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