Don't immediately drop ignored variables

Values assigned to the variable "_" shouldn't be dropped immediately.
Doing so means guard types assigned to such variables are dropped
immediately, effectively making them useless.

Changelog: fixed

compiler/src/hir.rs

@@ -753,6 +753,7 @@ pub(crate) struct DefineElse {
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub(crate) struct DefineVariable {
+    pub(crate) resolved_type: types::TypeRef,
     pub(crate) variable_id: Option<types::VariableId>,
     pub(crate) mutable: bool,
     pub(crate) name: Identifier,
@@ -2327,6 +2328,7 @@ impl<'a> LowerToHir<'a> {
         node: ast::DefineVariable,
     ) -> Box<DefineVariable> {
         Box::new(DefineVariable {
+            resolved_type: types::TypeRef::Unknown,
             mutable: node.mutable,
             variable_id: None,
             name: self.identifier(node.name),
@@ -5371,6 +5373,7 @@ mod tests {
         assert_eq!(
             hir,
             Expression::DefineVariable(Box::new(DefineVariable {
+                resolved_type: types::TypeRef::Unknown,
                 variable_id: None,
                 name: Identifier {
                     name: "a".to_string(),

compiler/src/mir/passes.rs

@@ -2382,8 +2382,11 @@ impl<'a> LowerMethod<'a> {
             self.current_block_mut().move_register(reg, src, loc);
         } else {
             let src = self.input_expression(node.value, None);
+            let reg = self.new_register(node.resolved_type);
 
-            self.unconditional_drop_register(src, loc);
+            // We don't drop immediately as this would break e.g. guards bounds
+            // to `_` (e.g. `let _ = something_that_returns_a_guard`).
+            self.current_block_mut().move_register(reg, src, loc);
         }
 
         self.get_nil(loc)

compiler/src/type_check/expressions.rs

@@ -1707,6 +1707,8 @@ impl<'a> CheckMethodBody<'a> {
         let name = &node.name.name;
         let rtype = TypeRef::nil();
 
+        node.resolved_type = var_type;
+
         if name == IGNORE_VARIABLE {
             return rtype;
         }