parser, checker: fix var scope in lambda(fix #19860) (#19871)

vlib/v/checker/checker.v

@@ -79,6 +79,7 @@ pub mut:
 	inside_unsafe              bool // true inside `unsafe {}` blocks
 	inside_const               bool // true inside `const ( ... )` blocks
 	inside_anon_fn             bool // true inside `fn() { ... }()`
+	inside_lambda              bool // true inside `|...| ...`
 	inside_ref_lit             bool // true inside `a := &something`
 	inside_defer               bool // true inside `defer {}` blocks
 	inside_fn_arg              bool // `a`, `b` in `a.f(b)`
@@ -2799,6 +2800,10 @@ pub fn (mut c Checker) expr(mut node ast.Expr) ast.Type {
 			return c.int_lit(mut node)
 		}
 		ast.LambdaExpr {
+			c.inside_lambda = true
+			defer {
+				c.inside_lambda = false
+			}
 			return c.lambda_expr(mut node, c.expected_type)
 		}
 		ast.LockExpr {
@@ -3732,8 +3737,13 @@ fn (mut c Checker) ident(mut node ast.Ident) ast.Type {
 					found_var := c.fn_scope.find_var(node.name)
 
 					if found_var != none {
-						c.error('`${node.name}` must be added to the capture list for the closure to be used inside',
-							node.pos)
+						if c.inside_lambda {
+							// Lambdas don't support capturing variables yet, so that's the only hint.
+							c.error('undefined variable `${node.name}`', node.pos)
+						} else {
+							c.error('`${node.name}` must be added to the capture list for the closure to be used inside',
+								node.pos)
+						}
 						return ast.void_type
 					}
 				}

vlib/v/checker/tests/lambda_undefined_variables_err.out

@@ -0,0 +1,33 @@
+vlib/v/checker/tests/lambda_undefined_variables_err.vv:7:2: warning: unused variable: `s2`
+    5 |     f(|x| s1)
+    6 | 
+    7 |     s2 := 'abc'
+      |     ~~
+    8 |     f(|x| s2)
+    9 | }
+vlib/v/checker/tests/lambda_undefined_variables_err.vv:5:8: error: undefined ident: `s1`
+    3 | 
+    4 | fn main() {
+    5 |     f(|x| s1)
+      |           ~~
+    6 | 
+    7 |     s2 := 'abc'
+vlib/v/checker/tests/lambda_undefined_variables_err.vv:5:4: error: `s1` used as value
+    3 | 
+    4 | fn main() {
+    5 |     f(|x| s1)
+      |       ^
+    6 | 
+    7 |     s2 := 'abc'
+vlib/v/checker/tests/lambda_undefined_variables_err.vv:8:8: error: undefined variable `s2`
+    6 | 
+    7 |     s2 := 'abc'
+    8 |     f(|x| s2)
+      |           ~~
+    9 | }
+vlib/v/checker/tests/lambda_undefined_variables_err.vv:8:4: error: `s2` used as value
+    6 | 
+    7 |     s2 := 'abc'
+    8 |     f(|x| s2)
+      |       ^
+    9 | }

vlib/v/checker/tests/lambda_undefined_variables_err.vv

@@ -0,0 +1,9 @@
+fn f(g fn (int) string) {
+}
+
+fn main() {
+	f(|x| s1)
+
+	s2 := 'abc'
+	f(|x| s2)
+}

vlib/v/parser/expr.v

@@ -852,6 +852,7 @@ fn (mut p Parser) lambda_expr() ?ast.LambdaExpr {
 	defer {
 		p.close_scope()
 	}
+	p.scope.detached_from_parent = true
 
 	mut pos := p.tok.pos()
 	mut params := []ast.Ident{}