1 use if_chain::if_chain;
2 use rustc::declare_lint_pass;
3 use rustc::lint::{in_external_macro, EarlyContext, EarlyLintPass, LintArray, LintContext, LintPass};
4 use rustc_errors::Applicability;
5 use rustc_session::declare_tool_lint;
7 use syntax::source_map::Span;
8 use syntax::visit::FnKind;
9 use syntax_pos::BytePos;
11 use crate::utils::{match_path_ast, snippet_opt, span_lint_and_then};
13 declare_clippy_lint! {
14 /// **What it does:** Checks for return statements at the end of a block.
16 /// **Why is this bad?** Removing the `return` and semicolon will make the code
19 /// **Known problems:** If the computation returning the value borrows a local
20 /// variable, removing the `return` may run afoul of the borrow checker.
24 /// fn foo(x: usize) -> usize {
30 /// fn foo(x: usize) -> usize {
36 "using a return statement like `return expr;` where an expression would suffice"
39 declare_clippy_lint! {
40 /// **What it does:** Checks for `let`-bindings, which are subsequently
43 /// **Why is this bad?** It is just extraneous code. Remove it to make your code
46 /// **Known problems:** None.
50 /// fn foo() -> String {
51 /// let x = String::new();
57 /// fn foo() -> String {
63 "creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block"
66 declare_clippy_lint! {
67 /// **What it does:** Checks for unit (`()`) expressions that can be removed.
69 /// **Why is this bad?** Such expressions add no value, but can make the code
70 /// less readable. Depending on formatting they can make a `break` or `return`
71 /// statement look like a function call.
73 /// **Known problems:** The lint currently misses unit return types in types,
74 /// e.g., the `F` in `fn generic_unit<F: Fn() -> ()>(f: F) { .. }`.
78 /// fn return_unit() -> () {
84 "needless unit expression"
87 #[derive(PartialEq, Eq, Copy, Clone)]
93 declare_lint_pass!(Return => [NEEDLESS_RETURN, LET_AND_RETURN, UNUSED_UNIT]);
96 // Check the final stmt or expr in a block for unnecessary return.
97 fn check_block_return(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
98 if let Some(stmt) = block.stmts.last() {
100 ast::StmtKind::Expr(ref expr) | ast::StmtKind::Semi(ref expr) => {
101 self.check_final_expr(cx, expr, Some(stmt.span), RetReplacement::Empty);
108 // Check a the final expression in a block if it's a return.
111 cx: &EarlyContext<'_>,
114 replacement: RetReplacement,
117 // simple return is always "bad"
118 ast::ExprKind::Ret(ref inner) => {
119 // allow `#[cfg(a)] return a; #[cfg(b)] return b;`
120 if !expr.attrs.iter().any(attr_is_cfg) {
121 Self::emit_return_lint(
123 span.expect("`else return` is not possible"),
124 inner.as_ref().map(|i| i.span),
129 // a whole block? check it!
