parser: improve the position of mut receiver warning / error (#8240)

vlib/v/checker/tests/mut_receiver.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/mut_receiver.vv:5:4: warning: use `(mut f Foo)` instead of `(f mut Foo)`
+    3 |     name string
+    4 | }
+    5 | fn (f mut Foo) info() {
+      |    ~~~~~~~~~~~
+    6 |     f.name = 'foo'
+    7 | }
+vlib/v/checker/tests/mut_receiver.vv:8:4: error: use `(mut f Foo)` or `(f &Foo)` instead of `(mut f &Foo)`
+    6 |     f.name = 'foo'
+    7 | }
+    8 | fn (mut f &Foo) info2() {
+      |    ~~~~~~~~~~~~
+    9 |     f.name = 'foo'
+   10 | }

vlib/v/checker/tests/mut_receiver.vv

@@ -5,6 +5,9 @@ mut:
 fn (f mut Foo) info() {
 	f.name = 'foo'
 }
+fn (mut f &Foo) info2() {
+	f.name = 'foo'
+}
 fn main() {
 	println('hello, world')
 }

vlib/v/checker/tests/mut_receiver_warning.out

@@ -1,7 +0,0 @@
-vlib/v/checker/tests/mut_receiver_warning.vv:5:7: warning: use `(mut f Foo)` instead of `(f mut Foo)`
-    3 |     name string
-    4 | }
-    5 | fn (f mut Foo) info() {
-      |       ~~~
-    6 |     f.name = 'foo'
-    7 | }

vlib/v/parser/fn.v

@@ -195,6 +195,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	mut rec_mut := false
 	mut params := []table.Param{}
 	if p.tok.kind == .lpar {
+		lpar_pos := p.tok.position()
 		p.next() // (
 		is_method = true
 		is_shared := p.tok.kind == .key_shared
@@ -208,7 +209,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		if !rec_mut {
 			rec_mut = p.tok.kind == .key_mut
 			if rec_mut {
-				p.warn_with_pos('use `(mut f Foo)` instead of `(f mut Foo)`', p.tok.position())
+				p.warn_with_pos('use `(mut f Foo)` instead of `(f mut Foo)`', lpar_pos.extend(p.peek_tok2.position()))
 			}
 		}
 		receiver_pos = rec_start_pos.extend(p.tok.position())
@@ -231,7 +232,8 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		}
 		rec_type_pos = rec_type_pos.extend(p.prev_tok.position())
 		if is_amp && rec_mut {
-			p.error('use `(mut f Foo)` or `(f &Foo)` instead of `(mut f &Foo)`')
+			p.error_with_pos('use `(mut f Foo)` or `(f &Foo)` instead of `(mut f &Foo)`',
+				lpar_pos.extend(p.tok.position()))
 			return ast.FnDecl{
 				scope: 0
 			}