checker: check expr evaluated but not used (fix #21436) (#21816)

vlib/v/checker/checker.v

@@ -2219,6 +2219,12 @@ fn (mut c Checker) block(mut node ast.Block) {
 		c.stmts(mut node.stmts)
 		c.inside_unsafe = prev_unsafe
 	} else {
+		if node.stmts.len > 0 && node.stmts.last() is ast.ExprStmt {
+			last_stmt := node.stmts.last() as ast.ExprStmt
+			if last_stmt.expr !in [ast.CallExpr, ast.IfExpr, ast.MatchExpr, ast.InfixExpr] {
+				c.warn('expression evaluated but not used', node.stmts.last().pos)
+			}
+		}
 		c.stmts(mut node.stmts)
 	}
 }

vlib/v/checker/tests/expr_evaluated_but_not_used.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/expr_evaluated_but_not_used.vv:7:3: warning: expression evaluated but not used
+    5 |         println(3)
+    6 |         println(4)
+    7 |         5
+      |         ^
+    8 |     }
+    9 |     println('works')

vlib/v/checker/tests/expr_evaluated_but_not_used.vv

@@ -0,0 +1,10 @@
+fn main() {
+	{
+		println(1)
+		println(2)
+		println(3)
+		println(4)
+		5
+	}
+	println('works')
+}