clippy_lints/src/returns.rs

   1 use if_chain::if_chain;
   2 use rustc::lint::in_external_macro;
   3 use rustc_errors::Applicability;
   4 use rustc_lint::{EarlyContext, EarlyLintPass, LintContext};
   5 use rustc_session::{declare_lint_pass, declare_tool_lint};
   6 use rustc_span::source_map::Span;
   7 use rustc_span::BytePos;
   8 use syntax::ast;
   9 use syntax::visit::FnKind;
  10
  11 use crate::utils::{in_macro, match_path_ast, snippet_opt, span_lint_and_then};
  12
  13 declare_clippy_lint! {
  14     /// **What it does:** Checks for return statements at the end of a block.
  15     ///
  16     /// **Why is this bad?** Removing the `return` and semicolon will make the code
  17     /// more rusty.
  18     ///
  19     /// **Known problems:** If the computation returning the value borrows a local
  20     /// variable, removing the `return` may run afoul of the borrow checker.
  21     ///
  22     /// **Example:**
  23     /// ```rust
  24     /// fn foo(x: usize) -> usize {
  25     ///     return x;
  26     /// }
  27     /// ```
  28     /// simplify to
  29     /// ```rust
  30     /// fn foo(x: usize) -> usize {
  31     ///     x
  32     /// }
  33     /// ```
  34     pub NEEDLESS_RETURN,
  35     style,
  36     "using a return statement like `return expr;` where an expression would suffice"
  37 }
  38
  39 declare_clippy_lint! {
  40     /// **What it does:** Checks for `let`-bindings, which are subsequently
  41     /// returned.
  42     ///
  43     /// **Why is this bad?** It is just extraneous code. Remove it to make your code
  44     /// more rusty.
  45     ///
  46     /// **Known problems:** None.
  47     ///
  48     /// **Example:**
  49     /// ```rust
  50     /// fn foo() -> String {
  51     ///     let x = String::new();
  52     ///     x
  53     /// }
  54     /// ```
  55     /// instead, use
  56     /// ```
  57     /// fn foo() -> String {
  58     ///     String::new()
  59     /// }
  60     /// ```
  61     pub LET_AND_RETURN,
  62     style,
  63     "creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block"
  64 }
  65
  66 declare_clippy_lint! {
  67     /// **What it does:** Checks for unit (`()`) expressions that can be removed.
  68     ///
  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.
  72     ///
  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) { .. }`.
  75     ///
  76     /// **Example:**
  77     /// ```rust
  78     /// fn return_unit() -> () {
  79     ///     ()
  80     /// }
  81     /// ```
  82     pub UNUSED_UNIT,
  83     style,
  84     "needless unit expression"
  85 }
  86
  87 #[derive(PartialEq, Eq, Copy, Clone)]
  88 enum RetReplacement {
  89     Empty,
  90     Block,
  91 }
  92
  93 declare_lint_pass!(Return => [NEEDLESS_RETURN, LET_AND_RETURN, UNUSED_UNIT]);
  94
  95 impl Return {
  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() {
  99             match stmt.kind {
 100                 ast::StmtKind::Expr(ref expr) | ast::StmtKind::Semi(ref expr) => {
 101                     self.check_final_expr(cx, expr, Some(stmt.span), RetReplacement::Empty);
 102                 },
 103                 _ => (),
 104             }
 105         }
 106     }
 107
 108     // Check a the final expression in a block if it's a return.
 109     fn check_final_expr(
 110         &mut self,
 111         cx: &EarlyContext<'_>,
 112         expr: &ast::Expr,
 113         span: Option<Span>,
 114         replacement: RetReplacement,
 115     ) {
 116         match expr.kind {
 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(
 122                         cx,
 123                         span.expect("`else return` is not possible"),
 124                         inner.as_ref().map(|i| i.span),
 125                         replacement,
 126                     );
 127                 }
 128             },
 129             // a whole block? check it!
