clippy_lints/src/returns.rs

   1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution.
   3 //
   4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   7 // option. This file may not be copied, modified, or distributed
   8 // except according to those terms.
   9
  10 use crate::rustc::lint::{in_external_macro, EarlyContext, EarlyLintPass, LintArray, LintContext, LintPass};
  11 use crate::rustc::{declare_tool_lint, lint_array};
  12 use crate::rustc_errors::Applicability;
  13 use crate::syntax::ast;
  14 use crate::syntax::source_map::Span;
  15 use crate::syntax::visit::FnKind;
  16 use crate::syntax_pos::BytePos;
  17 use crate::utils::{in_macro, match_path_ast, snippet_opt, span_lint_and_then, span_note_and_lint};
  18 use if_chain::if_chain;
  19
  20 /// **What it does:** Checks for return statements at the end of a block.
  21 ///
  22 /// **Why is this bad?** Removing the `return` and semicolon will make the code
  23 /// more rusty.
  24 ///
  25 /// **Known problems:** If the computation returning the value borrows a local
  26 /// variable, removing the `return` may run afoul of the borrow checker.
  27 ///
  28 /// **Example:**
  29 /// ```rust
  30 /// fn foo(x: usize) {
  31 ///     return x;
  32 /// }
  33 /// ```
  34 /// simplify to
  35 /// ```rust
  36 /// fn foo(x: usize) {
  37 ///     x
  38 /// }
  39 /// ```
  40 declare_clippy_lint! {
  41     pub NEEDLESS_RETURN,
  42     style,
  43     "using a return statement like `return expr;` where an expression would suffice"
  44 }
  45
  46 /// **What it does:** Checks for `let`-bindings, which are subsequently
  47 /// returned.
  48 ///
  49 /// **Why is this bad?** It is just extraneous code. Remove it to make your code
  50 /// more rusty.
  51 ///
  52 /// **Known problems:** None.
  53 ///
  54 /// **Example:**
  55 /// ```rust
  56 /// fn foo() -> String {
  57 ///     let x = String::new();
  58 ///     x
  59 /// }
  60 /// ```
  61 /// instead, use
  62 /// ```
  63 /// fn foo() -> String {
  64 ///     String::new()
  65 /// }
  66 /// ```
  67 declare_clippy_lint! {
  68     pub LET_AND_RETURN,
  69     style,
  70     "creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block"
  71 }
  72
  73 /// **What it does:** Checks for unit (`()`) expressions that can be removed.
  74 ///
  75 /// **Why is this bad?** Such expressions add no value, but can make the code
  76 /// less readable. Depending on formatting they can make a `break` or `return`
  77 /// statement look like a function call.
  78 ///
  79 /// **Known problems:** The lint currently misses unit return types in types,
  80 /// e.g. the `F` in `fn generic_unit<F: Fn() -> ()>(f: F) { .. }`.
  81 ///
  82 /// **Example:**
  83 /// ```rust
  84 /// fn return_unit() -> () {
  85 ///     ()
  86 /// }
  87 /// ```
  88 declare_clippy_lint! {
  89     pub UNUSED_UNIT,
  90     style,
  91     "needless unit expression"
  92 }
  93
  94 #[derive(Copy, Clone)]
  95 pub struct ReturnPass;
  96
  97 impl ReturnPass {
  98     // Check the final stmt or expr in a block for unnecessary return.
  99     fn check_block_return(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
 100         if let Some(stmt) = block.stmts.last() {
 101             match stmt.node {
 102                 ast::StmtKind::Expr(ref expr) | ast::StmtKind::Semi(ref expr) => {
 103                     self.check_final_expr(cx, expr, Some(stmt.span));
 104                 },
 105                 _ => (),
 106             }
 107         }
 108     }
 109
 110     // Check a the final expression in a block if it's a return.
 111     fn check_final_expr(&mut self, cx: &EarlyContext<'_>, expr: &ast::Expr, span: Option<Span>) {
 112         match expr.node {
 113             // simple return is always "bad"
 114             ast::ExprKind::Ret(Some(ref inner)) => {
 115                 // allow `#[cfg(a)] return a; #[cfg(b)] return b;`
 116                 if !expr.attrs.iter().any(attr_is_cfg) {
 117                     self.emit_return_lint(cx, span.expect("`else return` is not possible"), inner.span);
 118                 }
 119             },
 120             // a whole block? check it!
