1 use if_chain::if_chain;
2 use rustc::lint::{in_external_macro, EarlyContext, EarlyLintPass, LintArray, LintContext, LintPass};
3 use rustc::{declare_lint_pass, declare_tool_lint};
4 use rustc_errors::Applicability;
6 use syntax::source_map::Span;
7 use syntax::visit::FnKind;
8 use syntax_pos::BytePos;
10 use crate::utils::{match_path_ast, snippet_opt, span_lint_and_then};
12 declare_clippy_lint! {
13 /// **What it does:** Checks for return statements at the end of a block.
15 /// **Why is this bad?** Removing the `return` and semicolon will make the code
18 /// **Known problems:** If the computation returning the value borrows a local
19 /// variable, removing the `return` may run afoul of the borrow checker.
23 /// fn foo(x: usize) -> usize {
29 /// fn foo(x: usize) -> usize {
35 "using a return statement like `return expr;` where an expression would suffice"
38 declare_clippy_lint! {
39 /// **What it does:** Checks for `let`-bindings, which are subsequently
42 /// **Why is this bad?** It is just extraneous code. Remove it to make your code
45 /// **Known problems:** None.
49 /// fn foo() -> String {
50 /// let x = String::new();
56 /// fn foo() -> String {
62 "creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block"
65 declare_clippy_lint! {
66 /// **What it does:** Checks for unit (`()`) expressions that can be removed.
68 /// **Why is this bad?** Such expressions add no value, but can make the code
69 /// less readable. Depending on formatting they can make a `break` or `return`
70 /// statement look like a function call.
72 /// **Known problems:** The lint currently misses unit return types in types,
73 /// e.g., the `F` in `fn generic_unit<F: Fn() -> ()>(f: F) { .. }`.
77 /// fn return_unit() -> () {
83 "needless unit expression"
86 #[derive(PartialEq, Eq, Copy, Clone)]
92 declare_lint_pass!(Return => [NEEDLESS_RETURN, LET_AND_RETURN, UNUSED_UNIT]);
95 // Check the final stmt or expr in a block for unnecessary return.
96 fn check_block_return(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
97 if let Some(stmt) = block.stmts.last() {
99 ast::StmtKind::Expr(ref expr) | ast::StmtKind::Semi(ref expr) => {
100 self.check_final_expr(cx, expr, Some(stmt.span), RetReplacement::Empty);
107 // Check a the final expression in a block if it's a return.
110 cx: &EarlyContext<'_>,
113 replacement: RetReplacement,
116 // simple return is always "bad"
117 ast::ExprKind::Ret(ref inner) => {
118 // allow `#[cfg(a)] return a; #[cfg(b)] return b;`
119 if !expr.attrs.iter().any(attr_is_cfg) {
120 Self::emit_return_lint(
122 span.expect("`else return` is not possible"),
123 inner.as_ref().map(|i| i.span),
128 // a whole block? check it!
