checker: add an interface check for mutability, fixes #1081, fixes #7038 (#11963)

.github/workflows/vinix-kernel.yml

@@ -20,7 +20,8 @@ jobs:
       - name: Build V
         run: make
       - name: Clone current Vinix
-        run: git clone https://github.com/vlang/vinix.git --depth=1
+        run: |
+          git clone https://github.com/vlang/vinix.git --depth 1
       - name: Clone current mlibc
         run: git clone https://github.com/managarm/mlibc.git --depth=1
       - name: Patch mlibc for Vinix

vlib/context/README.md

@@ -39,9 +39,9 @@ fn example_with_cancel() {
 	// The callers of gen need to cancel the context once
 	// they are done consuming generated integers not to leak
 	// the internal routine started by gen.
-	gen := fn (ctx context.Context) chan int {
+	gen := fn (mut ctx context.Context) chan int {
 		dst := chan int{}
-		go fn (ctx context.Context, dst chan int) {
+		go fn (mut ctx context.Context, dst chan int) {
 			mut v := 0
 			ch := ctx.done()
 			for {
@@ -55,16 +55,20 @@ fn example_with_cancel() {
 					}
 				}
 			}
-		}(ctx, dst)
+		}(mut ctx, dst)
 		return dst
 	}
 
-	ctx, cancel := context.with_cancel(context.background())
+	mut background := context.background()
+	mut b := &background
+	mut ctx, cancel := context.with_cancel(mut b)
 	defer {
 		cancel()
 	}
 
-	ch := gen(ctx)
+	mut mut_ctx := ctx
+	mut ctx2 := &mut_ctx
+	ch := gen(mut ctx2)
 	for i in 0 .. 5 {
 		v := <-ch
 		assert i == v
@@ -87,7 +91,9 @@ const (
 // function that it should abandon its work as soon as it gets to it.
 fn example_with_deadline() {
 	dur := time.now().add(short_duration)
-	ctx, cancel := context.with_deadline(context.background(), dur)
+	mut background := context.background()
+	mut b := &background
+	mut ctx, cancel := context.with_deadline(mut b, dur)
 
 	defer {
 		// Even though ctx will be expired, it is good practice to call its
@@ -122,7 +128,9 @@ const (
 fn example_with_timeout() {
 	// Pass a context with a timeout to tell a blocking function that it
 	// should abandon its work after the timeout elapses.
-	ctx, cancel := context.with_timeout(context.background(), short_duration)
+	mut background := context.background()
+	mut b := &background
+	mut ctx, cancel := context.with_timeout(mut b, short_duration)
 	defer {
 		cancel()
 	}

vlib/context/_context.v

@@ -43,23 +43,6 @@ pub interface Context {
 	// should be canceled. deadline returns none when no deadline is
 	// set. Successive calls to deadline return the same results.
 	deadline() ?time.Time
-	// done returns a channel that's closed when work done on behalf of this
-	// context should be canceled. done may return nil if this context can
-	// never be canceled. Successive calls to done return the same value.
-	// The close of the done channel may happen asynchronously,
-	// after the cancel function returns.
-	//
-	// with_cancel arranges for done to be closed when cancel is called;
-	// with_deadline arranges for done to be closed when the deadline
-	// expires; with_timeout arranges for done to be closed when the timeout
-	// elapses.
-	done() chan int
-	// If done is not yet closed, err returns nil.
-	// If done is closed, err returns a non-nil error explaining why:
-	// canceled if the context was canceled
-	// or deadline_exceeded if the context's deadline passed.
-	// After err returns a non-nil error, successive calls to err return the same error.
-	err() IError
 	// Value returns the value associated with this context for key, or nil
 	// if no value is associated with key. Successive calls to Value with
 	// the same key returns the same result.
@@ -76,6 +59,24 @@ pub interface Context {
 	// collisions.
 	value(key Key) ?Any
 	str() string
+	// done returns a channel that's closed when work done on behalf of this
+	// context should be canceled. done may return nil if this context can
+	// never be canceled. Successive calls to done return the same value.
+	// The close of the done channel may happen asynchronously,
+	// after the cancel function returns.
+	//
+	// with_cancel arranges for done to be closed when cancel is called;
+	// with_deadline arranges for done to be closed when the deadline
+	// expires; with_timeout arranges for done to be closed when the timeout
+	// elapses.
+mut:
+	done() chan int
+	// If done is not yet closed, err returns nil.
+	// If done is closed, err returns a non-nil error explaining why:
+	// canceled if the context was canceled
+	// or deadline_exceeded if the context's deadline passed.
+	// After err returns a non-nil error, successive calls to err return the same error.
+	err() IError
 }
 
