src/tools/clippy/clippy_lints/src/let_and_return.rs

   1 use if_chain::if_chain;
   2 use rustc_errors::Applicability;
   3 use rustc_hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
   4 use rustc_hir::{Block, Expr, ExprKind, PatKind, StmtKind};
   5 use rustc_lint::{LateContext, LateLintPass, LintContext};
   6 use rustc_middle::hir::map::Map;
   7 use rustc_middle::lint::in_external_macro;
   8 use rustc_middle::ty::subst::GenericArgKind;
   9 use rustc_session::{declare_lint_pass, declare_tool_lint};
  10
  11 use crate::utils::{fn_def_id, in_macro, match_qpath, snippet_opt, span_lint_and_then};
  12
  13 declare_clippy_lint! {
  14     /// **What it does:** Checks for `let`-bindings, which are subsequently
  15     /// returned.
  16     ///
  17     /// **Why is this bad?** It is just extraneous code. Remove it to make your code
  18     /// more rusty.
  19     ///
  20     /// **Known problems:** None.
  21     ///
  22     /// **Example:**
  23     /// ```rust
  24     /// fn foo() -> String {
  25     ///     let x = String::new();
  26     ///     x
  27     /// }
  28     /// ```
  29     /// instead, use
  30     /// ```
  31     /// fn foo() -> String {
  32     ///     String::new()
  33     /// }
  34     /// ```
  35     pub LET_AND_RETURN,
  36     style,
  37     "creating a let-binding and then immediately returning it like `let x = expr; x` at the end of a block"
  38 }
  39
  40 declare_lint_pass!(LetReturn => [LET_AND_RETURN]);
  41
  42 impl<'tcx> LateLintPass<'tcx> for LetReturn {
  43     fn check_block(&mut self, cx: &LateContext<'tcx>, block: &'tcx Block<'_>) {
  44         // we need both a let-binding stmt and an expr
  45         if_chain! {
  46             if let Some(retexpr) = block.expr;
  47             if let Some(stmt) = block.stmts.iter().last();
  48             if let StmtKind::Local(local) = &stmt.kind;
  49             if local.ty.is_none();
  50             if local.attrs.is_empty();
  51             if let Some(initexpr) = &local.init;
  52             if let PatKind::Binding(.., ident, _) = local.pat.kind;
  53             if let ExprKind::Path(qpath) = &retexpr.kind;
  54             if match_qpath(qpath, &[&*ident.name.as_str()]);
  55             if !last_statement_borrows(cx, initexpr);
  56             if !in_external_macro(cx.sess(), initexpr.span);
  57             if !in_external_macro(cx.sess(), retexpr.span);
  58             if !in_external_macro(cx.sess(), local.span);
  59             if !in_macro(local.span);
  60             then {
  61                 span_lint_and_then(
  62                     cx,
  63                     LET_AND_RETURN,
  64                     retexpr.span,
  65                     "returning the result of a `let` binding from a block",
  66                     |err| {
  67                         err.span_label(local.span, "unnecessary `let` binding");
  68
  69                         if let Some(snippet) = snippet_opt(cx, initexpr.span) {
  70                             err.multipart_suggestion(
  71                                 "return the expression directly",
  72                                 vec![
  73                                     (local.span, String::new()),
  74                                     (retexpr.span, snippet),
  75                                 ],
  76                                 Applicability::MachineApplicable,
  77                             );
  78                         } else {
  79                             err.span_help(initexpr.span, "this expression can be directly returned");
  80                         }
  81                     },
  82                 );
  83             }
  84         }
  85     }
  86 }
  87
  88 fn last_statement_borrows<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) -> bool {
  89     let mut visitor = BorrowVisitor { cx, borrows: false };
  90     walk_expr(&mut visitor, expr);
  91     visitor.borrows
  92 }
  93
  94 struct BorrowVisitor<'a, 'tcx> {
  95     cx: &'a LateContext<'tcx>,
  96     borrows: bool,
  97 }
  98
  99 impl<'tcx> Visitor<'tcx> for BorrowVisitor<'_, 'tcx> {
 100     type Map = Map<'tcx>;
 101
 102     fn visit_expr(&mut self, expr: &'tcx Expr<'_>) {
 103         if self.borrows {
 104             return;
 105         }
 106
 107         if let Some(def_id) = fn_def_id(self.cx, expr) {
 108             self.borrows = self
 109                 .cx
 110                 .tcx
 111                 .fn_sig(def_id)
 112                 .output()
 113                 .skip_binder()
 114                 .walk()
 115                 .any(|arg| matches!(arg.unpack(), GenericArgKind::Lifetime(_)));
 116         }
 117
 118         walk_expr(self, expr);
 119     }
 120
 121     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 122         NestedVisitorMap::None
 123     }
 124 }