cgen: fix `x.interface_field = value_implementing_interface` (closes #7620)

vlib/v/checker/checker.v

@@ -1720,7 +1720,8 @@ fn (mut c Checker) type_implements(typ table.Type, inter_typ table.Type, pos tok
 			}
 			continue
 		}
-		c.error("`$styp` doesn't implement method `$imethod.name`", pos)
+		c.error("`$styp` doesn't implement method `$imethod.name` of interface `$inter_sym.name`",
+			pos)
 	}
 	if mut inter_sym.info is table.Interface {
 		if typ !in inter_sym.info.types && typ_sym.kind != .interface_ {
@@ -2370,6 +2371,9 @@ pub fn (mut c Checker) assign_stmt(mut assign_stmt ast.AssignStmt) {
 				c.error('cannot assign to `$left`: $err', right.position())
 			}
 		}
+		if left_sym.kind == .interface_ {
+			c.type_implements(right_type, left_type, right.position())
+		}
 	}
 	// this needs to run after the assign stmt left exprs have been run through checker so that ident.obj is set
 	// Check `x := &y` and `mut x := <-ch`

vlib/v/checker/tests/is_type_invalid.out

@@ -5,7 +5,7 @@ vlib/v/checker/tests/is_type_invalid.vv:14:12: error: `IoS` has no variant `byte
       |               ~~
    15 |         println('not cool')
    16 |     }
-vlib/v/checker/tests/is_type_invalid.vv:18:2: error: `Cat` doesn't implement method `speak`
+vlib/v/checker/tests/is_type_invalid.vv:18:2: error: `Cat` doesn't implement method `speak` of interface `Animal`
    16 |     }
    17 |     a := Animal(Dog{})
    18 |     if a is Cat {

vlib/v/checker/tests/match_invalid_type.out

@@ -12,7 +12,7 @@ vlib/v/checker/tests/match_invalid_type.vv:4:2: error: match must be exhaustive
       |     ~~~~~~~~~~~~~~
     5 |         byte {
     6 |             println('not cool')
-vlib/v/checker/tests/match_invalid_type.vv:24:3: error: `Cat` doesn't implement method `speak`
+vlib/v/checker/tests/match_invalid_type.vv:24:3: error: `Cat` doesn't implement method `speak` of interface `Animal`
    22 |     a := Animal(Dog{})
    23 |     match a {
    24 |         Cat {

vlib/v/checker/tests/unimplemented_interface_a.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/unimplemented_interface_a.vv:10:6: error: `Cat` doesn't implement method `name`
+vlib/v/checker/tests/unimplemented_interface_a.vv:10:6: error: `Cat` doesn't implement method `name` of interface `Animal`
     8 | 
     9 | fn main() {
    10 |     foo(Cat{})

vlib/v/tests/interface_edge_cases/assign_to_interface_field_test.v

@@ -0,0 +1,28 @@
+interface Planet {
+	name() string
+}
+
+struct Moon {}
+
+fn (moon Moon) name() string {
+	return 'moon'
+}
+
+struct Mars {}
+
+fn (moon Mars) name() string {
+	return 'mars'
+}
+
+struct AnyPlanet {
+mut:
+	planet Planet
+}
+
+fn test_a_struct_implementing_an_interface_can_be_assigned_without_explicit_casts() {
+	mut anyplanet := AnyPlanet{}
+	anyplanet.planet = Moon{}
+	assert anyplanet.planet.name() == 'moon'
+	anyplanet.planet = Mars{}
+	assert anyplanet.planet.name() == 'mars'
+}