parser: smartcast mutable selector (#6881)

vlib/v/parser/if_match.v

@@ -87,8 +87,10 @@ fn (mut p Parser) if_expr(is_comptime bool) ast.IfExpr {
 		comments << p.eat_comments()
 		// `if mut name is T`
 		mut is_mut_name := false
-		if p.tok.kind == .key_mut && p.peek_tok2.kind == .key_is {
+		mut mut_pos := token.Position{}
+		if p.tok.kind == .key_mut {
 			is_mut_name = true
+			mut_pos = p.tok.position()
 			p.next()
 			comments << p.eat_comments()
 		}
@@ -135,6 +137,11 @@ fn (mut p Parser) if_expr(is_comptime bool) ast.IfExpr {
 			} else {
 				''
 			}
+			if !is_is_cast && is_mut_name {
+				p.error_with_pos('remove unnecessary `mut`', mut_pos)
+			}
+		} else if is_mut_name {
+			p.error_with_pos('remove unnecessary `mut`', mut_pos)
 		}
 		end_pos := p.prev_tok.position()
 		body_pos := p.tok.position()

vlib/v/parser/tests/unnecessary_mut.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/unnecessary_mut.vv:3:5: error: remove unnecessary `mut`
+    1 | fn main() {
+    2 |     mut x := 0
+    3 |     if mut x == 0 {
+      |        ~~~
+    4 |         println(true)
+    5 |     }

vlib/v/parser/tests/unnecessary_mut.vv

@@ -0,0 +1,6 @@
+fn main() {
+	mut x := 0
+	if mut x == 0 {
+		println(true)
+	}
+}

vlib/v/parser/tests/unnecessary_mut_2.out

@@ -0,0 +1,6 @@
+vlib/v/parser/tests/unnecessary_mut_2.vv:2:5: error: remove unnecessary `mut`
+    1 | fn main() {
+    2 |     if mut true {
+      |        ~~~
+    3 |         println(true)
+    4 |     }

vlib/v/parser/tests/unnecessary_mut_2.vv

@@ -0,0 +1,5 @@
+fn main() {
+	if mut true {
+		println(true)
+	}
+}

vlib/v/tests/union_sum_type_test.v

@@ -357,6 +357,11 @@ mut:
 
 __type Food = Milk | Eggs
 
+struct FoodWrapper {
+mut:
+	food Food
+}
+
 fn test_match_aggregate() {
 	f := Food(Milk{'test'})
 	match union f {
@@ -410,6 +415,28 @@ fn test_if_not_mut() {
 	}
 }
 
+fn test_match_mut_selector() {
+	mut f := FoodWrapper{Food(Milk{'test'})}
+	match union mut f.food {
+		Eggs {
+			f.food.name = 'eggs'
+			assert f.food.name == 'eggs'
+		}
+		Milk {
+			f.food.name = 'milk'
+			assert f.food.name == 'milk'
+		}
+	}
+}
+
+fn test_if_mut_selector() {
+	mut f := FoodWrapper{Food(Milk{'test'})}
+	if mut f.food is Milk {
+		f.food.name = 'milk'
+		assert f.food.name == 'milk'
+	}
+}
+
 fn test_sum_type_match() {
 	// TODO: Remove these casts
 	assert is_gt_simple('3', int(2))