 // background returns an empty Context. It is never canceled, has no

vlib/context/cancel.v

@@ -12,12 +12,13 @@ pub type CancelFn = fn ()
 
 pub interface Canceler {
 	id string
+mut:
 	cancel(remove_from_parent bool, err IError)
 	done() chan int
 }
 
 [deprecated]
-pub fn cancel(ctx Context) {
+pub fn cancel(mut ctx Context) {
 	match mut ctx {
 		CancelContext {
 			ctx.cancel(true, canceled)
@@ -47,9 +48,9 @@ mut:
 //
 // Canceling this context releases resources associated with it, so code should
 // call cancel as soon as the operations running in this Context complete.
-pub fn with_cancel(parent Context) (Context, CancelFn) {
+pub fn with_cancel(mut parent Context) (Context, CancelFn) {
 	mut c := new_cancel_context(parent)
-	propagate_cancel(parent, c)
+	propagate_cancel(mut parent, mut c)
 	return Context(c), fn [mut c] () {
 		c.cancel(true, canceled)
 	}
@@ -116,18 +117,20 @@ fn (mut ctx CancelContext) cancel(remove_from_parent bool, err IError) {
 
 	for _, child in ctx.children {
 		// NOTE: acquiring the child's lock while holding parent's lock.
-		child.cancel(false, err)
+		mut c := child
+		c.cancel(false, err)
 	}
 
 	ctx.children = map[string]Canceler{}
 	ctx.mutex.unlock()
 
 	if remove_from_parent {
-		remove_child(ctx.context, ctx)
+		mut cc := &ctx.context
+		remove_child(mut cc, ctx)
 	}
 }
 
-fn propagate_cancel(parent Context, child Canceler) {
+fn propagate_cancel(mut parent Context, mut child Canceler) {
 	done := parent.done()
 	select {
 		_ := <-done {
@@ -136,15 +139,15 @@ fn propagate_cancel(parent Context, child Canceler) {
 			return
 		}
 	}
-	mut p := parent_cancel_context(parent) or {
-		go fn (parent Context, child Canceler) {
+	mut p := parent_cancel_context(mut parent) or {
+		go fn (mut parent Context, mut child Canceler) {
 			pdone := parent.done()
 			select {
 				_ := <-pdone {
 					child.cancel(false, parent.err())
 				}
 			}
-		}(parent, child)
+		}(mut parent, mut child)
 		return
 	}
 
@@ -162,7 +165,7 @@ fn propagate_cancel(parent Context, child Canceler) {
 // parent.done() matches that CancelContext. (If not, the CancelContext
 // has been wrapped in a custom implementation providing a
 // different done channel, in which case we should not bypass it.)
-fn parent_cancel_context(parent Context) ?&CancelContext {
+fn parent_cancel_context(mut parent Context) ?&CancelContext {
 	done := parent.done()
 	if done.closed {
 		return none
@@ -181,7 +184,7 @@ fn parent_cancel_context(parent Context) ?&CancelContext {
 }
 
 // remove_child removes a context from its parent.
-fn remove_child(parent Context, child Canceler) {
-	mut p := parent_cancel_context(parent) or { return }
+fn remove_child(mut parent Context, child Canceler) {
+	mut p := parent_cancel_context(mut parent) or { return }
 	p.children.delete(child.id)
 }

vlib/context/cancel_test.v

@@ -10,9 +10,9 @@ fn test_with_cancel() {
 	// The callers of gen need to cancel the context once
 	// they are done consuming generated integers not to leak
 	// the internal routine started by gen.
-	gen := fn (ctx context.Context) chan int {
+	gen := fn (mut ctx context.Context) chan int {
 		dst := chan int{}
-		go fn (ctx context.Context, dst chan int) {
+		go fn (mut ctx context.Context, dst chan int) {
 			mut v := 0
 			ch := ctx.done()
 			for {
@@ -26,16 +26,20 @@ fn test_with_cancel() {
 					}
 				}
 			}
-		}(ctx, dst)
+		}(mut ctx, dst)
 		return dst
 	}
 
