3 use syntax::codemap::{Span, Spanned};
4 use syntax::visit::FnKind;
6 use utils::{span_note_and_lint, span_lint_and_then, snippet_opt, match_path_ast, in_macro,
9 /// **What it does:** Checks for return statements at the end of a block.
11 /// **Why is this bad?** Removing the `return` and semicolon will make the code
14 /// **Known problems:** None.
18 /// fn foo(x: usize) { return x; }
23 "using a return statement like `return expr;` where an expression would suffice"
26 /// **What it does:** Checks for `let`-bindings, which are subsequently returned.
28 /// **Why is this bad?** It is just extraneous code. Remove it to make your code
31 /// **Known problems:** None.
40 "creating a let-binding and then immediately returning it like `let x = expr; x` at \
44 #[derive(Copy, Clone)]
45 pub struct ReturnPass;
48 // Check the final stmt or expr in a block for unnecessary return.
49 fn check_block_return(&mut self, cx: &EarlyContext, block: &ast::Block) {
50 if let Some(stmt) = block.stmts.last() {
52 ast::StmtKind::Expr(ref expr) |
53 ast::StmtKind::Semi(ref expr) => {
54 self.check_final_expr(cx, expr, Some(stmt.span));
61 // Check a the final expression in a block if it's a return.
62 fn check_final_expr(&mut self, cx: &EarlyContext, expr: &ast::Expr, span: Option<Span>) {
64 // simple return is always "bad"
65 ast::ExprKind::Ret(Some(ref inner)) => {
66 // allow `#[cfg(a)] return a; #[cfg(b)] return b;`
67 if !expr.attrs.iter().any(attr_is_cfg) {
68 self.emit_return_lint(cx, span.expect("`else return` is not possible"), inner.span);
71 // a whole block? check it!
72 ast::ExprKind::Block(ref block) => {
73 self.check_block_return(cx, block);
75 // an if/if let expr, check both exprs
76 // note, if without else is going to be a type checking error anyways
77 // (except for unit type functions) so we don't match it
78 ast::ExprKind::If(_, ref ifblock, Some(ref elsexpr)) => {
79 self.check_block_return(cx, ifblock);
80 self.check_final_expr(cx, elsexpr, None);
82 // a match expr, check all arms
83 ast::ExprKind::Match(_, ref arms) => {
85 self.check_final_expr(cx, &arm.body, Some(arm.body.span));
92 fn emit_return_lint(&mut self, cx: &EarlyContext, ret_span: Span, inner_span: Span) {
93 if in_external_macro(cx, inner_span) || in_macro(inner_span) {
96 span_lint_and_then(cx,
99 "unneeded return statement",
100 |db| if let Some(snippet) = snippet_opt(cx, inner_span) {
101 db.span_suggestion(ret_span, "remove `return` as shown:", snippet);
105 // Check for "let x = EXPR; x"
106 fn check_let_return(&mut self, cx: &EarlyContext, block: &ast::Block) {
107 let mut it = block.stmts.iter();
109 // we need both a let-binding stmt and an expr
111 let Some(retexpr) = it.next_back(),
112 let ast::StmtKind::Expr(ref retexpr) = retexpr.node,
113 let Some(stmt) = it.next_back(),
114 let ast::StmtKind::Local(ref local) = stmt.node,
115 // don't lint in the presence of type inference
117 !local.attrs.iter().any(attr_is_cfg),
118 let Some(ref initexpr) = local.init,
119 let ast::PatKind::Ident(_, Spanned { node: id, .. }, _) = local.pat.node,
120 let ast::ExprKind::Path(_, ref path) = retexpr.node,
121 match_path_ast(path, &[&id.name.as_str()]),
122 !in_external_macro(cx, initexpr.span),
124 span_note_and_lint(cx,
127 "returning the result of a let binding from a block. \
128 Consider returning the expression directly.",
130 "this expression can be directly returned");
135 impl LintPass for ReturnPass {
136 fn get_lints(&self) -> LintArray {
137 lint_array!(NEEDLESS_RETURN, LET_AND_RETURN)
141 impl EarlyLintPass for ReturnPass {
142 fn check_fn(&mut self, cx: &EarlyContext, kind: FnKind, _: &ast::FnDecl, _: Span, _: ast::NodeId) {
144 FnKind::ItemFn(.., block) |
145 FnKind::Method(.., block) => self.check_block_return(cx, block),
146 FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span)),
150 fn check_block(&mut self, cx: &EarlyContext, block: &ast::Block) {
151 self.check_let_return(cx, block);
155 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
156 attr.meta_item_list().is_some() && attr.name().map_or(false, |n| n == "cfg")