tools.vet: add notice for empty strings conditions

cmd/tools/vvet/tests/empty_string.out

@@ -0,0 +1,8 @@
+cmd/tools/vvet/tests/empty_string.vv:3: notice: Use `foo == ''` instead of `foo.len == 0`
+cmd/tools/vvet/tests/empty_string.vv:4: notice: Use `foo != ''` instead of `foo.len != 0`
+cmd/tools/vvet/tests/empty_string.vv:5: notice: Use `foo == ''` instead of `foo.len < 1`
+cmd/tools/vvet/tests/empty_string.vv:6: notice: Use `foo != ''` instead of `foo.len > 0`
+cmd/tools/vvet/tests/empty_string.vv:9: notice: Use `'' == bar` instead of `0 == bar.len`
+cmd/tools/vvet/tests/empty_string.vv:10: notice: Use `'' != bar` instead of `0 != bar.len`
+cmd/tools/vvet/tests/empty_string.vv:11: notice: Use `'' == bar` instead of `1 > bar.len`
+cmd/tools/vvet/tests/empty_string.vv:12: notice: Use `'' != bar` instead of `0 < bar.len`

cmd/tools/vvet/tests/empty_string.vv

@@ -0,0 +1,13 @@
+fn main() {
+	foo := 'foo'
+	foo.len == 0
+	foo.len != 0
+	foo.len < 1
+	foo.len > 0
+
+	bar := 'bar'
+	0 == bar.len
+	0 != bar.len
+	1 > bar.len
+	0 < bar.len
+}

cmd/tools/vvet/vvet.v

@@ -290,6 +290,7 @@ fn (mut vt Vet) expr(expr ast.Expr) {
 			vt.filtered_lines.assigns(expr.pos)
 		}
 		ast.InfixExpr {
+			vt.vet_empty_str(expr)
 			vt.expr(expr.right)
 		}
 		ast.CallExpr {
@@ -310,6 +311,37 @@ fn (mut vt Vet) expr(expr ast.Expr) {
 	}
 }
 
+fn (mut vt Vet) vet_empty_str(expr ast.InfixExpr) {
+	if expr.left is ast.SelectorExpr && expr.right is ast.IntegerLiteral
+		&& expr.left.field_name == 'len' {
+		if expr.right.val == '0' && expr.op != .lt {
+			// TODO: remove as when values can be smart casted.
+			expr_str := (expr.left as ast.SelectorExpr).expr.str()
+			op := if expr.op == .gt { '!=' } else { expr.op.str() }
+			vt.notice("Use `${expr_str} ${op} ''` instead of `${expr_str}.len ${expr.op} 0`",
+				expr.pos.line_nr, .unknown)
+		} else if expr.right.val == '1' && expr.op in [.lt, .gt] {
+			expr_str := (expr.left as ast.SelectorExpr).expr.str()
+			op := if expr.op == .lt { '==' } else { '!=' }
+			vt.notice("Use `${expr_str} ${op} ''` instead of `${expr_str}.len ${expr.op} 1`",
+				expr.pos.line_nr, .unknown)
+		}
+	} else if expr.left is ast.IntegerLiteral && expr.right is ast.SelectorExpr
+		&& expr.right.field_name == 'len' {
+		if (expr.left as ast.IntegerLiteral).val == '0' && expr.op != .gt {
+			expr_str := expr.right.expr.str()
+			op := if expr.op == .lt { '!=' } else { expr.op.str() }
+			vt.notice("Use `'' ${op} ${expr_str}` instead of `0 ${expr.op} ${expr_str}.len`",
+				expr.pos.line_nr, .unknown)
+		} else if (expr.left as ast.IntegerLiteral).val == '1' && expr.op in [.lt, .gt] {
+			expr_str := expr.right.expr.str()
+			op := if expr.op == .lt { '!=' } else { '==' }
+			vt.notice("Use `'' ${op} ${expr_str}` instead of `1 ${expr.op} ${expr_str}.len`",
+				expr.pos.line_nr, .unknown)
+		}
+	}
+}
+
 fn (vt &Vet) vprintln(s string) {
 	if !vt.opt.is_verbose {
 		return