 130             ast::ExprKind::Block(ref block, _) => {
 131                 self.check_block_return(cx, block);
 132             },
 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);
 139             },
 140             // a match expr, check all arms
 141             ast::ExprKind::Match(_, ref arms) => {
 142                 for arm in arms {
 143                     self.check_final_expr(cx, &arm.body, Some(arm.body.span), RetReplacement::Block);
 144                 }
 145             },
 146             _ => (),
 147         }
 148     }
 149
 150     fn emit_return_lint(cx: &EarlyContext<'_>, ret_span: Span, inner_span: Option<Span>, replacement: RetReplacement) {
 151         match inner_span {
 152             Some(inner_span) => {
 153                 if in_external_macro(cx.sess(), inner_span) || inner_span.from_expansion() {
 154                     return;
 155                 }
 156
 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);
 160                     }
 161                 })
 162             },
 163             None => match replacement {
 164                 RetReplacement::Empty => {
 165                     span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded `return` statement", |db| {
 166                         db.span_suggestion(
 167                             ret_span,
 168                             "remove `return`",
 169                             String::new(),
 170                             Applicability::MachineApplicable,
 171                         );
 172                     });
 173                 },
 174                 RetReplacement::Block => {
 175                     span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded `return` statement", |db| {
 176                         db.span_suggestion(
 177                             ret_span,
 178                             "replace `return` with an empty block",
 179                             "{}".to_string(),
 180                             Applicability::MachineApplicable,
 181                         );
 182                     });
 183                 },
 184             },
 185         }
 186     }
 187
 188     // Check for "let x = EXPR; x"
 189     fn check_let_return(cx: &EarlyContext<'_>, block: &ast::Block) {
 190         let mut it = block.stmts.iter();
 191
 192         // we need both a let-binding stmt and an expr
 193         if_chain! {
 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);
 208             if !in_macro(local.span);
 209             then {
 210                 span_lint_and_then(
 211                     cx,
 212                     LET_AND_RETURN,
 213                     retexpr.span,
 214                     "returning the result of a `let` binding from a block",
 215                     |err| {
 216                         err.span_label(local.span, "unnecessary `let` binding");
 217
 218                         if let Some(snippet) = snippet_opt(cx, initexpr.span) {
 219                             err.multipart_suggestion(
 220                                 "return the expression directly",
 221                                 vec![
 222                                     (local.span, String::new()),
 223                                     (retexpr.span, snippet),
 224                                 ],
 225                                 Applicability::MachineApplicable,
 226                             );
 227                         } else {
 228                             err.span_help(initexpr.span, "this expression can be directly returned");
 229                         }
 230                     },
 231                 );
 232             }
 233         }
 234     }
 235 }
 236
 237 impl EarlyLintPass for Return {
 238     fn check_fn(&mut self, cx: &EarlyContext<'_>, kind: FnKind<'_>, decl: &ast::FnDecl, span: Span, _: ast::NodeId) {
 239         match kind {
 240             FnKind::ItemFn(.., block) | FnKind::Method(.., block) => self.check_block_return(cx, block),
 241             FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span), RetReplacement::Empty),
 242         }
 243         if_chain! {
 244             if let ast::FunctionRetTy::Ty(ref ty) = decl.output;
 245             if let ast::TyKind::Tup(ref vals) = ty.kind;
 246             if vals.is_empty() && !ty.span.from_expansion() && get_def(span) == get_def(ty.span);
 247             then {
 248                 let (rspan, appl) = if let Ok(fn_source) =
 249                         cx.sess().source_map()
 250                                  .span_to_snippet(span.with_hi(ty.span.hi())) {
 251                     if let Some(rpos) = fn_source.rfind("->") {
 252                         #[allow(clippy::cast_possible_truncation)]
 253                         (ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
 254                             Applicability::MachineApplicable)
 255                     } else {
 256                         (ty.span, Applicability::MaybeIncorrect)
 257                     }
 258                 } else {
 259                     (ty.span, Applicability::MaybeIncorrect)
 260                 };
 261                 span_lint_and_then(cx, UNUSED_UNIT, rspan, "unneeded unit return type", |db| {
 262                     db.span_suggestion(
 263                         rspan,
 264                         "remove the `-> ()`",
 265                         String::new(),
 266                         appl,
 267                     );
 268                 });
 269             }
 270         }
 271     }
 272
 273     fn check_block(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
 274         Self::check_let_return(cx, block);
 275         if_chain! {
 276             if let Some(ref stmt) = block.stmts.last();
 277             if let ast::StmtKind::Expr(ref expr) = stmt.kind;
 278             if is_unit_expr(expr) && !stmt.span.from_expansion();
 279             then {
 280                 let sp = expr.span;
 281                 span_lint_and_then(cx, UNUSED_UNIT, sp, "unneeded unit expression", |db| {
 282                     db.span_suggestion(
 283                         sp,
 284                         "remove the final `()`",
 285                         String::new(),
 286                         Applicability::MachineApplicable,
 287                     );
 288                 });
 289             }
 290         }
 291     }
 292
 293     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
 294         match e.kind {
 295             ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
 296                 if is_unit_expr(expr) && !expr.span.from_expansion() {
 297                     span_lint_and_then(cx, UNUSED_UNIT, expr.span, "unneeded `()`", |db| {
 298                         db.span_suggestion(
 299                             expr.span,
 300                             "remove the `()`",
 301                             String::new(),
 302                             Applicability::MachineApplicable,
 303                         );
 304                     });
 305                 }
 306             },
 307             _ => (),
 308         }
 309     }
 310 }
 311
 312 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
 313     attr.meta_item_list().is_some() && attr.check_name(sym!(cfg))
 314 }
 315
 316 // get the def site
 317 #[must_use]
 318 fn get_def(span: Span) -> Option<Span> {
 319     if span.from_expansion() {
 320         Some(span.ctxt().outer_expn_data().def_site)
 321     } else {
 322         None
 323     }
 324 }
 325
 326 // is this expr a `()` unit?
 327 fn is_unit_expr(expr: &ast::Expr) -> bool {
 328     if let ast::ExprKind::Tup(ref vals) = expr.kind {
 329         vals.is_empty()
 330     } else {
 331         false
 332     }
 333 }