-	ctx, cancel := context.with_cancel(context.background())
+	mut background := context.background()
+	mut b := &background
+	mut ctx, cancel := context.with_cancel(mut b)
 	defer {
 		cancel()
 	}
 
-	ch := gen(ctx)
+	mut mut_ctx := ctx
+	mut ctx2 := &mut_ctx
+	ch := gen(mut ctx2)
 	for i in 0 .. 5 {
 		v := <-ch
 		assert i == v

vlib/context/deadline.v

@@ -26,12 +26,12 @@ mut:
 //
 // Canceling this context releases resources associated with it, so code should
 // call cancel as soon as the operations running in this Context complete.
-pub fn with_deadline(parent Context, d time.Time) (Context, CancelFn) {
+pub fn with_deadline(mut parent Context, d time.Time) (Context, CancelFn) {
 	id := rand.uuid_v4()
 	if cur := parent.deadline() {
 		if cur < d {
 			// The current deadline is already sooner than the new one.
-			return with_cancel(parent)
+			return with_cancel(mut parent)
 		}
 	}
 	cancel_ctx := new_cancel_context(parent)
@@ -40,7 +40,7 @@ pub fn with_deadline(parent Context, d time.Time) (Context, CancelFn) {
 		deadline: d
 		id: id
 	}
-	propagate_cancel(parent, ctx)
+	propagate_cancel(mut parent, mut ctx)
 	dur := d - time.now()
 	if dur.nanoseconds() <= 0 {
 		ctx.cancel(true, deadline_exceeded) // deadline has already passed
@@ -64,8 +64,8 @@ pub fn with_deadline(parent Context, d time.Time) (Context, CancelFn) {
 //
 // Canceling this context releases resources associated with it, so code should
 // call cancel as soon as the operations running in this Context complete
-pub fn with_timeout(parent Context, timeout time.Duration) (Context, CancelFn) {
-	return with_deadline(parent, time.now().add(timeout))
+pub fn with_timeout(mut parent Context, timeout time.Duration) (Context, CancelFn) {
+	return with_deadline(mut parent, time.now().add(timeout))
 }
 
 pub fn (ctx &TimerContext) deadline() ?time.Time {
@@ -88,7 +88,8 @@ pub fn (mut ctx TimerContext) cancel(remove_from_parent bool, err IError) {
 	ctx.cancel_ctx.cancel(false, err)
 	if remove_from_parent {
 		// Remove this TimerContext from its parent CancelContext's children.
-		remove_child(ctx.cancel_ctx.context, ctx)
+		mut cc := &ctx.cancel_ctx.context
+		remove_child(mut cc, ctx)
 	}
 }
 

vlib/context/deadline_test.v

@@ -10,7 +10,9 @@ const (
 // function that it should abandon its work as soon as it gets to it.
 fn test_with_deadline() {
 	dur := time.now().add(short_duration)
-	ctx, cancel := context.with_deadline(context.background(), dur)
+	mut background := context.background()
+	mut b := &background
+	mut ctx, cancel := context.with_deadline(mut b, dur)
 
 	defer {
 		// Even though ctx will be expired, it is good practice to call its
@@ -33,7 +35,9 @@ fn test_with_deadline() {
 fn test_with_timeout() {
 	// Pass a context with a timeout to tell a blocking function that it
 	// should abandon its work after the timeout elapses.
-	ctx, cancel := context.with_timeout(context.background(), short_duration)
+	mut background := context.background()
+	mut b := &background
+	mut ctx, cancel := context.with_timeout(mut b, short_duration)
 	defer {
 		cancel()
 	}

vlib/context/value.v

@@ -37,11 +37,11 @@ pub fn (ctx &ValueContext) deadline() ?time.Time {
 	return ctx.context.deadline()
 }
 
