clippy_lints/src/returns.rs

   1 use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_and_then};
   2 use clippy_utils::source::snippet_opt;
   3 use clippy_utils::{fn_def_id, path_to_local_id};
   4 use if_chain::if_chain;
   5 use rustc_ast::ast::Attribute;
   6 use rustc_errors::Applicability;
   7 use rustc_hir::intravisit::{walk_expr, FnKind, NestedVisitorMap, Visitor};
   8 use rustc_hir::{Block, Body, Expr, ExprKind, FnDecl, HirId, MatchSource, PatKind, StmtKind};
   9 use rustc_lint::{LateContext, LateLintPass, LintContext};
  10 use rustc_middle::hir::map::Map;
  11 use rustc_middle::lint::in_external_macro;
  12 use rustc_middle::ty::subst::GenericArgKind;
  13 use rustc_session::{declare_lint_pass, declare_tool_lint};
  14 use rustc_span::hygiene::DesugaringKind;
  15 use rustc_span::source_map::Span;
  16 use rustc_span::sym;
  17
  18 declare_clippy_lint! {
  19     /// ### What it does
  20     /// Checks for `let`-bindings, which are subsequently
  21     /// returned.
  22     ///
  23     /// ### Why is this bad?
  24     /// It is just extraneous code. Remove it to make your code
  25     /// more rusty.
  26     ///
  27     /// ### Example
  28     /// ```rust
  29     /// fn foo() -> String {
  30     ///     let x = String::new();
  31     ///     x
  32     /// }
  33     /// ```
  34     /// instead, use
  35     /// ```
  36     /// fn foo() -> String {
  37     ///     String::new()
  38     /// }
  39     /// ```
  40     #[clippy::version = "pre 1.29.0"]
  41     pub LET_AND_RETURN,
  42     style,
  43     "creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block"
  44 }
  45
  46 declare_clippy_lint! {
  47     /// ### What it does
  48     /// Checks for return statements at the end of a block.
  49     ///
  50     /// ### Why is this bad?
  51     /// Removing the `return` and semicolon will make the code
  52     /// more rusty.
  53     ///
  54     /// ### Example
  55     /// ```rust
  56     /// fn foo(x: usize) -> usize {
  57     ///     return x;
  58     /// }
  59     /// ```
  60     /// simplify to
  61     /// ```rust
  62     /// fn foo(x: usize) -> usize {
  63     ///     x
  64     /// }
  65     /// ```
  66     #[clippy::version = "pre 1.29.0"]
  67     pub NEEDLESS_RETURN,
  68     style,
  69     "using a return statement like `return expr;` where an expression would suffice"
  70 }
  71
  72 #[derive(PartialEq, Eq, Copy, Clone)]
  73 enum RetReplacement {
  74     Empty,
  75     Block,
  76     Unit,
  77 }
  78
  79 declare_lint_pass!(Return => [LET_AND_RETURN, NEEDLESS_RETURN]);
  80
  81 impl<'tcx> LateLintPass<'tcx> for Return {
  82     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'_>) {
  83         // we need both a let-binding stmt and an expr
  84         if_chain! {
  85             if let Some(retexpr) = block.expr;
  86             if let Some(stmt) = block.stmts.iter().last();
  87             if let StmtKind::Local(local) = &stmt.kind;
  88             if local.ty.is_none();
  89             if cx.tcx.hir().attrs(local.hir_id).is_empty();
  90             if let Some(initexpr) = &local.init;
  91             if let PatKind::Binding(_, local_id, _, _) = local.pat.kind;
  92             if path_to_local_id(retexpr, local_id);
  93             if !last_statement_borrows(cx, initexpr);
  94             if !in_external_macro(cx.sess(), initexpr.span);
  95             if !in_external_macro(cx.sess(), retexpr.span);
  96             if !local.span.from_expansion();
  97             then {
  98                 span_lint_and_then(
  99                     cx,
 100                     LET_AND_RETURN,
 101                     retexpr.span,
 102                     "returning the result of a `let` binding from a block",
 103                     |err| {
