clippy_lints/src/returns.rs

   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;
   5 use syntax::ast;
   6 use syntax::source_map::Span;
   7 use syntax::visit::FnKind;
   8 use syntax_pos::BytePos;
   9
  10 use crate::utils::{in_macro_or_desugar, match_path_ast, snippet_opt, span_lint_and_then};
  11
  12 declare_clippy_lint! {
  13     /// **What it does:** Checks for return statements at the end of a block.
  14     ///
  15     /// **Why is this bad?** Removing the `return` and semicolon will make the code
  16     /// more rusty.
  17     ///
  18     /// **Known problems:** If the computation returning the value borrows a local
  19     /// variable, removing the `return` may run afoul of the borrow checker.
  20     ///
  21     /// **Example:**
  22     /// ```rust
  23     /// fn foo(x: usize) -> usize {
  24     ///     return x;
  25     /// }
  26     /// ```
  27     /// simplify to
  28     /// ```rust
  29     /// fn foo(x: usize) -> usize {
  30     ///     x
  31     /// }
  32     /// ```
  33     pub NEEDLESS_RETURN,
  34     style,
  35     "using a return statement like `return expr;` where an expression would suffice"
  36 }
  37
  38 declare_clippy_lint! {
  39     /// **What it does:** Checks for `let`-bindings, which are subsequently
  40     /// returned.
  41     ///
  42     /// **Why is this bad?** It is just extraneous code. Remove it to make your code
  43     /// more rusty.
  44     ///
  45     /// **Known problems:** None.
  46     ///
  47     /// **Example:**
  48     /// ```rust
  49     /// fn foo() -> String {
  50     ///     let x = String::new();
  51     ///     x
  52     /// }
  53     /// ```
  54     /// instead, use
  55     /// ```
  56     /// fn foo() -> String {
  57     ///     String::new()
  58     /// }
  59     /// ```
  60     pub LET_AND_RETURN,
  61     style,
  62     "creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block"
  63 }
  64
  65 declare_clippy_lint! {
  66     /// **What it does:** Checks for unit (`()`) expressions that can be removed.
  67     ///
  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.
  71     ///
  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) { .. }`.
  74     ///
  75     /// **Example:**
  76     /// ```rust
  77     /// fn return_unit() -> () {
  78     ///     ()
  79     /// }
  80     /// ```
  81     pub UNUSED_UNIT,
  82     style,
  83     "needless unit expression"
  84 }
  85
  86 #[derive(PartialEq, Eq, Copy, Clone)]
  87 enum RetReplacement {
  88     Empty,
  89     Unit
  90 }
  91
  92 declare_lint_pass!(Return => [NEEDLESS_RETURN, LET_AND_RETURN, UNUSED_UNIT]);
  93
  94 impl Return {
  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() {
  98             match stmt.node {
  99                 ast::StmtKind::Expr(ref expr) | ast::StmtKind::Semi(ref expr) => {
 100                     self.check_final_expr(cx, expr, Some(stmt.span), RetReplacement::Empty);
 101                 },
 102                 _ => (),
 103             }
 104         }
 105     }
 106
 107     // Check a the final expression in a block if it's a return.
 108     fn check_final_expr(&mut self, cx: &EarlyContext<'_>, expr: &ast::Expr, span: Option<Span>, replacement: RetReplacement) {
 109         match expr.node {
 110             // simple return is always "bad"
 111             ast::ExprKind::Ret(ref inner) => {
 112                 // allow `#[cfg(a)] return a; #[cfg(b)] return b;`
 113                 if !expr.attrs.iter().any(attr_is_cfg) {
 114                     self.emit_return_lint(cx, span.expect("`else return` is not possible"), inner.as_ref().map(|i| i.span), replacement);
 115                 }
 116             },
 117             // a whole block? check it!
 118             ast::ExprKind::Block(ref block, _) => {
 119                 self.check_block_return(cx, block);
 120             },
 121             // an if/if let expr, check both exprs
 122             // note, if without else is going to be a type checking error anyways
 123             // (except for unit type functions) so we don't match it
 124             ast::ExprKind::If(_, ref ifblock, Some(ref elsexpr)) => {
 125                 self.check_block_return(cx, ifblock);
 126                 self.check_final_expr(cx, elsexpr, None, RetReplacement::Empty);
 127             },
 128             // a match expr, check all arms
 129             ast::ExprKind::Match(_, ref arms) => {
 130                 for arm in arms {
 131                     self.check_final_expr(cx, &arm.body, Some(arm.body.span), RetReplacement::Unit);
 132                 }
 133             },
 134             _ => (),
 135         }
 136     }
 137
 138     fn emit_return_lint(&mut self, cx: &EarlyContext<'_>, ret_span: Span, inner_span: Option<Span>, replacement: RetReplacement) {
 139         match inner_span {
 140             Some(inner_span) => {
 141                 if in_external_macro(cx.sess(), inner_span) || in_macro_or_desugar(inner_span) {
 142                     return;
 143                 }
 144
 145                 span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
 146                     if let Some(snippet) = snippet_opt(cx, inner_span) {
 147                         db.span_suggestion(
 148                             ret_span,
 149                             "remove `return` as shown",
 150                             snippet,
 151                             Applicability::MachineApplicable,
 152                         );
 153                     }
 154                 })
 155             },
 156             None => {
 157                 match replacement {
 158                     RetReplacement::Empty => {
 159                         span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
 160                             db.span_suggestion(
 161                                 ret_span,
 162                                 "remove `return`",
 163                                 String::new(),
 164                                 Applicability::MachineApplicable,
 165                             );
 166                         });
 167                     }
 168                     RetReplacement::Unit => {
 169                         span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
 170                             db.span_suggestion(
 171                                 ret_span,
 172                                 "replace `return` with the unit type `()`",
