checker: check error of implementing other module private interface (fix

 #19620) (#19688)

vlib/crypto/cipher/cipher.v

@@ -6,7 +6,7 @@ module cipher
 // using a given key. It provides the capability to encrypt
 // or decrypt individual blocks. The mode implementations
 // extend that capability to streams of blocks.
-interface Block {
+pub interface Block {
 	block_size int // block_size returns the cipher's block size.
 	encrypt(mut dst []u8, src []u8) // Encrypt encrypts the first block in src into dst.
 	// Dst and src must overlap entirely or not at all.
@@ -15,7 +15,7 @@ interface Block {
 }
 
 // A Stream represents a stream cipher.
-interface Stream {
+pub interface Stream {
 	// xor_key_stream XORs each byte in the given slice with a byte from the
 	// cipher's key stream. Dst and src must overlap entirely or not at all.
 	//
@@ -31,7 +31,7 @@ interface Stream {
 
 // A BlockMode represents a block cipher running in a block-based mode (CBC,
 // ECB etc).
-interface BlockMode {
+pub interface BlockMode {
 	block_size int // block_size returns the mode's block size.
 	crypt_blocks(mut dst []u8, src []u8) // crypt_blocks encrypts or decrypts a number of blocks. The length of
 	// src must be a multiple of the block size. Dst and src must overlap

vlib/net/http/server.v

@@ -18,7 +18,7 @@ pub enum ServerStatus {
 	stopped
 }
 
-interface Handler {
+pub interface Handler {
 mut:
 	handle(Request) Response
 }

vlib/v/checker/checker.v

@@ -958,8 +958,14 @@ fn (mut c Checker) type_implements(typ ast.Type, interface_type ast.Type, pos to
 		eprintln('> type_implements typ: ${typ.debug()} (`${c.table.type_to_str(typ)}`) | inter_typ: ${interface_type.debug()} (`${c.table.type_to_str(interface_type)}`)')
 	}
 	utyp := c.unwrap_generic(typ)
+	styp := c.table.type_to_str(utyp)
 	typ_sym := c.table.sym(utyp)
 	mut inter_sym := c.table.sym(interface_type)
+	if inter_sym.mod !in [typ_sym.mod, c.mod] && !inter_sym.is_pub && typ_sym.mod != 'builtin' {
+		c.error('`${styp}` cannot implement private interface `${inter_sym.name}` of other module',
+			pos)
+		return false
+	}
 
 	// small hack for JS.Any type. Since `any` in regular V is getting deprecated we have our own JS.Any type for JS backend.
 	if typ_sym.name == 'JS.Any' {
@@ -1006,7 +1012,6 @@ fn (mut c Checker) type_implements(typ ast.Type, interface_type ast.Type, pos to
 		// `none` "implements" the Error interface
 		return true
 	}
-	styp := c.table.type_to_str(utyp)
 	if typ_sym.kind == .interface_ && inter_sym.kind == .interface_ && !styp.starts_with('JS.')
 		&& !inter_sym.name.starts_with('JS.') {
 		c.error('cannot implement interface `${inter_sym.name}` with a different interface `${styp}`',

vlib/v/checker/tests/modules/implement_private_interface.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/modules/implement_private_interface/main.v:7:16: error: `Bar` cannot implement private interface `baz.Foo` of other module
+    5 | fn main() {
+    6 |     b := Bar{10}
+    7 |     baz.print_foo(b) // prints 10
+      |                   ^
+    8 | }
+    9 |

vlib/v/checker/tests/modules/implement_private_interface/baz.v

@@ -0,0 +1,10 @@
+// sub module
+module baz
+
+interface Foo {
+	a int
+}
+
+pub fn print_foo(f Foo) {
+	println(f.a)
+}

vlib/v/checker/tests/modules/implement_private_interface/main.v

@@ -0,0 +1,12 @@
+module main
+
+import baz
+
+fn main() {
+	b := Bar{10}
+	baz.print_foo(b) // prints 10
+}
+
+struct Bar {
+	a int
+}

vlib/v/embed_file/decoder.v

@@ -1,7 +1,7 @@
 [has_globals]
 module embed_file
 
-interface Decoder {
+pub interface Decoder {
 	decompress([]u8) ![]u8
 }