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
  11 use crate::rustc::lint::{EarlyContext, EarlyLintPass, LintArray, LintPass, in_external_macro, LintContext};
  12 use crate::rustc::{declare_tool_lint, lint_array};
  13 use if_chain::if_chain;
  14 use crate::syntax::ast;
  15 use crate::syntax::source_map::Span;
  16 use crate::syntax::visit::FnKind;
  17 use crate::syntax_pos::BytePos;
  18 use crate::rustc_errors::Applicability;
  19 use crate::utils::{in_macro, match_path_ast, snippet_opt, span_lint_and_then, span_note_and_lint};
  20
  21 /// **What it does:** Checks for return statements at the end of a block.
  22 ///
  23 /// **Why is this bad?** Removing the `return` and semicolon will make the code
  24 /// more rusty.
  25 ///
  26 /// **Known problems:** If the computation returning the value borrows a local
  27 /// variable, removing the `return` may run afoul of the borrow checker.
  28 ///
  29 /// **Example:**
  30 /// ```rust
  31 /// fn foo(x: usize) { return x; }
  32 /// ```
  33 /// simplify to
  34 /// ```rust
  35 /// fn foo(x: usize) { x }
  36 /// ```
  37 declare_clippy_lint! {
  38     pub NEEDLESS_RETURN,
  39     style,
  40     "using a return statement like `return expr;` where an expression would suffice"
  41 }
  42
  43 /// **What it does:** Checks for `let`-bindings, which are subsequently
  44 /// returned.
  45 ///
  46 /// **Why is this bad?** It is just extraneous code. Remove it to make your code
  47 /// more rusty.
  48 ///
  49 /// **Known problems:** None.
  50 ///
  51 /// **Example:**
  52 /// ```rust
  53 /// fn foo() -> String {
  54 ///    let x = String::new();
  55 ///    x
  56 ///}
  57 /// ```
  58 /// instead, use
  59 /// ```
  60 /// fn foo() -> String {
  61 ///    String::new()
  62 ///}
  63 /// ```
  64 declare_clippy_lint! {
  65     pub LET_AND_RETURN,
  66     style,
  67     "creating a let-binding and then immediately returning it like `let x = expr; x` at \
  68      the end of a block"
  69 }
  70
  71 /// **What it does:** Checks for unit (`()`) expressions that can be removed.
  72 ///
  73 /// **Why is this bad?** Such expressions add no value, but can make the code
  74 /// less readable. Depending on formatting they can make a `break` or `return`
  75 /// statement look like a function call.
  76 ///
  77 /// **Known problems:** The lint currently misses unit return types in types,
  78 /// e.g. the `F` in `fn generic_unit<F: Fn() -> ()>(f: F) { .. }`.
  79 ///
  80 /// **Example:**
  81 /// ```rust
  82 /// fn return_unit() -> () { () }
  83 /// ```
  84 declare_clippy_lint! {
  85     pub UNUSED_UNIT,
  86     style,
  87     "needless unit expression"
  88 }
  89
  90 #[derive(Copy, Clone)]
  91 pub struct ReturnPass;
  92
  93 impl ReturnPass {
  94     // Check the final stmt or expr in a block for unnecessary return.
  95     fn check_block_return(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
  96         if let Some(stmt) = block.stmts.last() {
  97             match stmt.node {
  98                 ast::StmtKind::Expr(ref expr) | ast::StmtKind::Semi(ref expr) => {
  99                     self.check_final_expr(cx, expr, Some(stmt.span));
 100                 },
 101                 _ => (),
 102             }
 103         }
 104     }
 105
 106     // Check a the final expression in a block if it's a return.
 107     fn check_final_expr(&mut self, cx: &EarlyContext<'_>, expr: &ast::Expr, span: Option<Span>) {
 108         match expr.node {
 109             // simple return is always "bad"
 110             ast::ExprKind::Ret(Some(ref inner)) => {
 111                 // allow `#[cfg(a)] return a; #[cfg(b)] return b;`
 112                 if !expr.attrs.iter().any(attr_is_cfg) {
 113                     self.emit_return_lint(cx, span.expect("`else return` is not possible"), inner.span);
 114                 }
 115             },
 116             // a whole block? check it!
