checker - improve error for `as` casting on non-sum type (#6587)

vlib/v/checker/checker.v

@@ -2575,9 +2575,11 @@ pub fn (mut c Checker) expr(node ast.Expr) table.Type {
 					// c.error('only $info.variants can be casted to `$typ`', node.pos)
 				}
 			} else {
-				//
-				c.error('cannot cast non sum type `$type_sym.source_name` using `as`',
-					node.pos)
+				mut s := 'cannot cast non-sum type `$expr_type_sym.source_name` using `as`'
+				if type_sym.kind == .sum_type {
+					s += ' - use e.g. `${type_sym.source_name}(some_expr)` instead.'
+				}
+				c.error(s, node.pos)
 			}
 			return node.typ.to_ptr()
 			// return node.typ

vlib/v/checker/tests/sum.out

@@ -0,0 +1,27 @@
+vlib/v/checker/tests/sum.vv:5:8: error: cannot cast non-sum type `int` using `as`
+    3 | fn non_sum() {
+    4 |     v := 4
+    5 |     _ = v as rune
+      |           ~~
+    6 |     _ = v as Var
+    7 | }
+vlib/v/checker/tests/sum.vv:6:8: error: cannot cast non-sum type `int` using `as` - use e.g. `Var(some_expr)` instead.
+    4 |     v := 4
+    5 |     _ = v as rune
+    6 |     _ = v as Var
+      |           ~~
+    7 | }
+    8 |
+vlib/v/checker/tests/sum.vv:10:11: error: cannot cast `rune` to `Var`
+    8 |
+    9 | fn sum() {
+   10 |     _ := Var(`J`)
+      |              ~~~
+   11 |     mut s2 := Var('')
+   12 |     s2 = true
+vlib/v/checker/tests/sum.vv:12:7: error: cannot assign `bool` to `s2` of type `Var`
+   10 |     _ := Var(`J`)
+   11 |     mut s2 := Var('')
+   12 |     s2 = true
+      |          ~~~~
+   13 | }

vlib/v/checker/tests/sum.vv

@@ -0,0 +1,13 @@
+type Var = int | string
+
+fn non_sum() {
+	v := 4
+	_ = v as rune
+	_ = v as Var
+}
+
+fn sum() {
+	_ := Var(`J`)
+	mut s2 := Var('')
+	s2 = true
+}