 104                         err.span_label(local.span, "unnecessary `let` binding");
 105
 106                         if let Some(mut snippet) = snippet_opt(cx, initexpr.span) {
 107                             if !cx.typeck_results().expr_adjustments(retexpr).is_empty() {
 108                                 snippet.push_str(" as _");
 109                             }
 110                             err.multipart_suggestion(
 111                                 "return the expression directly",
 112                                 vec![
 113                                     (local.span, String::new()),
 114                                     (retexpr.span, snippet),
 115                                 ],
 116                                 Applicability::MachineApplicable,
 117                             );
 118                         } else {
 119                             err.span_help(initexpr.span, "this expression can be directly returned");
 120                         }
 121                     },
 122                 );
 123             }
 124         }
 125     }
 126
 127     fn check_fn(
 128         &mut self,
 129         cx: &LateContext<'tcx>,
 130         kind: FnKind<'tcx>,
 131         _: &'tcx FnDecl<'tcx>,
 132         body: &'tcx Body<'tcx>,
 133         _: Span,
 134         _: HirId,
 135     ) {
 136         match kind {
 137             FnKind::Closure => {
 138                 // when returning without value in closure, replace this `return`
 139                 // with an empty block to prevent invalid suggestion (see #6501)
 140                 let replacement = if let ExprKind::Ret(None) = &body.value.kind {
 141                     RetReplacement::Block
 142                 } else {
 143                     RetReplacement::Empty
 144                 };
 145                 check_final_expr(cx, &body.value, Some(body.value.span), replacement);
 146             },
 147             FnKind::ItemFn(..) | FnKind::Method(..) => {
 148                 if let ExprKind::Block(block, _) = body.value.kind {
 149                     check_block_return(cx, block);
 150                 }
 151             },
 152         }
 153     }
 154 }
 155
 156 fn attr_is_cfg(attr: &Attribute) -> bool {
 157     attr.meta_item_list().is_some() && attr.has_name(sym::cfg)
 158 }
 159
 160 fn check_block_return<'tcx>(cx: &LateContext<'tcx>, block: &Block<'tcx>) {
 161     if let Some(expr) = block.expr {
 162         check_final_expr(cx, expr, Some(expr.span), RetReplacement::Empty);
 163     } else if let Some(stmt) = block.stmts.iter().last() {
 164         match stmt.kind {
 165             StmtKind::Expr(expr) | StmtKind::Semi(expr) => {
 166                 check_final_expr(cx, expr, Some(stmt.span), RetReplacement::Empty);
 167             },
 168             _ => (),
 169         }
 170     }
 171 }
 172
 173 fn check_final_expr<'tcx>(
 174     cx: &LateContext<'tcx>,
 175     expr: &'tcx Expr<'tcx>,
 176     span: Option<Span>,
 177     replacement: RetReplacement,
 178 ) {
 179     match expr.kind {
 180         // simple return is always "bad"
 181         ExprKind::Ret(ref inner) => {
 182             // allow `#[cfg(a)] return a; #[cfg(b)] return b;`
 183             let attrs = cx.tcx.hir().attrs(expr.hir_id);
 184             if !attrs.iter().any(attr_is_cfg) {
 185                 let borrows = inner.map_or(false, |inner| last_statement_borrows(cx, inner));
 186                 if !borrows {
 187                     emit_return_lint(
 188                         cx,
 189                         span.expect("`else return` is not possible"),
 190                         inner.as_ref().map(|i| i.span),
 191                         replacement,
 192                     );
 193                 }
 194             }
 195         },
 196         // a whole block? check it!