 121             ast::ExprKind::Block(ref block, _) => {
 122                 self.check_block_return(cx, block);
 123             },
 124             // an if/if let expr, check both exprs
 125             // note, if without else is going to be a type checking error anyways
 126             // (except for unit type functions) so we don't match it
 127             ast::ExprKind::If(_, ref ifblock, Some(ref elsexpr)) => {
 128                 self.check_block_return(cx, ifblock);
 129                 self.check_final_expr(cx, elsexpr, None);
 130             },
 131             // a match expr, check all arms
 132             ast::ExprKind::Match(_, ref arms) => {
 133                 for arm in arms {
 134                     self.check_final_expr(cx, &arm.body, Some(arm.body.span));
 135                 }
 136             },
 137             _ => (),
 138         }
 139     }
 140
 141     fn emit_return_lint(&mut self, cx: &EarlyContext<'_>, ret_span: Span, inner_span: Span) {
 142         if in_external_macro(cx.sess(), inner_span) || in_macro(inner_span) {
 143             return;
 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_with_applicability(
 148                     ret_span,
 149                     "remove `return` as shown",
 150                     snippet,
 151                     Applicability::MachineApplicable,
 152                 );
 153             }
 154         });
 155     }
 156
 157     // Check for "let x = EXPR; x"
 158     fn check_let_return(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
 159         let mut it = block.stmts.iter();
 160
 161         // we need both a let-binding stmt and an expr
 162         if_chain! {
 163             if let Some(retexpr) = it.next_back();
 164             if let ast::StmtKind::Expr(ref retexpr) = retexpr.node;
 165             if let Some(stmt) = it.next_back();
 166             if let ast::StmtKind::Local(ref local) = stmt.node;
 167             // don't lint in the presence of type inference
 168             if local.ty.is_none();
 169             if !local.attrs.iter().any(attr_is_cfg);
 170             if let Some(ref initexpr) = local.init;
 171             if let ast::PatKind::Ident(_, ident, _) = local.pat.node;
 172             if let ast::ExprKind::Path(_, ref path) = retexpr.node;
 173             if match_path_ast(path, &[&ident.as_str()]);
 174             if !in_external_macro(cx.sess(), initexpr.span);
 175             then {
 176                     span_note_and_lint(cx,
 177                                        LET_AND_RETURN,
 178                                        retexpr.span,
 179                                        "returning the result of a let binding from a block. \
 180                                        Consider returning the expression directly.",
 181                                        initexpr.span,
 182                                        "this expression can be directly returned");
 183             }
 184         }
 185     }
 186 }
 187
 188 impl LintPass for ReturnPass {
 189     fn get_lints(&self) -> LintArray {
 190         lint_array!(NEEDLESS_RETURN, LET_AND_RETURN, UNUSED_UNIT)
 191     }
 192 }
 193
 194 impl EarlyLintPass for ReturnPass {
 195     fn check_fn(&mut self, cx: &EarlyContext<'_>, kind: FnKind<'_>, decl: &ast::FnDecl, span: Span, _: ast::NodeId) {
 196         match kind {
 197             FnKind::ItemFn(.., block) | FnKind::Method(.., block) => self.check_block_return(cx, block),
 198             FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span)),
 199         }
 200         if_chain! {
 201             if let ast::FunctionRetTy::Ty(ref ty) = decl.output;
 202             if let ast::TyKind::Tup(ref vals) = ty.node;
 203             if vals.is_empty() && !in_macro(ty.span) && get_def(span) == get_def(ty.span);
 204             then {
 205                 let (rspan, appl) = if let Ok(fn_source) =
 206                         cx.sess().source_map()
 207                                  .span_to_snippet(span.with_hi(ty.span.hi())) {
 208                     if let Some(rpos) = fn_source.rfind("->") {
 209                         #[allow(clippy::cast_possible_truncation)]
 210                         (ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
 211                             Applicability::MachineApplicable)
 212                     } else {
 213                         (ty.span, Applicability::MaybeIncorrect)
 214                     }
 215                 } else {
 216                     (ty.span, Applicability::MaybeIncorrect)
 217                 };
 218                 span_lint_and_then(cx, UNUSED_UNIT, rspan, "unneeded unit return type", |db| {
 219                     db.span_suggestion_with_applicability(
 220                         rspan,
 221                         "remove the `-> ()`",
 222                         String::new(),
 223                         appl,
 224                     );
 225                 });
 226             }
 227         }
 228     }
 229
 230     fn check_block(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
 231         self.check_let_return(cx, block);
 232         if_chain! {
 233             if let Some(ref stmt) = block.stmts.last();
 234             if let ast::StmtKind::Expr(ref expr) = stmt.node;
 235             if is_unit_expr(expr) && !in_macro(expr.span);
 236             then {
 237                 let sp = expr.span;
 238                 span_lint_and_then(cx, UNUSED_UNIT, sp, "unneeded unit expression", |db| {
 239                     db.span_suggestion_with_applicability(
 240                         sp,
 241                         "remove the final `()`",
 242                         String::new(),
 243                         Applicability::MachineApplicable,
 244                     );
 245                 });
 246             }
 247         }
 248     }
 249
 250     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
 251         match e.node {
 252             ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
 253                 if is_unit_expr(expr) && !in_macro(expr.span) {
 254                     span_lint_and_then(cx, UNUSED_UNIT, expr.span, "unneeded `()`", |db| {
 255                         db.span_suggestion_with_applicability(
 256                             expr.span,
 257                             "remove the `()`",
 258                             String::new(),
 259                             Applicability::MachineApplicable,
 260                         );
 261                     });
 262                 }
 263             },
 264             _ => (),
 265         }
 266     }
 267 }
 268
 269 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
 270     attr.meta_item_list().is_some() && attr.name() == "cfg"
 271 }
 272
 273 // get the def site
 274 fn get_def(span: Span) -> Option<Span> {
 275     span.ctxt().outer().expn_info().and_then(|info| info.def_site)
 276 }
 277
 278 // is this expr a `()` unit?
 279 fn is_unit_expr(expr: &ast::Expr) -> bool {
 280     if let ast::ExprKind::Tup(ref vals) = expr.node {
 281         vals.is_empty()
 282     } else {
 283         false
 284     }
 285 }