clippy_lints/src/returns.rs

   1 use if_chain::if_chain;
   2 use rustc_ast::ast::Attribute;
   3 use rustc_errors::Applicability;
   4 use rustc_hir::intravisit::{walk_expr, FnKind, NestedVisitorMap, Visitor};
   5 use rustc_hir::{Block, Body, Expr, ExprKind, FnDecl, HirId, MatchSource, PatKind, StmtKind};
   6 use rustc_lint::{LateContext, LateLintPass, LintContext};
   7 use rustc_middle::hir::map::Map;
   8 use rustc_middle::lint::in_external_macro;
   9 use rustc_middle::ty::subst::GenericArgKind;
  10 use rustc_session::{declare_lint_pass, declare_tool_lint};
  11 use rustc_span::source_map::Span;
  12 use rustc_span::sym;
  13
  14 use crate::utils::{fn_def_id, in_macro, match_qpath, snippet_opt, span_lint_and_sugg, span_lint_and_then};
  15
  16 declare_clippy_lint! {
  17     /// **What it does:** Checks for `let`-bindings, which are subsequently
  18     /// returned.
  19     ///
  20     /// **Why is this bad?** It is just extraneous code. Remove it to make your code
  21     /// more rusty.
  22     ///
  23     /// **Known problems:** None.
  24     ///
  25     /// **Example:**
  26     /// ```rust
  27     /// fn foo() -> String {
  28     ///     let x = String::new();
  29     ///     x
  30     /// }
  31     /// ```
  32     /// instead, use
  33     /// ```
  34     /// fn foo() -> String {
  35     ///     String::new()
  36     /// }
  37     /// ```
  38     pub LET_AND_RETURN,
  39     style,
  40     "creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block"
  41 }
  42
  43 declare_clippy_lint! {
  44     /// **What it does:** Checks for return statements at the end of a block.
  45     ///
  46     /// **Why is this bad?** Removing the `return` and semicolon will make the code
  47     /// more rusty.
  48     ///
  49     /// **Known problems:** None.
  50     ///
  51     /// **Example:**
  52     /// ```rust
  53     /// fn foo(x: usize) -> usize {
  54     ///     return x;
  55     /// }
  56     /// ```
  57     /// simplify to
  58     /// ```rust
  59     /// fn foo(x: usize) -> usize {
  60     ///     x
  61     /// }
  62     /// ```
  63     pub NEEDLESS_RETURN,
  64     style,
  65     "using a return statement like `return expr;` where an expression would suffice"
  66 }
  67
  68 #[derive(PartialEq, Eq, Copy, Clone)]
  69 enum RetReplacement {
  70     Empty,
  71     Block,
  72 }
  73
  74 declare_lint_pass!(Return => [LET_AND_RETURN, NEEDLESS_RETURN]);
  75
  76 impl<'tcx> LateLintPass<'tcx> for Return {
  77     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'_>) {
  78         // we need both a let-binding stmt and an expr
  79         if_chain! {
  80             if let Some(retexpr) = block.expr;
  81             if let Some(stmt) = block.stmts.iter().last();
  82             if let StmtKind::Local(local) = &stmt.kind;
  83             if local.ty.is_none();
  84             if local.attrs.is_empty();
  85             if let Some(initexpr) = &local.init;
  86             if let PatKind::Binding(.., ident, _) = local.pat.kind;
  87             if let ExprKind::Path(qpath) = &retexpr.kind;
  88             if match_qpath(qpath, &[&*ident.name.as_str()]);
  89             if !last_statement_borrows(cx, initexpr);
  90             if !in_external_macro(cx.sess(), initexpr.span);
  91             if !in_external_macro(cx.sess(), retexpr.span);
  92             if !in_external_macro(cx.sess(), local.span);
  93             if !in_macro(local.span);
  94             then {
  95                 span_lint_and_then(
  96                     cx,
  97                     LET_AND_RETURN,
  98                     retexpr.span,
  99                     "returning the result of a `let` binding from a block",
 100                     |err| {
 101                         err.span_label(local.span, "unnecessary `let` binding");
 102
 103                         if let Some(mut snippet) = snippet_opt(cx, initexpr.span) {
 104                             if !cx.typeck_results().expr_adjustments(&retexpr).is_empty() {
 105                                 snippet.push_str(" as _");
 106                             }
 107                             err.multipart_suggestion(
 108                                 "return the expression directly",
 109                                 vec![
 110                                     (local.span, String::new()),
 111                                     (retexpr.span, snippet),
 112                                 ],
 113                                 Applicability::MachineApplicable,
 114                             );
 115                         } else {
 116                             err.span_help(initexpr.span, "this expression can be directly returned");
 117                         }
 118                     },
 119                 );
 120             }
 121         }
 122     }
 123
 124     fn check_fn(
 125         &mut self,
 126         cx: &LateContext<'tcx>,
 127         kind: FnKind<'tcx>,
 128         _: &'tcx FnDecl<'tcx>,
 129         body: &'tcx Body<'tcx>,
 130         _: Span,
 131         _: HirId,
 132     ) {
 133         match kind {
 134             FnKind::Closure(_) => check_final_expr(cx, &body.value, Some(body.value.span), RetReplacement::Empty),
 135             FnKind::ItemFn(..) | FnKind::Method(..) => {
 136                 if let ExprKind::Block(ref block, _) = body.value.kind {
 137                     check_block_return(cx, block);
 138                 }
 139             },
 140         }
 141     }
 142 }
 143
 144 fn attr_is_cfg(attr: &Attribute) -> bool {
 145     attr.meta_item_list().is_some() && attr.has_name(sym::cfg)
 146 }
 147
 148 fn check_block_return<'tcx>(cx: &LateContext<'tcx>, block: &Block<'tcx>) {
 149     if let Some(expr) = block.expr {
 150         check_final_expr(cx, expr, Some(expr.span), RetReplacement::Empty);
 151     } else if let Some(stmt) = block.stmts.iter().last() {
 152         match stmt.kind {
 153             StmtKind::Expr(ref expr) | StmtKind::Semi(ref expr) => {
 154                 check_final_expr(cx, expr, Some(stmt.span), RetReplacement::Empty);
 155             },
 156             _ => (),
 157         }
 158     }
 159 }
 160
 161 fn check_final_expr<'tcx>(
 162     cx: &LateContext<'tcx>,
 163     expr: &'tcx Expr<'tcx>,
 164     span: Option<Span>,
 165     replacement: RetReplacement,
 166 ) {
 167     match expr.kind {
 168         // simple return is always "bad"
 169         ExprKind::Ret(ref inner) => {
 170             // allow `#[cfg(a)] return a; #[cfg(b)] return b;`
 171             if !expr.attrs.iter().any(attr_is_cfg) {
 172                 let borrows = inner.map_or(false, |inner| last_statement_borrows(cx, inner));
 173                 if !borrows {
 174                     emit_return_lint(
 175                         cx,
 176                         span.expect("`else return` is not possible"),
 177                         inner.as_ref().map(|i| i.span),
 178                         replacement,
 179                     );
 180                 }
 181             }
 182         },
 183         // a whole block? check it!
