3 use syntax::codemap::{Span, Spanned};
4 use syntax::visit::FnKind;
6 use utils::{span_lint, span_lint_and_then, snippet_opt, match_path_ast, in_external_macro};
8 /// **What it does:** This lint checks for return statements at the end of a block.
10 /// **Why is this bad?** Removing the `return` and semicolon will make the code more rusty.
12 /// **Known problems:** Following this lint's advice may currently run afoul of Rust issue [#31439](https://github.com/rust-lang/rust/issues/31439), so if you get lifetime errors, please roll back the change until that issue is fixed.
14 /// **Example:** `fn foo(x: usize) { return x; }`
16 pub NEEDLESS_RETURN, Warn,
17 "using a return statement like `return expr;` where an expression would suffice"
20 /// **What it does:** This lint checks for `let`-bindings, which are subsequently returned.
22 /// **Why is this bad?** It is just extraneous code. Remove it to make your code more rusty.
24 /// **Known problems:** Following this lint's advice may currently run afoul of Rust issue [#31439](https://github.com/rust-lang/rust/issues/31439), so if you get lifetime errors, please roll back the change until that issue is fixed.
26 /// **Example:** `{ let x = ..; x }`
28 pub LET_AND_RETURN, Warn,
29 "creating a let-binding and then immediately returning it like `let x = expr; x` at \
33 #[derive(Copy, Clone)]
34 pub struct ReturnPass;
37 // Check the final stmt or expr in a block for unnecessary return.
38 fn check_block_return(&mut self, cx: &EarlyContext, block: &Block) {
39 if let Some(ref expr) = block.expr {
40 self.check_final_expr(cx, expr);
41 } else if let Some(stmt) = block.stmts.last() {
42 if let StmtKind::Semi(ref expr, _) = stmt.node {
43 if let ExprKind::Ret(Some(ref inner)) = expr.node {
44 self.emit_return_lint(cx, (stmt.span, inner.span));
50 // Check a the final expression in a block if it's a return.
51 fn check_final_expr(&mut self, cx: &EarlyContext, expr: &Expr) {
53 // simple return is always "bad"
54 ExprKind::Ret(Some(ref inner)) => {
55 self.emit_return_lint(cx, (expr.span, inner.span));
57 // a whole block? check it!
58 ExprKind::Block(ref block) => {
59 self.check_block_return(cx, block);
61 // an if/if let expr, check both exprs
62 // note, if without else is going to be a type checking error anyways
63 // (except for unit type functions) so we don't match it
64 ExprKind::If(_, ref ifblock, Some(ref elsexpr)) => {
65 self.check_block_return(cx, ifblock);
66 self.check_final_expr(cx, elsexpr);
68 // a match expr, check all arms
69 ExprKind::Match(_, ref arms) => {
71 self.check_final_expr(cx, &arm.body);
78 fn emit_return_lint(&mut self, cx: &EarlyContext, spans: (Span, Span)) {
79 if in_external_macro(cx, spans.1) {
82 span_lint_and_then(cx, NEEDLESS_RETURN, spans.0, "unneeded return statement", |db| {
83 if let Some(snippet) = snippet_opt(cx, spans.1) {
84 db.span_suggestion(spans.0, "remove `return` as shown:", snippet);
89 // Check for "let x = EXPR; x"
90 fn check_let_return(&mut self, cx: &EarlyContext, block: &Block) {
91 // we need both a let-binding stmt and an expr
94 let Some(stmt) = block.stmts.last(),
95 let Some(ref retexpr) = block.expr,
96 let StmtKind::Decl(ref decl, _) = stmt.node,
97 let DeclKind::Local(ref local) = decl.node,
98 let Some(ref initexpr) = local.init,
99 let PatKind::Ident(_, Spanned { node: id, .. }, _) = local.pat.node,
100 let ExprKind::Path(_, ref path) = retexpr.node,
101 match_path_ast(path, &[&id.name.as_str()])
103 self.emit_let_lint(cx, retexpr.span, initexpr.span);
108 fn emit_let_lint(&mut self, cx: &EarlyContext, lint_span: Span, note_span: Span) {
109 if in_external_macro(cx, note_span) {
112 let mut db = span_lint(cx,
115 "returning the result of a let binding from a block. Consider returning the \
116 expression directly.");
117 if cx.current_level(LET_AND_RETURN) != Level::Allow {
118 db.span_note(note_span, "this expression can be directly returned");
123 impl LintPass for ReturnPass {
124 fn get_lints(&self) -> LintArray {
125 lint_array!(NEEDLESS_RETURN, LET_AND_RETURN)
129 impl EarlyLintPass for ReturnPass {
130 fn check_fn(&mut self, cx: &EarlyContext, _: FnKind, _: &FnDecl, block: &Block, _: Span, _: NodeId) {
131 self.check_block_return(cx, block);
134 fn check_block(&mut self, cx: &EarlyContext, block: &Block) {
135 self.check_let_return(cx, block);