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 if_chain::if_chain;
   4 use rustc_ast::ast::Attribute;
   5 use rustc_errors::Applicability;
   6 use rustc_hir::intravisit::{walk_expr, FnKind, NestedVisitorMap, Visitor};
   7 use rustc_hir::{Block, Body, Expr, ExprKind, FnDecl, HirId, MatchSource, PatKind, StmtKind};
   8 use rustc_lint::{LateContext, LateLintPass, LintContext};
   9 use rustc_middle::hir::map::Map;
  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::source_map::Span;
  14 use rustc_span::sym;
  15
  16 use crate::utils::{fn_def_id, in_macro, match_qpath};
  17
  18 declare_clippy_lint! {
  19     /// **What it does:** Checks for `let`-bindings, which are subsequently
  20     /// returned.
  21     ///
  22     /// **Why is this bad?** It is just extraneous code. Remove it to make your code
  23     /// more rusty.
  24     ///
  25     /// **Known problems:** None.
  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     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:** Checks for return statements at the end of a block.
  47     ///
  48     /// **Why is this bad?** Removing the `return` and semicolon will make the code
  49     /// more rusty.
  50     ///
  51     /// **Known problems:** None.
  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     pub NEEDLESS_RETURN,
  66     style,
  67     "using a return statement like `return expr;` where an expression would suffice"
  68 }
  69
  70 #[derive(PartialEq, Eq, Copy, Clone)]
  71 enum RetReplacement {
  72     Empty,
  73     Block,
  74 }
  75
  76 declare_lint_pass!(Return => [LET_AND_RETURN, NEEDLESS_RETURN]);
  77
  78 impl<'tcx> LateLintPass<'tcx> for Return {
  79     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'_>) {
  80         // we need both a let-binding stmt and an expr
  81         if_chain! {
  82             if let Some(retexpr) = block.expr;
  83             if let Some(stmt) = block.stmts.iter().last();
  84             if let StmtKind::Local(local) = &stmt.kind;
  85             if local.ty.is_none();
  86             if cx.tcx.hir().attrs(local.hir_id).is_empty();
  87             if let Some(initexpr) = &local.init;
  88             if let PatKind::Binding(.., ident, _) = local.pat.kind;
  89             if let ExprKind::Path(qpath) = &retexpr.kind;
  90             if match_qpath(qpath, &[&*ident.name.as_str()]);
  91             if !last_statement_borrows(cx, initexpr);
  92             if !in_external_macro(cx.sess(), initexpr.span);
  93             if !in_external_macro(cx.sess(), retexpr.span);
  94             if !in_external_macro(cx.sess(), local.span);
  95             if !in_macro(local.span);
  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(ref 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(ref expr) | StmtKind::Semi(ref 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(ref block, _) => {
 197             check_block_return(cx, block);
 198         },
 199         ExprKind::If(_, then, else_clause_opt) => {
 200             if let ExprKind::Block(ref ifblock, _) = then.kind {
 201                 check_block_return(cx, ifblock);
 202             }
 203             if let Some(else_clause) = else_clause_opt {
 204                 check_final_expr(cx, else_clause, None, RetReplacement::Empty);
 205             }
 206         },
 207         // a match expr, check all arms
 208         // an if/if let expr, check both exprs
 209         // note, if without else is going to be a type checking error anyways
 210         // (except for unit type functions) so we don't match it
 211         ExprKind::Match(_, ref arms, source) => match source {
 212             MatchSource::Normal => {
 213                 for arm in arms.iter() {
 214                     check_final_expr(cx, &arm.body, Some(arm.body.span), RetReplacement::Block);
 215                 }
 216             },
 217             MatchSource::IfLetDesugar {
 218                 contains_else_clause: true,
 219             } => {
 220                 if let ExprKind::Block(ref ifblock, _) = arms[0].body.kind {
 221                     check_block_return(cx, ifblock);
 222                 }
 223                 check_final_expr(cx, arms[1].body, None, RetReplacement::Empty);
 224             },
 225             _ => (),
 226         },
 227         _ => (),
 228     }
 229 }
 230
 231 fn emit_return_lint(cx: &LateContext<'_>, ret_span: Span, inner_span: Option<Span>, replacement: RetReplacement) {
 232     if ret_span.from_expansion() {
 233         return;
 234     }
 235     match inner_span {
 236         Some(inner_span) => {
 237             if in_external_macro(cx.tcx.sess, inner_span) || inner_span.from_expansion() {
 238                 return;
 239             }
 240
 241             span_lint_and_then(cx, NEEDLESS_RETURN, ret_span, "unneeded `return` statement", |diag| {
 242                 if let Some(snippet) = snippet_opt(cx, inner_span) {
 243                     diag.span_suggestion(ret_span, "remove `return`", snippet, Applicability::MachineApplicable);
 244                 }
 245             })
 246         },
 247         None => match replacement {
 248             RetReplacement::Empty => {
 249                 span_lint_and_sugg(
 250                     cx,
 251                     NEEDLESS_RETURN,
 252                     ret_span,
 253                     "unneeded `return` statement",
 254                     "remove `return`",
 255                     String::new(),
 256                     Applicability::MachineApplicable,
 257                 );
 258             },
 259             RetReplacement::Block => {
 260                 span_lint_and_sugg(
 261                     cx,
 262                     NEEDLESS_RETURN,
 263                     ret_span,
 264                     "unneeded `return` statement",
 265                     "replace `return` with an empty block",
 266                     "{}".to_string(),
 267                     Applicability::MachineApplicable,
 268                 );
 269             },
 270         },
 271     }
 272 }
 273
 274 fn last_statement_borrows<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
 275     let mut visitor = BorrowVisitor { cx, borrows: false };
 276     walk_expr(&mut visitor, expr);
 277     visitor.borrows
 278 }
 279
 280 struct BorrowVisitor<'a, 'tcx> {
 281     cx: &'a LateContext<'tcx>,
 282     borrows: bool,
 283 }
 284
 285 impl<'tcx> Visitor<'tcx> for BorrowVisitor<'_, 'tcx> {
 286     type Map = Map<'tcx>;
 287
 288     fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
 289         if self.borrows {
 290             return;
 291         }
 292
 293         if let Some(def_id) = fn_def_id(self.cx, expr) {
 294             self.borrows = self
 295                 .cx
 296                 .tcx
 297                 .fn_sig(def_id)
 298                 .output()
 299                 .skip_binder()
 300                 .walk()
 301                 .any(|arg| matches!(arg.unpack(), GenericArgKind::Lifetime(_)));
 302         }
 303
 304         walk_expr(self, expr);
 305     }
 306
 307     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 308         NestedVisitorMap::None
 309     }
 310 }