parser: add error for variables, that are evaluated, but not used

vlib/v/checker/tests/var_eval_not_used.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/var_eval_not_used.v:6:2: error: `c` evaluated but not used 
+    4 | 
+    5 | fn main() {
+    6 |     c
+      |     ^
+    7 | }

vlib/v/checker/tests/var_eval_not_used.vv

@@ -0,0 +1,7 @@
+const (
+	c = 1
+)
+
+fn main() {
+	c
+}

vlib/v/checker/tests/var_eval_not_used_scope.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/var_eval_not_used_scope.v:7:3: error: `c` evaluated but not used 
+    5 | fn main() {
+    6 |     {
+    7 |         c
+      |         ^
+    8 |     }
+    9 | }

vlib/v/checker/tests/var_eval_not_used_scope.vv

@@ -0,0 +1,9 @@
+const (
+	c = 1
+)
+
+fn main() {
+	{
+		c
+	}
+}

vlib/v/parser/fn.v

@@ -37,6 +37,7 @@ pub fn (mut p Parser) call_expr(is_c, is_js bool, mod string) ast.CallExpr {
 	mut or_stmts := []ast.Stmt{}
 	mut is_or_block_used := false
 	if p.tok.kind == .key_orelse {
+		p.inside_or_expr = true
 		p.next()
 		p.open_scope()
 		p.scope.register('err', ast.Var{
@@ -54,6 +55,7 @@ pub fn (mut p Parser) call_expr(is_c, is_js bool, mod string) ast.CallExpr {
 		is_or_block_used = true
 		or_stmts = p.parse_block_no_scope()
 		p.close_scope()
+		p.inside_or_expr = false
 	}
 	node := ast.CallExpr{
 		name: fn_name

vlib/v/parser/if.v

@@ -8,6 +8,8 @@ import v.table
 import v.token
 
 fn (mut p Parser) if_expr() ast.IfExpr {
+	p.inside_if_expr = true
+	defer { p.inside_if_expr = false }
 	pos := p.tok.position()
 	mut branches := []ast.IfBranch{}
 	mut has_else := false

vlib/v/parser/parser.v

@@ -26,6 +26,8 @@ mut:
 	is_c              bool
 	is_js             bool
 	inside_if         bool
+	inside_if_expr    bool
+	inside_or_expr    bool
 	inside_for        bool
 	inside_fn         bool
 	pref              &pref.Preferences
@@ -479,6 +481,8 @@ pub fn (mut p Parser) stmt() ast.Stmt {
 				}
 			} else if p.tok.kind == .name && p.peek_tok.kind == .name {
 				p.error_with_pos('unexpected name `$p.peek_tok.lit`', p.peek_tok.position())
+			} else if p.tok.kind == .name && !p.inside_if_expr && !p.inside_or_expr && p.peek_tok.kind in [.rcbr, .eof] {
+				p.error_with_pos('`$p.tok.lit` evaluated but not used', p.tok.position())
 			}
 			epos := p.tok.position()
 			expr := p.expr(0)