 117             ast::ExprKind::Block(ref block, _) => {
 118                 self.check_block_return(cx, block);
 119             },
 120             // an if/if let expr, check both exprs
 121             // note, if without else is going to be a type checking error anyways
 122             // (except for unit type functions) so we don't match it
 123             ast::ExprKind::If(_, ref ifblock, Some(ref elsexpr)) => {
 124                 self.check_block_return(cx, ifblock);
 125                 self.check_final_expr(cx, elsexpr, None);
 126             },
 127             // a match expr, check all arms
 128             ast::ExprKind::Match(_, ref arms) => for arm in arms {
 129                 self.check_final_expr(cx, &arm.body, Some(arm.body.span));
 130             },
 131             _ => (),
 132         }
 133     }
 134
 135     fn emit_return_lint(&mut self, cx: &EarlyContext<'_>, ret_span: Span, inner_span: Span) {
 136         if in_external_macro(cx.sess(), inner_span) || in_macro(inner_span) {
 137             return;
 138         }
 139         span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded return statement", |db| {
 140             if let Some(snippet) = snippet_opt(cx, inner_span) {
 141                 db.span_suggestion_with_applicability(
 142                     ret_span,
 143                     "remove `return` as shown",
 144                     snippet,
 145                     Applicability::MachineApplicable,
 146                 );
 147             }
 148         });
 149     }
 150
 151     // Check for "let x = EXPR; x"
 152     fn check_let_return(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
 153         let mut it = block.stmts.iter();
 154
 155         // we need both a let-binding stmt and an expr
 156         if_chain! {
 157             if let Some(retexpr) = it.next_back();
 158             if let ast::StmtKind::Expr(ref retexpr) = retexpr.node;
 159             if let Some(stmt) = it.next_back();
 160             if let ast::StmtKind::Local(ref local) = stmt.node;
 161             // don't lint in the presence of type inference
 162             if local.ty.is_none();
 163             if !local.attrs.iter().any(attr_is_cfg);
 164             if let Some(ref initexpr) = local.init;
 165             if let ast::PatKind::Ident(_, ident, _) = local.pat.node;
 166             if let ast::ExprKind::Path(_, ref path) = retexpr.node;
 167             if match_path_ast(path, &[&ident.as_str()]);
 168             if !in_external_macro(cx.sess(), initexpr.span);
 169             then {
 170                     span_note_and_lint(cx,
 171                                        LET_AND_RETURN,
 172                                        retexpr.span,
 173                                        "returning the result of a let binding from a block. \
 174                                        Consider returning the expression directly.",
 175                                        initexpr.span,
 176                                        "this expression can be directly returned");
 177             }
 178         }
 179     }
 180 }
 181
 182 impl LintPass for ReturnPass {
 183     fn get_lints(&self) -> LintArray {
 184         lint_array!(NEEDLESS_RETURN, LET_AND_RETURN, UNUSED_UNIT)
 185     }
 186 }
 187
 188 impl EarlyLintPass for ReturnPass {
 189     fn check_fn(&mut self, cx: &EarlyContext<'_>, kind: FnKind<'_>, decl: &ast::FnDecl, span: Span, _: ast::NodeId) {
 190         match kind {
 191             FnKind::ItemFn(.., block) | FnKind::Method(.., block) => self.check_block_return(cx, block),
 192             FnKind::Closure(body) => self.check_final_expr(cx, body, Some(body.span)),
 193         }
 194         if_chain! {
 195             if let ast::FunctionRetTy::Ty(ref ty) = decl.output;
 196             if let ast::TyKind::Tup(ref vals) = ty.node;
 197             if vals.is_empty() && !in_macro(ty.span) && get_def(span) == get_def(ty.span);
 198             then {
 199                 let (rspan, appl) = if let Ok(fn_source) =
 200                         cx.sess().source_map()
 201                                  .span_to_snippet(span.with_hi(ty.span.hi())) {
 202                     if let Some(rpos) = fn_source.rfind("->") {
 203                         #[allow(clippy::cast_possible_truncation)]
 204                         (ty.span.with_lo(BytePos(span.lo().0 + rpos as u32)),
 205                             Applicability::MachineApplicable)
 206                     } else {
 207                         (ty.span, Applicability::MaybeIncorrect)
 208                     }
 209                 } else {
 210                     (ty.span, Applicability::MaybeIncorrect)
 211                 };
 212                 span_lint_and_then(cx, UNUSED_UNIT, rspan, "unneeded unit return type", |db| {
 213                     db.span_suggestion_with_applicability(
 214                         rspan,
 215                         "remove the `-> ()`",
 216                         String::new(),
 217                         appl,
 218                     );
 219                 });
 220             }
 221         }
 222     }
 223
 224     fn check_block(&mut self, cx: &EarlyContext<'_>, block: &ast::Block) {
 225         self.check_let_return(cx, block);
 226         if_chain! {
 227             if let Some(ref stmt) = block.stmts.last();
 228             if let ast::StmtKind::Expr(ref expr) = stmt.node;
 229             if is_unit_expr(expr) && !in_macro(expr.span);
 230             then {
 231                 let sp = expr.span;
 232                 span_lint_and_then(cx, UNUSED_UNIT, sp, "unneeded unit expression", |db| {
 233                     db.span_suggestion_with_applicability(
 234                         sp,
 235                         "remove the final `()`",
 236                         String::new(),
 237                         Applicability::MachineApplicable,
 238                     );
 239                 });
 240             }
 241         }
 242     }
 243
 244     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
 245         match e.node {
 246             ast::ExprKind::Ret(Some(ref expr)) | ast::ExprKind::Break(_, Some(ref expr)) => {
 247                 if is_unit_expr(expr) && !in_macro(expr.span) {
 248                     span_lint_and_then(cx, UNUSED_UNIT, expr.span, "unneeded `()`", |db| {
 249                         db.span_suggestion_with_applicability(
 250                             expr.span,
 251                             "remove the `()`",
 252                             String::new(),
 253                             Applicability::MachineApplicable,
 254                         );
 255                     });
 256                 }
 257             }
 258             _ => ()
 259         }
 260     }
 261 }
 262
 263 fn attr_is_cfg(attr: &ast::Attribute) -> bool {
 264     attr.meta_item_list().is_some() && attr.name() == "cfg"
 265 }
 266
 267 // get the def site
 268 fn get_def(span: Span) -> Option<Span> {
 269     span.ctxt().outer().expn_info().and_then(|info| info.def_site)
 270 }
 271
 272 // is this expr a `()` unit?
 273 fn is_unit_expr(expr: &ast::Expr) -> bool {
 274     if let ast::ExprKind::Tup(ref vals) = expr.node {
 275         vals.is_empty()
 276     } else {
 277         false
 278     }
 279 }