 184         ExprKind::Block(ref block, _) => {
 185             check_block_return(cx, block);
 186         },
 187         ExprKind::If(_, then, else_clause_opt) => {
 188             if let ExprKind::Block(ref ifblock, _) = then.kind {
 189                 check_block_return(cx, ifblock);
 190             }
 191             if let Some(else_clause) = else_clause_opt {
 192                 check_final_expr(cx, else_clause, None, RetReplacement::Empty);
 193             }
 194         },
 195         // a match expr, check all arms
 196         // an if/if let expr, check both exprs
 197         // note, if without else is going to be a type checking error anyways
 198         // (except for unit type functions) so we don't match it
 199         ExprKind::Match(_, ref arms, source) => match source {
 200             MatchSource::Normal => {
 201                 for arm in arms.iter() {
 202                     check_final_expr(cx, &arm.body, Some(arm.body.span), RetReplacement::Block);
 203                 }
 204             },
 205             MatchSource::IfLetDesugar {
 206                 contains_else_clause: true,
 207             } => {
 208                 if let ExprKind::Block(ref ifblock, _) = arms[0].body.kind {
 209                     check_block_return(cx, ifblock);
 210                 }
 211                 check_final_expr(cx, arms[1].body, None, RetReplacement::Empty);
 212             },
 213             _ => (),
 214         },
 215         _ => (),
 216     }
 217 }
 218
 219 fn emit_return_lint(cx: &LateContext<'_>, ret_span: Span, inner_span: Option<Span>, replacement: RetReplacement) {
 220     if ret_span.from_expansion() {
 221         return;
 222     }
 223     match inner_span {
 224         Some(inner_span) => {
 225             if in_external_macro(cx.tcx.sess, inner_span) || inner_span.from_expansion() {
 226                 return;
 227             }
 228
 229             span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded `return` statement", |diag| {
 230                 if let Some(snippet) = snippet_opt(cx, inner_span) {
 231                     diag.span_suggestion(ret_span, "remove `return`", snippet, Applicability::MachineApplicable);
 232                 }
 233             })
 234         },
 235         None => match replacement {
 236             RetReplacement::Empty => {
 237                 span_lint_and_sugg(
 238                     cx,
 239                     NEEDLESS_RETURN,
 240                     ret_span,
 241                     "unneeded `return` statement",
 242                     "remove `return`",
 243                     String::new(),
 244                     Applicability::MachineApplicable,
 245                 );
 246             },
 247             RetReplacement::Block => {
 248                 span_lint_and_sugg(
 249                     cx,
 250                     NEEDLESS_RETURN,
 251                     ret_span,
 252                     "unneeded `return` statement",
 253                     "replace `return` with an empty block",
 254                     "{}".to_string(),
 255                     Applicability::MachineApplicable,
 256                 );
 257             },
 258         },
 259     }
 260 }
 261
 262 fn last_statement_borrows<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
 263     let mut visitor = BorrowVisitor { cx, borrows: false };
 264     walk_expr(&mut visitor, expr);
 265     visitor.borrows
 266 }
 267
 268 struct BorrowVisitor<'a, 'tcx> {
 269     cx: &'a LateContext<'tcx>,
 270     borrows: bool,
 271 }
 272
 273 impl<'tcx> Visitor<'tcx> for BorrowVisitor<'_, 'tcx> {
 274     type Map = Map<'tcx>;
 275
 276     fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
 277         if self.borrows {
 278             return;
 279         }
 280
 281         if let Some(def_id) = fn_def_id(self.cx, expr) {
 282             self.borrows = self
 283                 .cx
 284                 .tcx
 285                 .fn_sig(def_id)
 286                 .output()
 287                 .skip_binder()
 288                 .walk()
 289                 .any(|arg| matches!(arg.unpack(), GenericArgKind::Lifetime(_)));
 290         }
 291
 292         walk_expr(self, expr);
 293     }
 294
 295     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 296         NestedVisitorMap::None
 297     }
 298 }