 173                                 "()".to_string(),
 174                                 Applicability::MachineApplicable,
 175                             );
 176                         });
 177                     }
 178                 }
 179             }
 180         }
 181     }
 182
 183     // Check for "let x = EXPR; x"
 184     fn check_let_return(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
 185         let mut it = block.stmts.iter();
 186
 187         // we need both a let-binding stmt and an expr
 188         if_chain! {
 189             if let Some(retexpr) = it.next_back();
 190             if let ast::StmtKind::Expr(ref retexpr) = retexpr.node;
 191             if let Some(stmt) = it.next_back();
 192             if let ast::StmtKind::Local(ref local) = stmt.node;
 193             // don't lint in the presence of type inference
 194             if local.ty.is_none();
 195             if local.attrs.is_empty();
 196             if let Some(ref initexpr) = local.init;
 197             if let ast::PatKind::Ident(_, ident, _) = local.pat.node;
 198             if let ast::ExprKind::Path(_, ref path) = retexpr.node;
 199             if match_path_ast(path, &[&*ident.name.as_str()]);
 200             if !in_external_macro(cx.sess(), initexpr.span);
 201             then {
 202                 span_lint_and_then(
 203                     cx,
 204                     LET_AND_RETURN,
 205                     retexpr.span,
 206                     "returning the result of a let binding from a block",
 207                     |err| {
 208                         err.span_label(local.span, "unnecessary let binding");
 209
 210                         if let Some(snippet) = snippet_opt(cx, initexpr.span) {
 211                             err.multipart_suggestion(
 212                                 "return the expression directly",
 213                                 vec![
 214                                     (local.span, String::new()),
 215                                     (retexpr.span, snippet),
 216                                 ],
 217                                 Applicability::MachineApplicable,
 218                             );
 219                         } else {
 220                             err.span_help(initexpr.span, "this expression can be directly returned");
 221                         }
 222                     },
 223                 );
 224             }
 225         }
 226     }
 227 }
 228
 229 impl EarlyLintPass for Return {
 230     fn check_fn(&mut self, cx: &EarlyContext<'_>, kind: FnKind<'_>, decl: &ast::FnDecl, span: Span, _: ast::NodeId) {
 231         match kind {
 232             FnKind::ItemFn(.., block) | FnKind::Method(.., block) => self.check_block_return(cx, block),
 233             FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span), RetReplacement::Empty),
 234         }
 235         if_chain! {
 236             if let ast::FunctionRetTy::Ty(ref ty) = decl.output;
 237             if let ast::TyKind::Tup(ref vals) = ty.node;
 238             if vals.is_empty() && !in_macro_or_desugar(ty.span) && get_def(span) == get_def(ty.span);
 239             then {
 240                 let (rspan, appl) = if let Ok(fn_source) =
 241                         cx.sess().source_map()
 242                                  .span_to_snippet(span.with_hi(ty.span.hi())) {
 243                     if let Some(rpos) = fn_source.rfind("->") {
 244                         #[allow(clippy::cast_possible_truncation)]
 245                         (ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
 246                             Applicability::MachineApplicable)
 247                     } else {
 248                         (ty.span, Applicability::MaybeIncorrect)
 249                     }
 250                 } else {
 251                     (ty.span, Applicability::MaybeIncorrect)
 252                 };
 253                 span_lint_and_then(cx, UNUSED_UNIT, rspan, "unneeded unit return type", |db| {
 254                     db.span_suggestion(
 255                         rspan,
 256                         "remove the `-> ()`",
 257                         String::new(),
 258                         appl,
 259                     );
 260                 });
 261             }
 262         }
 263     }
 264
 265     fn check_block(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
 266         self.check_let_return(cx, block);
 267         if_chain! {
 268             if let Some(ref stmt) = block.stmts.last();
 269             if let ast::StmtKind::Expr(ref expr) = stmt.node;
 270             if is_unit_expr(expr) && !in_macro_or_desugar(expr.span);
 271             then {
 272                 let sp = expr.span;
 273                 span_lint_and_then(cx, UNUSED_UNIT, sp, "unneeded unit expression", |db| {
 274                     db.span_suggestion(
 275                         sp,
 276                         "remove the final `()`",
 277                         String::new(),
 278                         Applicability::MachineApplicable,
 279                     );
 280                 });
 281             }
 282         }
 283     }
 284
 285     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
 286         match e.node {
 287             ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
 288                 if is_unit_expr(expr) && !in_macro_or_desugar(expr.span) {
 289                     span_lint_and_then(cx, UNUSED_UNIT, expr.span, "unneeded `()`", |db| {
 290                         db.span_suggestion(
 291                             expr.span,
 292                             "remove the `()`",
 293                             String::new(),
 294                             Applicability::MachineApplicable,
 295                         );
 296                     });
 297                 }
 298             },
 299             _ => (),
 300         }
 301     }
 302 }
 303
 304 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
 305     attr.meta_item_list().is_some() && attr.check_name(sym!(cfg))
 306 }
 307
 308 // get the def site
 309 fn get_def(span: Span) -> Option<Span> {
 310     span.ctxt().outer_expn_info().and_then(|info| info.def_site)
 311 }
 312
 313 // is this expr a `()` unit?
 314 fn is_unit_expr(expr: &ast::Expr) -> bool {
 315     if let ast::ExprKind::Tup(ref vals) = expr.node {
 316         vals.is_empty()
 317     } else {
 318         false
 319     }
 320 }