 197         ExprKind::Block(block, _) => {
 198             check_block_return(cx, block);
 199         },
 200         ExprKind::If(_, then, else_clause_opt) => {
 201             if let ExprKind::Block(ifblock, _) = then.kind {
 202                 check_block_return(cx, ifblock);
 203             }
 204             if let Some(else_clause) = else_clause_opt {
 205                 if expr.span.desugaring_kind() != Some(DesugaringKind::LetElse) {
 206                     check_final_expr(cx, else_clause, None, RetReplacement::Empty);
 207                 }
 208             }
 209         },
 210         // a match expr, check all arms
 211         // an if/if let expr, check both exprs
 212         // note, if without else is going to be a type checking error anyways
 213         // (except for unit type functions) so we don't match it
 214         ExprKind::Match(_, arms, MatchSource::Normal) => {
 215             for arm in arms.iter() {
 216                 check_final_expr(cx, arm.body, Some(arm.body.span), RetReplacement::Unit);
 217             }
 218         },
 219         ExprKind::DropTemps(expr) => check_final_expr(cx, expr, None, RetReplacement::Empty),
 220         _ => (),
 221     }
 222 }
 223
 224 fn emit_return_lint(cx: &LateContext<'_>, ret_span: Span, inner_span: Option<Span>, replacement: RetReplacement) {
 225     if ret_span.from_expansion() {
 226         return;
 227     }
 228     match inner_span {
 229         Some(inner_span) => {
 230             if in_external_macro(cx.tcx.sess, inner_span) || inner_span.from_expansion() {
 231                 return;
 232             }
 233
 234             span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded `return` statement", |diag| {
 235                 if let Some(snippet) = snippet_opt(cx, inner_span) {
 236                     diag.span_suggestion(ret_span, "remove `return`", snippet, Applicability::MachineApplicable);
 237                 }
 238             });
 239         },
 240         None => match replacement {
 241             RetReplacement::Empty => {
 242                 span_lint_and_sugg(
 243                     cx,
 244                     NEEDLESS_RETURN,
 245                     ret_span,
 246                     "unneeded `return` statement",
 247                     "remove `return`",
 248                     String::new(),
 249                     Applicability::MachineApplicable,
 250                 );
 251             },
 252             RetReplacement::Block => {
 253                 span_lint_and_sugg(
 254                     cx,
 255                     NEEDLESS_RETURN,
 256                     ret_span,
 257                     "unneeded `return` statement",
 258                     "replace `return` with an empty block",
 259                     "{}".to_string(),
 260                     Applicability::MachineApplicable,
 261                 );
 262             },
 263             RetReplacement::Unit => {
 264                 span_lint_and_sugg(
 265                     cx,
 266                     NEEDLESS_RETURN,
 267                     ret_span,
 268                     "unneeded `return` statement",
 269                     "replace `return` with a unit value",
 270                     "()".to_string(),
 271                     Applicability::MachineApplicable,
 272                 );
 273             },
 274         },
 275     }
 276 }
 277
 278 fn last_statement_borrows<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
 279     let mut visitor = BorrowVisitor { cx, borrows: false };
 280     walk_expr(&mut visitor, expr);
 281     visitor.borrows
 282 }
 283
 284 struct BorrowVisitor<'a, 'tcx> {
 285     cx: &'a LateContext<'tcx>,
 286     borrows: bool,
 287 }
 288
 289 impl<'tcx> Visitor<'tcx> for BorrowVisitor<'_, 'tcx> {
 290     type Map = Map<'tcx>;
 291
 292     fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
 293         if self.borrows {
 294             return;
 295         }
 296
 297         if let Some(def_id) = fn_def_id(self.cx, expr) {
 298             self.borrows = self
 299                 .cx
 300                 .tcx
 301                 .fn_sig(def_id)
 302                 .output()
 303                 .skip_binder()
 304                 .walk(self.cx.tcx)
 305                 .any(|arg| matches!(arg.unpack(), GenericArgKind::Lifetime(_)));
 306         }
 307
 308         walk_expr(self, expr);
 309     }
 310
 311     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 312         NestedVisitorMap::None
 313     }
 314 }