clippy_lints/src/returns.rs

   1 use if_chain::if_chain;
   2 use rustc_ast::ast;
   3 use rustc_ast::visit::FnKind;
   4 use rustc_errors::Applicability;
   5 use rustc_lint::{EarlyContext, EarlyLintPass, LintContext};
   6 use rustc_middle::lint::in_external_macro;
   7 use rustc_session::{declare_lint_pass, declare_tool_lint};
   8 use rustc_span::source_map::Span;
   9 use rustc_span::BytePos;
  10
  11 use crate::utils::{in_macro, match_path_ast, snippet_opt, span_lint_and_sugg, 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", |diag| {
 158                     if let Some(snippet) = snippet_opt(cx, inner_span) {
 159                         diag.span_suggestion(ret_span, "remove `return`", snippet, Applicability::MachineApplicable);
 160                     }
 161                 })
 162             },
 163             None => match replacement {
 164                 RetReplacement::Empty => {
 165                     span_lint_and_sugg(
 166                         cx,
 167                         NEEDLESS_RETURN,
 168                         ret_span,
 169                         "unneeded `return` statement",
 170                         "remove `return`",
 171                         String::new(),
 172                         Applicability::MachineApplicable,
 173                     );
 174                 },
 175                 RetReplacement::Block => {
 176                     span_lint_and_sugg(
 177                         cx,
 178                         NEEDLESS_RETURN,
 179                         ret_span,
 180                         "unneeded `return` statement",
 181                         "replace `return` with an empty block",
 182                         "{}".to_string(),
 183                         Applicability::MachineApplicable,
 184                     );
 185                 },
 186             },
 187         }
 188     }
 189
 190     // Check for "let x = EXPR; x"
 191     fn check_let_return(cx: &EarlyContext<'_>, block: &ast::Block) {
 192         let mut it = block.stmts.iter();
 193
 194         // we need both a let-binding stmt and an expr
 195         if_chain! {
 196             if let Some(retexpr) = it.next_back();
 197             if let ast::StmtKind::Expr(ref retexpr) = retexpr.kind;
 198             if let Some(stmt) = it.next_back();
 199             if let ast::StmtKind::Local(ref local) = stmt.kind;
 200             // don't lint in the presence of type inference
 201             if local.ty.is_none();
 202             if local.attrs.is_empty();
 203             if let Some(ref initexpr) = local.init;
 204             if let ast::PatKind::Ident(_, ident, _) = local.pat.kind;
 205             if let ast::ExprKind::Path(_, ref path) = retexpr.kind;
 206             if match_path_ast(path, &[&*ident.name.as_str()]);
 207             if !in_external_macro(cx.sess(), initexpr.span);
 208             if !in_external_macro(cx.sess(), retexpr.span);
 209             if !in_external_macro(cx.sess(), local.span);
 210             if !in_macro(local.span);
 211             then {
 212                 span_lint_and_then(
 213                     cx,
 214                     LET_AND_RETURN,
 215                     retexpr.span,
 216                     "returning the result of a `let` binding from a block",
 217                     |err| {
 218                         err.span_label(local.span, "unnecessary `let` binding");
 219
 220                         if let Some(snippet) = snippet_opt(cx, initexpr.span) {
 221                             err.multipart_suggestion(
 222                                 "return the expression directly",
 223                                 vec![
 224                                     (local.span, String::new()),
 225                                     (retexpr.span, snippet),
 226                                 ],
 227                                 Applicability::MachineApplicable,
 228                             );
 229                         } else {
 230                             err.span_help(initexpr.span, "this expression can be directly returned");
 231                         }
 232                     },
 233                 );
 234             }
 235         }
 236     }
 237 }
 238
 239 impl EarlyLintPass for Return {
 240     fn check_fn(&mut self, cx: &EarlyContext<'_>, kind: FnKind<'_>, span: Span, _: ast::NodeId) {
 241         match kind {
 242             FnKind::Fn(.., Some(block)) => self.check_block_return(cx, block),
 243             FnKind::Closure(_, body) => self.check_final_expr(cx, body, Some(body.span), RetReplacement::Empty),
 244             FnKind::Fn(.., None) => {},
 245         }
 246         if_chain! {
 247             if let ast::FnRetTy::Ty(ref ty) = kind.decl().output;
 248             if let ast::TyKind::Tup(ref vals) = ty.kind;
 249             if vals.is_empty() && !ty.span.from_expansion() && get_def(span) == get_def(ty.span);
 250             then {
 251                 let (rspan, appl) = if let Ok(fn_source) =
 252                         cx.sess().source_map()
 253                                  .span_to_snippet(span.with_hi(ty.span.hi())) {
 254                     if let Some(rpos) = fn_source.rfind("->") {
 255                         #[allow(clippy::cast_possible_truncation)]
 256                         (ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
 257                             Applicability::MachineApplicable)
 258                     } else {
 259                         (ty.span, Applicability::MaybeIncorrect)
 260                     }
 261                 } else {
 262                     (ty.span, Applicability::MaybeIncorrect)
 263                 };
 264                 span_lint_and_sugg(
 265                     cx,
 266                     UNUSED_UNIT,
 267                     rspan,
 268                     "unneeded unit return type",
 269                     "remove the `-> ()`",
 270                     String::new(),
 271                     appl,
 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_sugg(
 286                     cx,
 287                     UNUSED_UNIT,
 288                     sp,
 289                     "unneeded unit expression",
 290                     "remove the final `()`",
 291                     String::new(),
 292                     Applicability::MachineApplicable,
 293                 );
 294             }
 295         }
 296     }
 297
 298     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
 299         match e.kind {
 300             ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
 301                 if is_unit_expr(expr) && !expr.span.from_expansion() {
 302                     span_lint_and_sugg(
 303                         cx,
 304                         UNUSED_UNIT,
 305                         expr.span,
 306                         "unneeded `()`",
 307                         "remove the `()`",
 308                         String::new(),
 309                         Applicability::MachineApplicable,
 310                     );
 311                 }
 312             },
 313             _ => (),
 314         }
 315     }
 316 }
 317
 318 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
 319     attr.meta_item_list().is_some() && attr.check_name(sym!(cfg))
 320 }
 321
 322 // get the def site
 323 #[must_use]
 324 fn get_def(span: Span) -> Option<Span> {
 325     if span.from_expansion() {
 326         Some(span.ctxt().outer_expn_data().def_site)
 327     } else {
 328         None
 329     }
 330 }
 331
 332 // is this expr a `()` unit?
 333 fn is_unit_expr(expr: &ast::Expr) -> bool {
 334     if let ast::ExprKind::Tup(ref vals) = expr.kind {
 335         vals.is_empty()
 336     } else {
 337         false
 338     }
 339 }