checker: check casting struct -> interface; disallow casting struct -> interface pointer (#8110)

vlib/v/checker/checker.v

@@ -3481,10 +3481,13 @@ pub fn (mut c Checker) cast_expr(mut node ast.CastExpr) table.Type {
 	} else if node.expr_type == table.none_type {
 		type_name := c.table.type_to_str(node.typ)
 		c.error('cannot cast `none` to `$type_name`', node.pos)
-	} else if from_type_sym.kind == .struct_ && !node.expr_type.is_ptr() && to_type_sym.kind !in
+	} else if from_type_sym.kind == .struct_ && !node.expr_type.is_ptr() {
-		[.sum_type, .interface_] && !c.is_builtin_mod {
+		if (node.typ.is_ptr() || to_type_sym.kind !in [.sum_type, .interface_]) && !c.is_builtin_mod {
-		type_name := c.table.type_to_str(node.typ)
+			type_name := c.table.type_to_str(node.typ)
-		c.error('cannot cast `struct` to `$type_name`', node.pos)
+			c.error('cannot cast struct to `$type_name`', node.pos)
+		} else if to_type_sym.kind == .interface_ {
+			c.type_implements(node.expr_type, node.typ, node.pos)
+		}
 	} else if node.expr_type.has_flag(.optional) || node.expr_type.has_flag(.variadic) {
 		// variadic case can happen when arrays are converted into variadic
 		msg := if node.expr_type.has_flag(.optional) { 'an optional' } else { 'a variadic' }

vlib/v/checker/tests/struct_type_cast_err.out

@@ -1,32 +1,32 @@
-vlib/v/checker/tests/struct_type_cast_err.vv:5:10: error: cannot cast `struct` to `string`
+vlib/v/checker/tests/struct_type_cast_err.vv:5:10: error: cannot cast struct to `string`
     3 | fn main() {
     4 |     foo := Foo{}
     5 |     _ := string(foo)
       |          ~~~~~~~~~~~
     6 |     _ := int(foo)
     7 |     _ := u64(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:6:10: error: cannot cast `struct` to `int`
+vlib/v/checker/tests/struct_type_cast_err.vv:6:10: error: cannot cast struct to `int`
     4 |     foo := Foo{}
     5 |     _ := string(foo)
     6 |     _ := int(foo)
       |          ~~~~~~~~
     7 |     _ := u64(foo)
     8 |     _ := u32(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:7:10: error: cannot cast `struct` to `u64`
+vlib/v/checker/tests/struct_type_cast_err.vv:7:10: error: cannot cast struct to `u64`
     5 |     _ := string(foo)
     6 |     _ := int(foo)
     7 |     _ := u64(foo)
       |          ~~~~~~~~
     8 |     _ := u32(foo)
     9 |     _ := rune(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:8:10: error: cannot cast `struct` to `u32`
+vlib/v/checker/tests/struct_type_cast_err.vv:8:10: error: cannot cast struct to `u32`
     6 |     _ := int(foo)
     7 |     _ := u64(foo)
     8 |     _ := u32(foo)
       |          ~~~~~~~~
     9 |     _ := rune(foo)
    10 |     _ := byte(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:9:10: error: cannot cast `struct` to `rune`
+vlib/v/checker/tests/struct_type_cast_err.vv:9:10: error: cannot cast struct to `rune`
     7 |     _ := u64(foo)
     8 |     _ := u32(foo)
     9 |     _ := rune(foo)
@@ -40,24 +40,31 @@ vlib/v/checker/tests/struct_type_cast_err.vv:10:10: error: cannot cast type `Foo
       |          ~~~~~~~~~
    11 |     _ := i8(foo)
    12 |     _ := i64(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:11:10: error: cannot cast `struct` to `i8`
+vlib/v/checker/tests/struct_type_cast_err.vv:11:10: error: cannot cast struct to `i8`
     9 |     _ := rune(foo)
    10 |     _ := byte(foo)
    11 |     _ := i8(foo)
       |          ~~~~~~~
    12 |     _ := i64(foo)
    13 |     _ := int(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:12:10: error: cannot cast `struct` to `i64`
+vlib/v/checker/tests/struct_type_cast_err.vv:12:10: error: cannot cast struct to `i64`
    10 |     _ := byte(foo)
    11 |     _ := i8(foo)
    12 |     _ := i64(foo)
       |          ~~~~~~~~
    13 |     _ := int(foo)
-   14 | }
+   14 |     _ = &I1(foo)
-vlib/v/checker/tests/struct_type_cast_err.vv:13:10: error: cannot cast `struct` to `int`
+vlib/v/checker/tests/struct_type_cast_err.vv:13:10: error: cannot cast struct to `int`
    11 |     _ := i8(foo)
    12 |     _ := i64(foo)
    13 |     _ := int(foo)
       |          ~~~~~~~~
-   14 | }
+   14 |     _ = &I1(foo)
-   15 |
+   15 | }
+vlib/v/checker/tests/struct_type_cast_err.vv:14:10: error: cannot cast struct to `&I1`
+   12 |     _ := i64(foo)
+   13 |     _ := int(foo)
+   14 |     _ = &I1(foo)
+      |          ~~~~~~~
+   15 | }
+   16 |

vlib/v/checker/tests/struct_type_cast_err.vv

@@ -11,5 +11,7 @@ fn main() {
     _ := i8(foo)
     _ := i64(foo)
     _ := int(foo)
+    _ = &I1(foo)
 }
 
+interface I1{}

vlib/v/checker/tests/unimplemented_interface_e.out

@@ -3,5 +3,13 @@ vlib/v/checker/tests/unimplemented_interface_e.vv:12:6: error: `Cat` incorrectly
    11 | fn main() {
    12 |     foo(Cat{})
       |         ~~~~~
-   13 | }
+   13 |     _ = Animal(Cat{})
+   14 | }
+details: main.Animal has `speak(s string)`
+vlib/v/checker/tests/unimplemented_interface_e.vv:13:6: error: `Cat` incorrectly implements method `speak` of interface `Animal`: expected `string`, not `&string` for parameter 1
+   11 | fn main() {
+   12 |     foo(Cat{})
+   13 |     _ = Animal(Cat{})
+      |         ~~~~~~~~~~~~~
+   14 | }
 details: main.Animal has `speak(s string)`

vlib/v/checker/tests/unimplemented_interface_e.vv

@@ -10,4 +10,5 @@ fn foo(a Animal) {}
 
 fn main() {
 	foo(Cat{})
+	_ = Animal(Cat{})
 }