Allow assignment to _ inside macro expressions (#14452)

spec/compiler/semantic/macro_spec.cr

@@ -1688,6 +1688,12 @@ describe "Semantic: macro" do
     method.location.not_nil!.expanded_location.not_nil!.line_number.should eq(9)
   end
 
+  it "assigns to underscore" do
+    assert_no_errors <<-CRYSTAL
+      {% _ = 1 %}
+      CRYSTAL
+  end
+
   it "unpacks block parameters inside macros (#13742)" do
     assert_no_errors <<-CRYSTAL
       macro foo
@@ -1707,6 +1713,16 @@ describe "Semantic: macro" do
       CRYSTAL
   end
 
+  it "unpacks to underscore within block parameters inside macros" do
+    assert_type(<<-CRYSTAL) { bool }
+      {% begin %}
+        {% x = nil %}
+        {% [{1, true, 'a', ""}].each { |(_, y, _, _)| x = y } %}
+        {{ x }}
+      {% end %}
+      CRYSTAL
+  end
+
   it "executes OpAssign (#9356)" do
     assert_type(<<-CRYSTAL) { int32 }
       {% begin %}
@@ -1740,6 +1756,15 @@ describe "Semantic: macro" do
       CRYSTAL
   end
 
+  it "assigns to underscore in MultiAssign" do
+    assert_type(<<-CRYSTAL) { tuple_of([char, bool]) }
+      {% begin %}
+        {% _, x, *_, y = [1, 'a', "", nil, true] %}
+        { {{x}}, {{y}} }
+      {% end %}
+      CRYSTAL
+  end
+
   describe "@caller" do
     it "returns an array of each call" do
       assert_type(<<-CRYSTAL) { int32 }

src/compiler/crystal/macros/interpreter.cr

@@ -297,6 +297,8 @@ module Crystal
       when Var
         node.value.accept self
         @vars[target.name] = @last
+      when Underscore
+        node.value.accept self
       else
         node.raise "can only assign to variables, not #{target.class_desc}"
       end