-pub fn (ctx &ValueContext) done() chan int {
+pub fn (mut ctx ValueContext) done() chan int {
 	return ctx.context.done()
 }
 
-pub fn (ctx &ValueContext) err() IError {
+pub fn (mut ctx ValueContext) err() IError {
 	return ctx.context.err()
 }
 

vlib/io/io.v

@@ -4,7 +4,7 @@ const (
 	buf_max_len = 1024
 )
 
-pub fn cp(src Reader, mut dst Writer) ? {
+pub fn cp(mut src Reader, mut dst Writer) ? {
 	mut buf := []byte{len: io.buf_max_len}
 	for {
 		len := src.read(mut buf) or { break }

vlib/io/io_cp_test.v

@@ -8,6 +8,6 @@ fn test_cp() ? {
 	}
 	mut r := io.new_buffered_reader(reader: f)
 	mut stdout := os.stdout()
-	io.cp(r, mut stdout) ?
+	io.cp(mut r, mut stdout) ?
 	assert true
 }

vlib/io/io_test.v

@@ -30,12 +30,12 @@ fn (mut w Writ) write(buf []byte) ?int {
 }
 
 fn test_copy() {
-	src := Buf{
+	mut src := Buf{
 		bytes: 'abcdefghij'.repeat(10).bytes()
 	}
 	mut dst := Writ{
 		bytes: []byte{}
 	}
-	io.cp(src, mut dst) or { assert false }
+	io.cp(mut src, mut dst) or { assert false }
 	assert dst.bytes == src.bytes
 }

vlib/io/reader.v

@@ -6,6 +6,7 @@ pub interface Reader {
 	// them into buf.
 	// A type that implements this should return
 	// `none` on end of stream (EOF) instead of just returning 0
+mut:
 	read(mut buf []byte) ?int
 }
 
@@ -17,14 +18,15 @@ const (
 // ReadAllConfig allows options to be passed for the behaviour
 // of read_all
 pub struct ReadAllConfig {
-	reader                Reader
 	read_to_end_of_stream bool
+mut:
+	reader Reader
 }
 
 // read_all reads all bytes from a reader until either a 0 length read
 // or if read_to_end_of_stream is true then the end of the stream (`none`)
 pub fn read_all(config ReadAllConfig) ?[]byte {
-	r := config.reader
+	mut r := config.reader
 	read_till_eof := config.read_to_end_of_stream
 
 	mut b := []byte{len: io.read_all_len}
@@ -44,7 +46,7 @@ pub fn read_all(config ReadAllConfig) ?[]byte {
 
 // read_any reads any available bytes from a reader
 // (until the reader returns a read of 0 length)
-pub fn read_any(r Reader) ?[]byte {
+pub fn read_any(mut r Reader) ?[]byte {
 	mut b := []byte{len: io.read_all_len}
 	mut read := 0
 	for {

vlib/io/readerwriter.v

@@ -3,6 +3,7 @@ module io
 // ReaderWriter represents a stream that can be read from and wrote to
 pub interface ReaderWriter {
 	// from Reader
+mut:
 	read(mut buf []byte) ?int
 	// from Writer
 	write(buf []byte) ?int
@@ -11,8 +12,8 @@ pub interface ReaderWriter {
 // ReaderWriterImpl is a ReaderWriter that can be made from
 // a seperate reader and writer (see fn make_readerwriter)
 struct ReaderWriterImpl {
-	r Reader
 mut:
+	r Reader
 	w Writer
 }
 

vlib/io/writer.v

@@ -2,6 +2,7 @@ module io
 
 // Writer represents a stream of data that can be wrote to
 pub interface Writer {
+mut:
 	write(buf []byte) ?int
 }
 