130 ast::ExprKind::Block(ref block, _) => {
131 self.check_block_return(cx, block);
133 // an if/if let expr, check both exprs
134 // note, if without else is going to be a type checking error anyways
135 // (except for unit type functions) so we don't match it
136 ast::ExprKind::If(_, ref ifblock, Some(ref elsexpr)) => {
137 self.check_block_return(cx, ifblock);
138 self.check_final_expr(cx, elsexpr, None, RetReplacement::Empty);
140 // a match expr, check all arms
141 ast::ExprKind::Match(_, ref arms) => {
143 self.check_final_expr(cx, &arm.body, Some(arm.body.span), RetReplacement::Block);
150 fn emit_return_lint(cx: &EarlyContext<'_>, ret_span: Span, inner_span: Option<Span>, replacement: RetReplacement) {
152 Some(inner_span) => {
153 if in_external_macro(cx.sess(), inner_span) || inner_span.from_expansion() {
157 span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
158 if let Some(snippet) = snippet_opt(cx, inner_span) {
159 db.span_suggestion(ret_span, "remove `return`", snippet, Applicability::MachineApplicable);
163 None => match replacement {
164 RetReplacement::Empty => {
165 span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
170 Applicability::MachineApplicable,
174 RetReplacement::Block => {
175 span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
178 "replace `return` with an empty block",
180 Applicability::MachineApplicable,
188 // Check for "let x = EXPR; x"
189 fn check_let_return(cx: &EarlyContext<'_>, block: &ast::Block) {
190 let mut it = block.stmts.iter();
192 // we need both a let-binding stmt and an expr
194 if let Some(retexpr) = it.next_back();
195 if let ast::StmtKind::Expr(ref retexpr) = retexpr.kind;
196 if let Some(stmt) = it.next_back();
197 if let ast::StmtKind::Local(ref local) = stmt.kind;
198 // don't lint in the presence of type inference
199 if local.ty.is_none();
200 if local.attrs.is_empty();
201 if let Some(ref initexpr) = local.init;
202 if let ast::PatKind::Ident(_, ident, _) = local.pat.kind;
203 if let ast::ExprKind::Path(_, ref path) = retexpr.kind;
204 if match_path_ast(path, &[&*ident.name.as_str()]);
205 if !in_external_macro(cx.sess(), initexpr.span);
206 if !in_external_macro(cx.sess(), retexpr.span);
207 if !in_external_macro(cx.sess(), local.span);
213 "returning the result of a let binding from a block",
215 err.span_label(local.span, "unnecessary let binding");
217 if let Some(snippet) = snippet_opt(cx, initexpr.span) {
218 err.multipart_suggestion(
219 "return the expression directly",
221 (local.span, String::new()),
222 (retexpr.span, snippet),
224 Applicability::MachineApplicable,
227 err.span_help(initexpr.span, "this expression can be directly returned");
236 impl EarlyLintPass for Return {
237 fn check_fn(&mut self, cx: &EarlyContext<'_>, kind: FnKind<'_>, decl: &ast::FnDecl, span: Span, _: ast::NodeId) {
239 FnKind::ItemFn(.., block) | FnKind::Method(.., block) => self.check_block_return(cx, block),
240 FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span), RetReplacement::Empty),
243 if let ast::FunctionRetTy::Ty(ref ty) = decl.output;
244 if let ast::TyKind::Tup(ref vals) = ty.kind;
245 if vals.is_empty() && !ty.span.from_expansion() && get_def(span) == get_def(ty.span);
247 let (rspan, appl) = if let Ok(fn_source) =
248 cx.sess().source_map()
249 .span_to_snippet(span.with_hi(ty.span.hi())) {
250 if let Some(rpos) = fn_source.rfind("->") {
251 #[allow(clippy::cast_possible_truncation)]
252 (ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
253 Applicability::MachineApplicable)
255 (ty.span, Applicability::MaybeIncorrect)
258 (ty.span, Applicability::MaybeIncorrect)
260 span_lint_and_then(cx, UNUSED_UNIT, rspan, "unneeded unit return type", |db| {
263 "remove the `-> ()`",
272 fn check_block(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
273 Self::check_let_return(cx, block);
275 if let Some(ref stmt) = block.stmts.last();
276 if let ast::StmtKind::Expr(ref expr) = stmt.kind;
277 if is_unit_expr(expr) && !stmt.span.from_expansion();
280 span_lint_and_then(cx, UNUSED_UNIT, sp, "unneeded unit expression", |db| {
283 "remove the final `()`",
285 Applicability::MachineApplicable,
292 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
294 ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
295 if is_unit_expr(expr) && !expr.span.from_expansion() {
296 span_lint_and_then(cx, UNUSED_UNIT, expr.span, "unneeded `()`", |db| {
301 Applicability::MachineApplicable,
311 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
312 attr.meta_item_list().is_some() && attr.check_name(sym!(cfg))
317 fn get_def(span: Span) -> Option<Span> {
318 if span.from_expansion() {
319 Some(span.ctxt().outer_expn_data().def_site)
325 // is this expr a `()` unit?
326 fn is_unit_expr(expr: &ast::Expr) -> bool {
327 if let ast::ExprKind::Tup(ref vals) = expr.kind {