 0 deletions

diff --git a/lib/compiler/src/beam_dead.erl b/lib/compiler/src/beam_dead.erl
index dbbaae05eb..762c7bdf9e 100644
--- a/lib/compiler/src/beam_dead.erl
+++ b/lib/compiler/src/beam_dead.erl
@@ -392,6 +392,26 @@ backward([{bif,'or',{f,To0},[Dst,{atom,false}],Dst}=I|Is], D,
 	_ ->
 	    backward(Is, D, [I|Acc])
     end;
+backward([{bif,map_get,{f,FF},[Key,Map],_}=I0,
+          {test,has_map_fields,{f,FT}=F,[Map|Keys0]}=I1|Is], D, Acc) when FF =/= 0 ->
+    case shortcut_label(FF, D) of
+        FT ->
+            case lists:delete(Key, Keys0) of
+                [] ->
+                    backward([I0|Is], D, Acc);
+                Keys ->
+                    Test = {test,has_map_fields,F,[Map|Keys]},
+                    backward([Test|Is], D, [I0|Acc])
+            end;
+        _ ->
+            backward([I1|Is], D, [I0|Acc])
+    end;
+backward([{bif,map_get,{f,FF},[_,Map],_}=I0,
+          {test,is_map,{f,FT},[Map]}=I1|Is], D, Acc) when FF =/= 0 ->
+    case shortcut_label(FF, D) of
+        FT -> backward([I0|Is], D, Acc);
+        _ -> backward([I1|Is], D, [I0|Acc])
+    end;
 backward([I|Is], D, Acc) ->
     backward(Is, D, [I|Acc]);
 backward([], _D, Acc) -> Acc.