vlib/net/http/server.v

@@ -18,6 +18,7 @@ pub enum ServerStatus {
 }
 
 interface Handler {
+mut:
 	handle(Request) Response
 }
 
@@ -80,7 +81,7 @@ pub fn (s &Server) status() ServerStatus {
 	return s.state
 }
 
-fn (s &Server) parse_and_respond(mut conn net.TcpConn) {
+fn (mut s Server) parse_and_respond(mut conn net.TcpConn) {
 	defer {
 		conn.close() or { eprintln('close() failed: $err') }
 	}

vlib/net/tcp.v

@@ -48,7 +48,7 @@ pub fn (mut c TcpConn) close() ? {
 	c.sock.close() ?
 }
 
-pub fn (mut c TcpConn) read_ptr(buf_ptr &byte, len int) ?int {
+pub fn (c TcpConn) read_ptr(buf_ptr &byte, len int) ?int {
 	mut res := wrap_read_result(C.recv(c.sock.handle, voidptr(buf_ptr), len, 0)) ?
 	$if trace_tcp ? {
 		eprintln('<<< TcpConn.read_ptr  | c.sock.handle: $c.sock.handle | buf_ptr: ${ptr_str(buf_ptr)} len: $len | res: $res')
@@ -80,7 +80,7 @@ pub fn (mut c TcpConn) read_ptr(buf_ptr &byte, len int) ?int {
 	return none
 }
 
-pub fn (mut c TcpConn) read(mut buf []byte) ?int {
+pub fn (c TcpConn) read(mut buf []byte) ?int {
 	return c.read_ptr(buf.data, buf.len)
 }
 
@@ -169,7 +169,7 @@ pub fn (mut c TcpConn) set_write_timeout(t time.Duration) {
 }
 
 [inline]
-pub fn (mut c TcpConn) wait_for_read() ? {
+pub fn (c TcpConn) wait_for_read() ? {
 	return wait_for_read(c.sock.handle, c.read_deadline, c.read_timeout)
 }
 

vlib/os/notify/notify.v

@@ -4,6 +4,7 @@ import time
 
 // Backends should provide a `new() ?FdNotifier` function
 pub interface FdNotifier {
+mut:
 	add(fd int, events FdEventType, conf ...FdConfigFlags) ?
 	modify(fd int, events FdEventType, conf ...FdConfigFlags) ?
 	remove(fd int) ?

vlib/os/notify/notify_test.v

@@ -27,8 +27,9 @@ fn test_level_trigger() ? {
 		notifier.add(reader, .read) ?
 
 		os.fd_write(writer, 'foobar')
-		check_read_event(notifier, reader, 'foo')
-		check_read_event(notifier, reader, 'bar')
+		mut n := &notifier
+		check_read_event(mut n, reader, 'foo')
+		check_read_event(mut n, reader, 'bar')
 
 		assert notifier.wait(0).len == 0
 	}
@@ -46,8 +47,10 @@ fn test_edge_trigger() ? {
 		}
 		notifier.add(reader, .read, .edge_trigger) ?
 
+		mut n := &notifier
+
 		os.fd_write(writer, 'foobar')
-		check_read_event(notifier, reader, 'foo')
+		check_read_event(mut n, reader, 'foo')
 
 		assert notifier.wait(0).len == 0
 
@@ -71,15 +74,17 @@ fn test_one_shot() ? {
 		}
 		notifier.add(reader, .read, .one_shot) ?
 
+		mut n := &notifier
+
 		os.fd_write(writer, 'foobar')
-		check_read_event(notifier, reader, 'foo')
+		check_read_event(mut n, reader, 'foo')
 		os.fd_write(writer, 'baz')
 
 		assert notifier.wait(0).len == 0
 
 		// rearm
 		notifier.modify(reader, .read) ?
-		check_read_event(notifier, reader, 'barbaz')
+		check_read_event(mut n, reader, 'barbaz')
 	}
 }
 
@@ -148,7 +153,7 @@ fn test_remove() ? {
 	}
 }
 
-fn check_read_event(notifier notify.FdNotifier, reader_fd int, expected string) {
+fn check_read_event(mut notifier notify.FdNotifier, reader_fd int, expected string) {
 	events := notifier.wait(0)
 	assert events.len == 1
 	assert events[0].fd == reader_fd

vlib/pg/pg.v

@@ -216,7 +216,7 @@ fn (db DB) handle_error_or_result(res voidptr, elabel string) ?[]Row {
 
