checker: prevent interface instantiation

vlib/v/checker/checker.v

@@ -255,6 +255,9 @@ pub fn (mut c Checker) struct_init(mut struct_init ast.StructInit) table.Type {
 		struct_init.typ = c.expected_type
 	}
 	type_sym := c.table.get_type_symbol(struct_init.typ)
+	if type_sym.kind == .interface_ {
+		c.error('cannot instantiate interface `$type_sym.name`', struct_init.pos)
+	}
 	if !type_sym.is_public && type_sym.kind != .placeholder && type_sym.mod != c.mod {
 		c.error('type `$type_sym.name` is private', struct_init.pos)
 	}

vlib/v/checker/tests/no_interface_instantiation_a.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/no_interface_instantiation_a.v:4:10: error: cannot instantiate interface `Speaker`
+    2 |
+    3 | fn main() {
+    4 |     _ := Speaker{}
+      |          ~~~~~~~~~
+    5 | }

vlib/v/checker/tests/no_interface_instantiation_a.vv

@@ -0,0 +1,5 @@
+interface Speaker {}
+
+fn main() {
+    _ := Speaker{}
+}

vlib/v/checker/tests/no_interface_instantiation_b.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/no_interface_instantiation_b.v:6:12: error: cannot instantiate interface `Speaker`
+    4 |
+    5 | fn main() {
+    6 |     my_fn({})
+      |            ^
+    7 | }

vlib/v/checker/tests/no_interface_instantiation_b.vv

@@ -0,0 +1,7 @@
+interface Speaker {}
+
+fn my_fn(s Speaker) {}
+
+fn main() {
+    my_fn({})
+}

vlib/v/checker/tests/no_interface_instantiation_c.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/no_interface_instantiation_c.v:9:9: error: cannot instantiate interface `Speaker`
+    7 | fn main() {
+    8 |     my_fn(
+    9 |         speak: 1
+      |         ~~~~~~~~
+   10 |     )
+   11 | }

vlib/v/checker/tests/no_interface_instantiation_c.vv

@@ -0,0 +1,11 @@
+interface Speaker {
+    speak()
+}
+
+fn my_fn(s Speaker) {}
+
+fn main() {
+    my_fn(
+        speak: 1
+    )
+}