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, in_external_macro};
8 /// **What it does:** Checks for return statements at the end of a block.
10 /// **Why is this bad?** Removing the `return` and semicolon will make the code
13 /// **Known problems:** If the computation returning the value borrows a local
14 /// variable, removing the `return` may run afoul of the borrow checker.
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
29 /// **Why is this bad?** It is just extraneous code. Remove it to make your code
32 /// **Known problems:** None.
41 "creating a let-binding and then immediately returning it like `let x = expr; x` at \
45 #[derive(Copy, Clone)]
46 pub struct ReturnPass;
49 // Check the final stmt or expr in a block for unnecessary return.
50 fn check_block_return(&mut self, cx: &EarlyContext, block: &ast::Block) {
51 if let Some(stmt) = block.stmts.last() {
53 ast::StmtKind::Expr(ref expr) |
54 ast::StmtKind::Semi(ref expr) => {
55 self.check_final_expr(cx, expr, Some(stmt.span));
62 // Check a the final expression in a block if it's a return.
63 fn check_final_expr(&mut self, cx: &EarlyContext, expr: &ast::Expr, span: Option<Span>) {
65 // simple return is always "bad"
66 ast::ExprKind::Ret(Some(ref inner)) => {
67 // allow `#[cfg(a)] return a; #[cfg(b)] return b;`
68 if !expr.attrs.iter().any(attr_is_cfg) {
69 self.emit_return_lint(cx, span.expect("`else return` is not possible"), inner.span);
72 // a whole block? check it!
73 ast::ExprKind::Block(ref block) => {
74 self.check_block_return(cx, block);
76 // an if/if let expr, check both exprs
77 // note, if without else is going to be a type checking error anyways
78 // (except for unit type functions) so we don't match it
79 ast::ExprKind::If(_, ref ifblock, Some(ref elsexpr)) => {
80 self.check_block_return(cx, ifblock);
81 self.check_final_expr(cx, elsexpr, None);
83 // a match expr, check all arms
84 ast::ExprKind::Match(_, ref arms) => {
86 self.check_final_expr(cx, &arm.body, Some(arm.body.span));
93 fn emit_return_lint(&mut self, cx: &EarlyContext, ret_span: Span, inner_span: Span) {
94 if in_external_macro(cx, inner_span) || in_macro(inner_span) {
97 span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
98 if let Some(snippet) = snippet_opt(cx, inner_span) {
99 db.span_suggestion(ret_span, "remove `return` as shown", snippet);
104 // Check for "let x = EXPR; x"
105 fn check_let_return(&mut self, cx: &EarlyContext, block: &ast::Block) {
106 let mut it = block.stmts.iter();
108 // we need both a let-binding stmt and an expr
110 let Some(retexpr) = it.next_back(),
111 let ast::StmtKind::Expr(ref retexpr) = retexpr.node,
112 let Some(stmt) = it.next_back(),
113 let ast::StmtKind::Local(ref local) = stmt.node,
114 // don't lint in the presence of type inference
116 !local.attrs.iter().any(attr_is_cfg),
117 let Some(ref initexpr) = local.init,
118 let ast::PatKind::Ident(_, Spanned { node: id, .. }, _) = local.pat.node,
119 let ast::ExprKind::Path(_, ref path) = retexpr.node,
120 match_path_ast(path, &[&id.name.as_str()]),
121 !in_external_macro(cx, initexpr.span),
123 span_note_and_lint(cx,
126 "returning the result of a let binding from a block. \
127 Consider returning the expression directly.",
129 "this expression can be directly returned");
134 impl LintPass for ReturnPass {
135 fn get_lints(&self) -> LintArray {
136 lint_array!(NEEDLESS_RETURN, LET_AND_RETURN)
140 impl EarlyLintPass for ReturnPass {
141 fn check_fn(&mut self, cx: &EarlyContext, kind: FnKind, _: &ast::FnDecl, _: Span, _: ast::NodeId) {
143 FnKind::ItemFn(.., block) |
144 FnKind::Method(.., block) => self.check_block_return(cx, block),
145 FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span)),
149 fn check_block(&mut self, cx: &EarlyContext, block: &ast::Block) {
150 self.check_let_return(cx, block);
154 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
155 attr.meta_item_list().is_some() && attr.name().map_or(false, |n| n == "cfg")