1 use crate::utils::{in_macro, match_path_ast, snippet_opt, span_lint_and_then, span_note_and_lint};
2 use if_chain::if_chain;
3 use rustc::lint::{in_external_macro, EarlyContext, EarlyLintPass, LintArray, LintContext, LintPass};
4 use rustc::{declare_tool_lint, lint_array};
5 use rustc_errors::Applicability;
7 use syntax::source_map::Span;
8 use syntax::visit::FnKind;
9 use syntax_pos::BytePos;
11 /// **What it does:** Checks for return statements at the end of a block.
13 /// **Why is this bad?** Removing the `return` and semicolon will make the code
16 /// **Known problems:** If the computation returning the value borrows a local
17 /// variable, removing the `return` may run afoul of the borrow checker.
21 /// fn foo(x: usize) {
27 /// fn foo(x: usize) {
31 declare_clippy_lint! {
34 "using a return statement like `return expr;` where an expression would suffice"
37 /// **What it does:** Checks for `let`-bindings, which are subsequently
40 /// **Why is this bad?** It is just extraneous code. Remove it to make your code
43 /// **Known problems:** None.
47 /// fn foo() -> String {
48 /// let x = String::new();
54 /// fn foo() -> String {
58 declare_clippy_lint! {
61 "creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block"
64 /// **What it does:** Checks for unit (`()`) expressions that can be removed.
66 /// **Why is this bad?** Such expressions add no value, but can make the code
67 /// less readable. Depending on formatting they can make a `break` or `return`
68 /// statement look like a function call.
70 /// **Known problems:** The lint currently misses unit return types in types,
71 /// e.g. the `F` in `fn generic_unit<F: Fn() -> ()>(f: F) { .. }`.
75 /// fn return_unit() -> () {
79 declare_clippy_lint! {
82 "needless unit expression"
85 #[derive(Copy, Clone)]
86 pub struct ReturnPass;
89 // Check the final stmt or expr in a block for unnecessary return.
90 fn check_block_return(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
91 if let Some(stmt) = block.stmts.last() {
93 ast::StmtKind::Expr(ref expr) | ast::StmtKind::Semi(ref expr) => {
94 self.check_final_expr(cx, expr, Some(stmt.span));
101 // Check a the final expression in a block if it's a return.
102 fn check_final_expr(&mut self, cx: &EarlyContext<'_>, expr: &ast::Expr, span: Option<Span>) {
104 // simple return is always "bad"
105 ast::ExprKind::Ret(Some(ref inner)) => {
106 // allow `#[cfg(a)] return a; #[cfg(b)] return b;`
107 if !expr.attrs.iter().any(attr_is_cfg) {
108 self.emit_return_lint(cx, span.expect("`else return` is not possible"), inner.span);
111 // a whole block? check it!
112 ast::ExprKind::Block(ref block, _) => {
113 self.check_block_return(cx, block);
115 // an if/if let expr, check both exprs
116 // note, if without else is going to be a type checking error anyways
117 // (except for unit type functions) so we don't match it
118 ast::ExprKind::If(_, ref ifblock, Some(ref elsexpr)) => {
119 self.check_block_return(cx, ifblock);
120 self.check_final_expr(cx, elsexpr, None);
122 // a match expr, check all arms
123 ast::ExprKind::Match(_, ref arms) => {
125 self.check_final_expr(cx, &arm.body, Some(arm.body.span));
132 fn emit_return_lint(&mut self, cx: &EarlyContext<'_>, ret_span: Span, inner_span: Span) {
133 if in_external_macro(cx.sess(), inner_span) || in_macro(inner_span) {
136 span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
137 if let Some(snippet) = snippet_opt(cx, inner_span) {
140 "remove `return` as shown",
142 Applicability::MachineApplicable,
148 // Check for "let x = EXPR; x"
149 fn check_let_return(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
150 let mut it = block.stmts.iter();
152 // we need both a let-binding stmt and an expr
154 if let Some(retexpr) = it.next_back();
155 if let ast::StmtKind::Expr(ref retexpr) = retexpr.node;
156 if let Some(stmt) = it.next_back();
157 if let ast::StmtKind::Local(ref local) = stmt.node;
158 // don't lint in the presence of type inference
159 if local.ty.is_none();
160 if !local.attrs.iter().any(attr_is_cfg);
161 if let Some(ref initexpr) = local.init;
162 if let ast::PatKind::Ident(_, ident, _) = local.pat.node;
163 if let ast::ExprKind::Path(_, ref path) = retexpr.node;
164 if match_path_ast(path, &[&ident.as_str()]);
165 if !in_external_macro(cx.sess(), initexpr.span);
167 span_note_and_lint(cx,
170 "returning the result of a let binding from a block. \
171 Consider returning the expression directly.",
173 "this expression can be directly returned");
179 impl LintPass for ReturnPass {
180 fn get_lints(&self) -> LintArray {
181 lint_array!(NEEDLESS_RETURN, LET_AND_RETURN, UNUSED_UNIT)
184 fn name(&self) -> &'static str {
189 impl EarlyLintPass for ReturnPass {
190 fn check_fn(&mut self, cx: &EarlyContext<'_>, kind: FnKind<'_>, decl: &ast::FnDecl, span: Span, _: ast::NodeId) {
192 FnKind::ItemFn(.., block) | FnKind::Method(.., block) => self.check_block_return(cx, block),
193 FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span)),
196 if let ast::FunctionRetTy::Ty(ref ty) = decl.output;
197 if let ast::TyKind::Tup(ref vals) = ty.node;
198 if vals.is_empty() && !in_macro(ty.span) && get_def(span) == get_def(ty.span);
200 let (rspan, appl) = if let Ok(fn_source) =
201 cx.sess().source_map()
202 .span_to_snippet(span.with_hi(ty.span.hi())) {
203 if let Some(rpos) = fn_source.rfind("->") {
204 #[allow(clippy::cast_possible_truncation)]
205 (ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
206 Applicability::MachineApplicable)
208 (ty.span, Applicability::MaybeIncorrect)
211 (ty.span, Applicability::MaybeIncorrect)
213 span_lint_and_then(cx, UNUSED_UNIT, rspan, "unneeded unit return type", |db| {
216 "remove the `-> ()`",
225 fn check_block(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
226 self.check_let_return(cx, block);
228 if let Some(ref stmt) = block.stmts.last();
229 if let ast::StmtKind::Expr(ref expr) = stmt.node;
230 if is_unit_expr(expr) && !in_macro(expr.span);
233 span_lint_and_then(cx, UNUSED_UNIT, sp, "unneeded unit expression", |db| {
236 "remove the final `()`",
238 Applicability::MachineApplicable,
245 fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
247 ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
248 if is_unit_expr(expr) && !in_macro(expr.span) {
249 span_lint_and_then(cx, UNUSED_UNIT, expr.span, "unneeded `()`", |db| {
254 Applicability::MachineApplicable,
264 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
265 attr.meta_item_list().is_some() && attr.name() == "cfg"
269 fn get_def(span: Span) -> Option<Span> {
270 span.ctxt().outer().expn_info().and_then(|info| info.def_site)
273 // is this expr a `()` unit?
274 fn is_unit_expr(expr: &ast::Expr) -> bool {
275 if let ast::ExprKind::Tup(ref vals) = expr.node {