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::{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     Block,
  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.kind {
  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(
 109         &mut self,
 110         cx: &EarlyContext<'_>,
 111         expr: &ast::Expr,
 112         span: Option<Span>,
 113         replacement: RetReplacement,
 114     ) {
 115         match expr.kind {
 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(
 121                         cx,
 122                         span.expect("`else return` is not possible"),
 123                         inner.as_ref().map(|i| i.span),
 124                         replacement,
 125                     );
 126                 }
 127             },
 128             // a whole block? check it!
 129             ast::ExprKind::Block(ref block, _) => {
 130                 self.check_block_return(cx, block);
 131             },
 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);
 138             },
 139             // a match expr, check all arms
 140             ast::ExprKind::Match(_, ref arms) => {
 141                 for arm in arms {
 142                     self.check_final_expr(cx, &arm.body, Some(arm.body.span), RetReplacement::Block);
 143                 }
 144             },
 145             _ => (),
 146         }
 147     }
 148
 149     fn emit_return_lint(
 150         &mut self,
 151         cx: &EarlyContext<'_>,
 152         ret_span: Span,
 153         inner_span: Option<Span>,
 154         replacement: RetReplacement,
 155     ) {
 156         match inner_span {
 157             Some(inner_span) => {
 158                 if in_external_macro(cx.sess(), inner_span) || inner_span.from_expansion() {
 159                     return;
 160                 }
 161
 162                 span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
 163                     if let Some(snippet) = snippet_opt(cx, inner_span) {
 164                         db.span_suggestion(ret_span, "remove `return`", snippet, Applicability::MachineApplicable);
 165                     }
 166                 })
 167             },
 168             None => match replacement {
 169                 RetReplacement::Empty => {
 170                     span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
 171                         db.span_suggestion(
 172                             ret_span,
 173                             "remove `return`",
 174                             String::new(),
 175                             Applicability::MachineApplicable,
 176                         );
 177                     });
 178                 },
 179                 RetReplacement::Block => {
 180                     span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
 181                         db.span_suggestion(
 182                             ret_span,
 183                             "replace `return` with an empty block",
 184                             "{}".to_string(),
 185                             Applicability::MachineApplicable,
 186                         );
 187                     });
 188                 },
 189             },
 190         }
 191     }
 192
 193     // Check for "let x = EXPR; x"
 194     fn check_let_return(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
 195         let mut it = block.stmts.iter();
 196
 197         // we need both a let-binding stmt and an expr
 198         if_chain! {
 199             if let Some(retexpr) = it.next_back();
 200             if let ast::StmtKind::Expr(ref retexpr) = retexpr.kind;
 201             if let Some(stmt) = it.next_back();
 202             if let ast::StmtKind::Local(ref local) = stmt.kind;
 203             // don't lint in the presence of type inference
 204             if local.ty.is_none();
 205             if local.attrs.is_empty();
 206             if let Some(ref initexpr) = local.init;
 207             if let ast::PatKind::Ident(_, ident, _) = local.pat.kind;
 208             if let ast::ExprKind::Path(_, ref path) = retexpr.kind;
 209             if match_path_ast(path, &[&*ident.name.as_str()]);
 210             if !in_external_macro(cx.sess(), initexpr.span);
 211             if !in_external_macro(cx.sess(), retexpr.span);
 212             if !in_external_macro(cx.sess(), local.span);
 213             then {
 214                 span_lint_and_then(
 215                     cx,
 216                     LET_AND_RETURN,
 217                     retexpr.span,
 218                     "returning the result of a let binding from a block",
 219                     |err| {
 220                         err.span_label(local.span, "unnecessary let binding");
 221
 222                         if let Some(snippet) = snippet_opt(cx, initexpr.span) {
 223                             err.multipart_suggestion(
 224                                 "return the expression directly",
 225                                 vec![
 226                                     (local.span, String::new()),
 227                                     (retexpr.span, snippet),
 228                                 ],
 229                                 Applicability::MachineApplicable,
 230                             );
 231                         } else {
 232                             err.span_help(initexpr.span, "this expression can be directly returned");
 233                         }
 234                     },
 235                 );
 236             }
 237         }
 238     }
 239 }
 240
 241 impl EarlyLintPass for Return {
 242     fn check_fn(&mut self, cx: &EarlyContext<'_>, kind: FnKind<'_>, decl: &ast::FnDecl, span: Span, _: ast::NodeId) {
 243         match kind {
 244             FnKind::ItemFn(.., block) | FnKind::Method(.., block) => self.check_block_return(cx, block),
 245             FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span), RetReplacement::Empty),
 246         }
 247         if_chain! {
 248             if let ast::FunctionRetTy::Ty(ref ty) = decl.output;
 249             if let ast::TyKind::Tup(ref vals) = ty.kind;
 250             if vals.is_empty() && !ty.span.from_expansion() && get_def(span) == get_def(ty.span);
 251             then {
 252                 let (rspan, appl) = if let Ok(fn_source) =
 253                         cx.sess().source_map()
 254                                  .span_to_snippet(span.with_hi(ty.span.hi())) {
 255                     if let Some(rpos) = fn_source.rfind("->") {
 256                         #[allow(clippy::cast_possible_truncation)]
 257                         (ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
 258                             Applicability::MachineApplicable)
 259                     } else {
 260                         (ty.span, Applicability::MaybeIncorrect)
 261                     }
 262                 } else {
 263                     (ty.span, Applicability::MaybeIncorrect)
 264                 };
 265                 span_lint_and_then(cx, UNUSED_UNIT, rspan, "unneeded unit return type", |db| {
 266                     db.span_suggestion(
 267                         rspan,
 268                         "remove the `-> ()`",
 269                         String::new(),
 270                         appl,
 271                     );
 272                 });
 273             }
 274         }
 275     }
 276
 277     fn check_block(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
 278         self.check_let_return(cx, block);
 279         if_chain! {
 280             if let Some(ref stmt) = block.stmts.last();
 281             if let ast::StmtKind::Expr(ref expr) = stmt.kind;
 282             if is_unit_expr(expr) && !stmt.span.from_expansion();
 283             then {
 284                 let sp = expr.span;
 285                 span_lint_and_then(cx, UNUSED_UNIT, sp, "unneeded unit expression", |db| {
 286                     db.span_suggestion(
 287                         sp,
 288                         "remove the final `()`",
 289                         String::new(),
 290                         Applicability::MachineApplicable,
 291                     );
 292                 });
 293             }
 294         }
 295     }
 296
 297     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
 298         match e.kind {
 299             ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
 300                 if is_unit_expr(expr) && !expr.span.from_expansion() {
 301                     span_lint_and_then(cx, UNUSED_UNIT, expr.span, "unneeded `()`", |db| {
 302                         db.span_suggestion(
 303                             expr.span,
 304                             "remove the `()`",
 305                             String::new(),
 306                             Applicability::MachineApplicable,
 307                         );
 308                     });
 309                 }
 310             },
 311             _ => (),
 312         }
 313     }
 314 }
 315
 316 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
 317     attr.meta_item_list().is_some() && attr.check_name(sym!(cfg))
 318 }
 319
 320 // get the def site
 321 fn get_def(span: Span) -> Option<Span> {
 322     if span.from_expansion() {
 323         Some(span.ctxt().outer_expn_data().def_site)
 324     } else {
 325         None
 326     }
 327 }
 328
 329 // is this expr a `()` unit?
 330 fn is_unit_expr(expr: &ast::Expr) -> bool {
 331     if let ast::ExprKind::Tup(ref vals) = expr.kind {
 332         vals.is_empty()
 333     } else {
 334         false
 335     }
 336 }