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     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.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(
 109         &mut self,
 110         cx: &EarlyContext<'_>,
 111         expr: &ast::Expr,
 112         span: Option<Span>,
 113         replacement: RetReplacement,
 114     ) {
 115         match expr.node {
 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) || in_macro_or_desugar(inner_span) {
 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.node;
 201             if let Some(stmt) = it.next_back();
 202             if let ast::StmtKind::Local(ref local) = stmt.node;
 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.node;
 208             if let ast::ExprKind::Path(_, ref path) = retexpr.node;
 209             if match_path_ast(path, &[&*ident.name.as_str()]);
 210             if !in_external_macro(cx.sess(), initexpr.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<'_>, decl: &ast::FnDecl, span: Span, _: ast::NodeId) {
 241         match kind {
 242             FnKind::ItemFn(.., block) | FnKind::Method(.., block) => self.check_block_return(cx, block),
 243             FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span), RetReplacement::Empty),
 244         }
 245         if_chain! {
 246             if let ast::FunctionRetTy::Ty(ref ty) = decl.output;
 247             if let ast::TyKind::Tup(ref vals) = ty.node;
 248             if vals.is_empty() && !in_macro_or_desugar(ty.span) && get_def(span) == get_def(ty.span);
 249             then {
 250                 let (rspan, appl) = if let Ok(fn_source) =
 251                         cx.sess().source_map()
 252                                  .span_to_snippet(span.with_hi(ty.span.hi())) {
 253                     if let Some(rpos) = fn_source.rfind("->") {
 254                         #[allow(clippy::cast_possible_truncation)]
 255                         (ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
 256                             Applicability::MachineApplicable)
 257                     } else {
 258                         (ty.span, Applicability::MaybeIncorrect)
 259                     }
 260                 } else {
 261                     (ty.span, Applicability::MaybeIncorrect)
 262                 };
 263                 span_lint_and_then(cx, UNUSED_UNIT, rspan, "unneeded unit return type", |db| {
 264                     db.span_suggestion(
 265                         rspan,
 266                         "remove the `-> ()`",
 267                         String::new(),
 268                         appl,
 269                     );
 270                 });
 271             }
 272         }
 273     }
 274
 275     fn check_block(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
 276         self.check_let_return(cx, block);
 277         if_chain! {
 278             if let Some(ref stmt) = block.stmts.last();
 279             if let ast::StmtKind::Expr(ref expr) = stmt.node;
 280             if is_unit_expr(expr) && !in_macro_or_desugar(expr.span);
 281             then {
 282                 let sp = expr.span;
 283                 span_lint_and_then(cx, UNUSED_UNIT, sp, "unneeded unit expression", |db| {
 284                     db.span_suggestion(
 285                         sp,
 286                         "remove the final `()`",
 287                         String::new(),
 288                         Applicability::MachineApplicable,
 289                     );
 290                 });
 291             }
 292         }
 293     }
 294
 295     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
 296         match e.node {
 297             ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
 298                 if is_unit_expr(expr) && !in_macro_or_desugar(expr.span) {
 299                     span_lint_and_then(cx, UNUSED_UNIT, expr.span, "unneeded `()`", |db| {
 300                         db.span_suggestion(
 301                             expr.span,
 302                             "remove the `()`",
 303                             String::new(),
 304                             Applicability::MachineApplicable,
 305                         );
 306                     });
 307                 }
 308             },
 309             _ => (),
 310         }
 311     }
 312 }
 313
 314 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
 315     attr.meta_item_list().is_some() && attr.check_name(sym!(cfg))
 316 }
 317
 318 // get the def site
 319 fn get_def(span: Span) -> Option<Span> {
 320     span.ctxt().outer_expn_info().and_then(|info| Some(info.def_site))
 321 }
 322
 323 // is this expr a `()` unit?
 324 fn is_unit_expr(expr: &ast::Expr) -> bool {
 325     if let ast::ExprKind::Tup(ref vals) = expr.node {
 326         vals.is_empty()
 327     } else {
 328         false
 329     }
 330 }