129 ast::ExprKind::Block(ref block, _) => {
130 self.check_block_return(cx, block);
132 // an if/if let expr, check both exprs
133 // note, if without else is going to be a type checking error anyways
134 // (except for unit type functions) so we don't match it
135 ast::ExprKind::If(_, ref ifblock, Some(ref elsexpr)) => {
136 self.check_block_return(cx, ifblock);
137 self.check_final_expr(cx, elsexpr, None, RetReplacement::Empty);
139 // a match expr, check all arms
140 ast::ExprKind::Match(_, ref arms) => {
142 self.check_final_expr(cx, &arm.body, Some(arm.body.span), RetReplacement::Block);
149 fn emit_return_lint(cx: &EarlyContext<'_>, ret_span: Span, inner_span: Option<Span>, replacement: RetReplacement) {
151 Some(inner_span) => {
152 if in_external_macro(cx.sess(), inner_span) || inner_span.from_expansion() {
156 span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
157 if let Some(snippet) = snippet_opt(cx, inner_span) {
158 db.span_suggestion(ret_span, "remove `return`", snippet, Applicability::MachineApplicable);
162 None => match replacement {
163 RetReplacement::Empty => {
164 span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
169 Applicability::MachineApplicable,
173 RetReplacement::Block => {
174 span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
177 "replace `return` with an empty block",
179 Applicability::MachineApplicable,
187 // Check for "let x = EXPR; x"
188 fn check_let_return(cx: &EarlyContext<'_>, block: &ast::Block) {
189 let mut it = block.stmts.iter();
191 // we need both a let-binding stmt and an expr
193 if let Some(retexpr) = it.next_back();
194 if let ast::StmtKind::Expr(ref retexpr) = retexpr.kind;
195 if let Some(stmt) = it.next_back();
196 if let ast::StmtKind::Local(ref local) = stmt.kind;
197 // don't lint in the presence of type inference
198 if local.ty.is_none();
199 if local.attrs.is_empty();
200 if let Some(ref initexpr) = local.init;
201 if let ast::PatKind::Ident(_, ident, _) = local.pat.kind;
202 if let ast::ExprKind::Path(_, ref path) = retexpr.kind;
203 if match_path_ast(path, &[&*ident.name.as_str()]);
204 if !in_external_macro(cx.sess(), initexpr.span);
205 if !in_external_macro(cx.sess(), retexpr.span);
206 if !in_external_macro(cx.sess(), local.span);
212 "returning the result of a let binding from a block",
214 err.span_label(local.span, "unnecessary let binding");
216 if let Some(snippet) = snippet_opt(cx, initexpr.span) {
217 err.multipart_suggestion(
218 "return the expression directly",
220 (local.span, String::new()),
221 (retexpr.span, snippet),
223 Applicability::MachineApplicable,
226 err.span_help(initexpr.span, "this expression can be directly returned");
235 impl EarlyLintPass for Return {
236 fn check_fn(&mut self, cx: &EarlyContext<'_>, kind: FnKind<'_>, decl: &ast::FnDecl, span: Span, _: ast::NodeId) {
238 FnKind::ItemFn(.., block) | FnKind::Method(.., block) => self.check_block_return(cx, block),
239 FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span), RetReplacement::Empty),
242 if let ast::FunctionRetTy::Ty(ref ty) = decl.output;
243 if let ast::TyKind::Tup(ref vals) = ty.kind;
244 if vals.is_empty() && !ty.span.from_expansion() && get_def(span) == get_def(ty.span);
246 let (rspan, appl) = if let Ok(fn_source) =
247 cx.sess().source_map()
248 .span_to_snippet(span.with_hi(ty.span.hi())) {
249 if let Some(rpos) = fn_source.rfind("->") {
250 #[allow(clippy::cast_possible_truncation)]
251 (ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
252 Applicability::MachineApplicable)
254 (ty.span, Applicability::MaybeIncorrect)
257 (ty.span, Applicability::MaybeIncorrect)
259 span_lint_and_then(cx, UNUSED_UNIT, rspan, "unneeded unit return type", |db| {
262 "remove the `-> ()`",
271 fn check_block(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
272 Self::check_let_return(cx, block);
274 if let Some(ref stmt) = block.stmts.last();
275 if let ast::StmtKind::Expr(ref expr) = stmt.kind;
276 if is_unit_expr(expr) && !stmt.span.from_expansion();
279 span_lint_and_then(cx, UNUSED_UNIT, sp, "unneeded unit expression", |db| {
282 "remove the final `()`",
284 Applicability::MachineApplicable,
291 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
293 ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
294 if is_unit_expr(expr) && !expr.span.from_expansion() {
295 span_lint_and_then(cx, UNUSED_UNIT, expr.span, "unneeded `()`", |db| {
300 Applicability::MachineApplicable,
310 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
311 attr.meta_item_list().is_some() && attr.check_name(sym!(cfg))
316 fn get_def(span: Span) -> Option<Span> {
317 if span.from_expansion() {
318 Some(span.ctxt().outer_expn_data().def_site)
324 // is this expr a `()` unit?
325 fn is_unit_expr(expr: &ast::Expr) -> bool {
326 if let ast::ExprKind::Tup(ref vals) = expr.kind {