 // copy_expert execute COPY commands
 // https://www.postgresql.org/docs/9.5/libpq-copy.html
-pub fn (db DB) copy_expert(query string, file io.ReaderWriter) ?int {
+pub fn (db DB) copy_expert(query string, mut file io.ReaderWriter) ?int {
 	res := C.PQexec(db.conn, query.str)
 	status := C.PQresultStatus(res)
 

vlib/rand/rand.v

@@ -10,6 +10,7 @@ import rand.wyrand
 // modules's API. It defines all the methods that a PRNG (in the vlib or custom made) must
 // implement in order to ensure that _all_ functions can be used with the generator.
 pub interface PRNG {
+mut:
 	seed(seed_data []u32)
 	u32() u32
 	u64() u64

vlib/v/ast/table.v

@@ -260,11 +260,23 @@ pub fn (t &Table) is_same_method(f &Fn, func &Fn) string {
 	if f.params.len != func.params.len {
 		return 'expected $f.params.len parameter(s), not $func.params.len'
 	}
-	for i in 1 .. f.params.len {
-		if f.params[i].typ != func.params[i].typ {
+
+	// interface name() other mut name() : error
+
+	for i in 0 .. f.params.len {
+		// don't check receiver for `.typ`
+		has_unexpected_type := i > 0 && f.params[i].typ != func.params[i].typ
+
+		has_unexpected_mutability := !f.params[i].is_mut && func.params[i].is_mut
+
+		if has_unexpected_type || has_unexpected_mutability {
 			exps := t.type_to_str(f.params[i].typ)
 			gots := t.type_to_str(func.params[i].typ)
-			return 'expected `$exps`, not `$gots` for parameter $i'
+			if has_unexpected_type {
+				return 'expected `$exps`, not `$gots` for parameter $i'
+			} else {
+				return 'expected `$exps` which is immutable, not `mut $gots`'
+			}
 		}
 	}
 	return ''

vlib/v/ast/walker/walker.v

@@ -4,6 +4,7 @@ import v.ast
 
 // Visitor defines a visit method which is invoked by the walker in each node it encounters.
 pub interface Visitor {
+mut:
 	visit(node &ast.Node) ?
 }
 
@@ -24,14 +25,15 @@ pub fn (i &Inspector) visit(node &ast.Node) ? {
 
 // inspect traverses and checks the AST node on a depth-first order and based on the data given
 pub fn inspect(node &ast.Node, data voidptr, inspector_callback InspectorFn) {
-	walk(Inspector{inspector_callback, data}, node)
+	mut inspector := Inspector{inspector_callback, data}
+	walk(mut inspector, node)
 }
 
 // walk traverses the AST using the given visitor
-pub fn walk(visitor Visitor, node &ast.Node) {
+pub fn walk(mut visitor Visitor, node &ast.Node) {
 	visitor.visit(node) or { return }
 	children := node.children()
 	for child_node in children {
-		walk(visitor, &child_node)
+		walk(mut visitor, &child_node)
 	}
 }

vlib/v/ast/walker/walker_test.v

@@ -35,7 +35,7 @@ struct Foo {
 	mut nbo := NodeByOffset{
 		pos: 13
 	}
-	walker.walk(nbo, file)
+	walker.walk(mut nbo, file)
 	assert nbo.node is ast.Stmt
 	stmt := nbo.node as ast.Stmt
 	assert stmt is ast.StructDecl

vlib/v/callgraph/callgraph.v

@@ -16,7 +16,7 @@ pub fn show(mut table ast.Table, pref &pref.Preferences, ast_files []&ast.File)
 	// Node14 [shape="box",label="PrivateBase",URL="$classPrivateBase.html"];
 	// Node15 -> Node9 [dir=back,color="midnightblue",fontsize=10,style="solid"];
 	for afile in ast_files {
-		walker.walk(mapper, afile)
+		walker.walk(mut mapper, afile)
 	}
 	mapper.dg.finish()
 }

