checker: suggest using `:=` when assigning to undefined variable (#5911)

vlib/v/checker/checker.v

@@ -2524,7 +2524,12 @@ pub fn (mut c Checker) ident(mut ident ast.Ident) table.Type {
 			}
 			ident.mod = saved_mod
 		}
-		c.error('undefined ident: `$ident.name`', ident.pos)
+		if ident.tok_kind == .assign {
+			c.error('undefined ident: `$ident.name` (use `:=` to assign a variable)',
+				ident.pos)
+		} else {
+			c.error('undefined ident: `$ident.name`', ident.pos)
+		}
 	}
 	if c.table.known_type(ident.name) {
 		// e.g. `User`  in `json.decode(User, '...')`

vlib/v/checker/tests/unknown_var_assign.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/unknown_var_assign.v:2:5: error: undefined ident: `x` (use `:=` to assign a variable)
+    1 | fn main() {
+    2 |     x = 'hello v'
+      |     ^
+    3 | }

vlib/v/checker/tests/unknown_var_assign.vv

@@ -0,0 +1,3 @@
+fn main() {
+    x = 'hello v'
+}

vlib/v/parser/parser.v

@@ -850,6 +850,7 @@ pub fn (mut p Parser) parse_ident(language table.Language) ast.Ident {
 		mut name := p.check_name()
 		if name == '_' {
 			return ast.Ident{
+				tok_kind: p.tok.kind
 				name: '_'
 				kind: .blank_ident
 				pos: pos
@@ -866,6 +867,7 @@ pub fn (mut p Parser) parse_ident(language table.Language) ast.Ident {
 			name = '${p.expr_mod}.$name'
 		}
 		return ast.Ident{
+			tok_kind: p.tok.kind
 			kind: .unresolved
 			name: name
 			language: language