3 use syntax::codemap::{Span, Spanned};
4 use syntax::visit::FnKind;
6 use utils::{in_external_macro, in_macro, match_path_ast, snippet_opt, span_lint_and_then, span_note_and_lint};
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) | 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) => for arm in arms {
84 self.check_final_expr(cx, &arm.body, Some(arm.body.span));
90 fn emit_return_lint(&mut self, cx: &EarlyContext, ret_span: Span, inner_span: Span) {
91 if in_external_macro(cx, inner_span) || in_macro(inner_span) {
94 span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
95 if let Some(snippet) = snippet_opt(cx, inner_span) {
96 db.span_suggestion(ret_span, "remove `return` as shown", snippet);
101 // Check for "let x = EXPR; x"
102 fn check_let_return(&mut self, cx: &EarlyContext, block: &ast::Block) {
103 let mut it = block.stmts.iter();
105 // we need both a let-binding stmt and an expr
107 let Some(retexpr) = it.next_back(),
108 let ast::StmtKind::Expr(ref retexpr) = retexpr.node,
109 let Some(stmt) = it.next_back(),
110 let ast::StmtKind::Local(ref local) = stmt.node,
111 // don't lint in the presence of type inference
113 !local.attrs.iter().any(attr_is_cfg),
114 let Some(ref initexpr) = local.init,
115 let ast::PatKind::Ident(_, Spanned { node: id, .. }, _) = local.pat.node,
116 let ast::ExprKind::Path(_, ref path) = retexpr.node,
117 match_path_ast(path, &[&id.name.as_str()]),
118 !in_external_macro(cx, initexpr.span),
120 span_note_and_lint(cx,
123 "returning the result of a let binding from a block. \
124 Consider returning the expression directly.",
126 "this expression can be directly returned");
131 impl LintPass for ReturnPass {
132 fn get_lints(&self) -> LintArray {
133 lint_array!(NEEDLESS_RETURN, LET_AND_RETURN)
137 impl EarlyLintPass for ReturnPass {
138 fn check_fn(&mut self, cx: &EarlyContext, kind: FnKind, _: &ast::FnDecl, _: Span, _: ast::NodeId) {
140 FnKind::ItemFn(.., block) | FnKind::Method(.., block) => self.check_block_return(cx, block),
141 FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span)),
145 fn check_block(&mut self, cx: &EarlyContext, block: &ast::Block) {
146 self.check_let_return(cx, block);
150 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
151 attr.meta_item_list().is_some() && attr.name().map_or(false, |n| n == "cfg")