vlib/v/checker/checker.v

@@ -3355,8 +3355,10 @@ fn (mut c Checker) type_implements(typ ast.Type, interface_type ast.Type, pos to
 			}
 			msg := c.table.is_same_method(imethod, method)
 			if msg.len > 0 {
-				sig := c.table.fn_signature(imethod, skip_receiver: true)
+				sig := c.table.fn_signature(imethod, skip_receiver: false)
+				typ_sig := c.table.fn_signature(method, skip_receiver: false)
 				c.add_error_detail('$inter_sym.name has `$sig`')
+				c.add_error_detail('         $typ_sym.name has `$typ_sig`')
 				c.error('`$styp` incorrectly implements method `$imethod.name` of interface `$inter_sym.name`: $msg',
 					pos)
 				return false

vlib/v/checker/tests/unimplemented_interface_b.out

@@ -4,4 +4,5 @@ vlib/v/checker/tests/unimplemented_interface_b.vv:13:6: error: `Cat` incorrectly
    13 |     foo(c)
       |         ^
    14 | }
-Details: main.Animal has `name() string`
+Details: main.Animal has `fn name(x main.Animal) string`
+         main.Cat has `fn name(c main.Cat)`

vlib/v/checker/tests/unimplemented_interface_c.out

@@ -4,4 +4,5 @@ vlib/v/checker/tests/unimplemented_interface_c.vv:12:6: error: `Cat` incorrectly
    12 |     foo(Cat{})
       |         ~~~~~
    13 | }
-Details: main.Animal has `name()`
+Details: main.Animal has `fn name(x main.Animal)`
+         main.Cat has `fn name(c main.Cat, s string)`

vlib/v/checker/tests/unimplemented_interface_d.out

@@ -4,4 +4,5 @@ vlib/v/checker/tests/unimplemented_interface_d.vv:12:6: error: `Cat` incorrectly
    12 |     foo(Cat{})
       |         ~~~~~
    13 | }
-Details: main.Animal has `speak(s string)`
+Details: main.Animal has `fn speak(x main.Animal, s string)`
+         main.Cat has `fn speak(c main.Cat)`

vlib/v/checker/tests/unimplemented_interface_e.out

@@ -5,11 +5,13 @@ vlib/v/checker/tests/unimplemented_interface_e.vv:12:6: error: `Cat` incorrectly
       |         ~~~~~
    13 |     _ = Animal(Cat{})
    14 | }
-Details: main.Animal has `speak(s string)`
+Details: main.Animal has `fn speak(x main.Animal, s string)`
+         main.Cat has `fn speak(c main.Cat, 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)`
+Details: main.Animal has `fn speak(x main.Animal, s string)`
+         main.Cat has `fn speak(c main.Cat, s &string)`

vlib/v/checker/tests/unimplemented_interface_f.out

@@ -4,4 +4,5 @@ vlib/v/checker/tests/unimplemented_interface_f.vv:11:13: error: `Cat` incorrectl
    11 |     animals << Cat{}
       |                ~~~~~
    12 | }
-Details: main.Animal has `speak(s string)`
+Details: main.Animal has `fn speak(x main.Animal, s string)`
+         main.Cat has `fn speak(c main.Cat)`

vlib/v/parser/tests/interface_mutability_receiver.out

@@ -0,0 +1,9 @@
+vlib/v/parser/tests/interface_mutability_receiver.vv:23:18: error: `Doggo` incorrectly implements method `set_name` of interface `Animal`: expected `Animal` which is immutable, not `mut &Doggo`
+   21 |     dog := Doggo{'Doggo'}
+   22 |     println(dog.name)
+   23 |     set_animal_name(dog, 'Pupper')
+      |                     ~~~
+   24 |     println(dog.name)
+   25 | }
+Details: main.Animal has `fn set_name(x main.Animal, name string)`
+         main.Doggo has `fn set_name(mut d main.Doggo, name string)`

vlib/v/parser/tests/interface_mutability_receiver.vv

@@ -0,0 +1,25 @@
+// fixes https://github.com/vlang/v/issues/1081 and https://github.com/vlang/v/issues/7338, code by https://github.com/nedpals
+//   copied from https://github.com/vlang/v/issues/7338 
+struct Doggo {
+pub mut:
+	name string
+}
+
+fn (mut d Doggo) set_name(name string) {
+	d.name = name
+}
+
+interface Animal {
+	set_name(name string)
+}
+
+fn set_animal_name(a Animal, name string) {
+	a.set_name(name)
+}
+
+fn main() {
+	dog := Doggo{'Doggo'}
+	println(dog.name)
+	set_animal_name(dog, 'Pupper')
+	println(dog.name)
+}

vlib/v/tests/generic_interface_test.v

@@ -66,6 +66,7 @@ fn test_extract_basic() {
 
 //////
 interface Iterator<T> {
+mut:
 	next() ?T
 }
 

vlib/v/tests/generics_fn_return_generic_interface_test.v

@@ -1,4 +1,5 @@
 interface Iter<T> {
+mut:
 	next() ?T
 }
 
@@ -24,7 +25,7 @@ fn iter<T>(arr []T) Iter<T> {
 }
 
 fn test_generics_fn_return_generic_interface() {
-	x := iter([1, 2, 3])
+	mut x := iter([1, 2, 3])
 	println(x)
 	y := x.next() or { 0 }
 	println(y)

vlib/v/tests/generics_interface_method_test.v

@@ -1,4 +1,5 @@
 interface Iter<T> {
+mut:
 	next() ?T
 }
 

vlib/v/tests/generics_interface_with_multi_generic_structs_test.v

@@ -1,4 +1,5 @@
 interface Iter<T> {
+mut:
 	next() ?T
 }
 
@@ -39,7 +40,7 @@ fn (mut it SkipIter<T>) next<T>() ?T {
 }
 
 fn test_generics_interface_with_multi_generic_structs() {
-	x := Iter<int>(ArrIter<int>{
+	mut x := Iter<int>(ArrIter<int>{
 		data: [1, 2, 3]
 	})
 	println(x)

vlib/v/tests/generics_interface_with_multi_generic_types_test.v

@@ -1,4 +1,5 @@
 interface Iter<T, U> {
+mut:
 	next() ?(T, U)
 }
 

vlib/v/tests/interface_embedding_test.v

@@ -9,11 +9,13 @@ interface WalkerTalker {
 
 interface Talker {
 	nspeeches int
+mut:
 	talk(msg string)
 }
 
 interface Walker {
 	nsteps int
+mut:
 	walk(newx int, newy int)
 }
 

vlib/v/tests/interface_test.v

@@ -196,6 +196,7 @@ interface Speaker2 {
 	name() string
 	speak()
 	return_speaker() Speaker2
+mut:
 	return_speaker2() ?Speaker2
 }
 
@@ -250,6 +251,7 @@ interface Animal {
 	name() string
 	name_detailed(pet_name string) string
 	speak(s string)
+mut:
 	set_breed(s string)
 }
 
@@ -327,6 +329,31 @@ mut:
 	my_field int
 }
 
+// the opposite example of interface_mutability_receiver.vv
+// 	 related to https://github.com/vlang/v/issues/1081 and https://github.com/vlang/v/issues/7338
+//   test example code by https://github.com/nedpals copied and adapted from https://github.com/vlang/v/issues/7338
+// we accept immutable get_name even if the interface specified `mut` as it is consistent with function param behavior
+struct Dog2 {
+pub mut:
+	name string
+}
+
+fn (d Dog2) get_name() string {
+	return d.name
+}
+
+// mut get_name might be a bad example
+// we try to show that an interface can be more liberal in mut than the implementor,
+// while our error check is for the opposite case
+interface Animal2 {
+mut:
+	get_name() string
+}
+
+fn get_animal_name(mut a Animal2) string {
+	return a.get_name()
+}
+
 fn main() {
 	mut aa := AA{}
 	mut ii := II(aa)
@@ -335,4 +362,7 @@ fn main() {
 	assert ii.my_field == 123
 	ii.my_field = 1234
 	assert aa.my_field == 1234
+	mut dog := Dog2{'Doggo'}
+	println(dog.name)
+	println(get_animal_name(mut dog))
 }