src/tools/clippy/clippy_lints/src/default_instead_of_iter_empty.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::last_path_segment;
   3 use clippy_utils::source::snippet_with_applicability;
   4 use clippy_utils::{match_def_path, paths};
   5 use rustc_errors::Applicability;
   6 use rustc_hir::{def, Expr, ExprKind, GenericArg, QPath, TyKind};
   7 use rustc_lint::{LateContext, LateLintPass};
   8 use rustc_session::{declare_lint_pass, declare_tool_lint};
   9
  10 declare_clippy_lint! {
  11     /// ### What it does
  12     /// It checks for `std::iter::Empty::default()` and suggests replacing it with
  13     /// `std::iter::empty()`.
  14     /// ### Why is this bad?
  15     /// `std::iter::empty()` is the more idiomatic way.
  16     /// ### Example
  17     /// ```rust
  18     /// let _ = std::iter::Empty::<usize>::default();
  19     /// let iter: std::iter::Empty<usize> = std::iter::Empty::default();
  20     /// ```
  21     /// Use instead:
  22     /// ```rust
  23     /// let _ = std::iter::empty::<usize>();
  24     /// let iter: std::iter::Empty<usize> = std::iter::empty();
  25     /// ```
  26     #[clippy::version = "1.64.0"]
  27     pub DEFAULT_INSTEAD_OF_ITER_EMPTY,
  28     style,
  29     "check `std::iter::Empty::default()` and replace with `std::iter::empty()`"
  30 }
  31 declare_lint_pass!(DefaultIterEmpty => [DEFAULT_INSTEAD_OF_ITER_EMPTY]);
  32
  33 impl<'tcx> LateLintPass<'tcx> for DefaultIterEmpty {
  34     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
  35         if let ExprKind::Call(iter_expr, []) = &expr.kind
  36             && let ExprKind::Path(QPath::TypeRelative(ty, _)) = &iter_expr.kind
  37             && let TyKind::Path(ty_path) = &ty.kind
  38             && let QPath::Resolved(None, path) = ty_path
  39             && let def::Res::Def(_, def_id) = &path.res
  40             && match_def_path(cx, *def_id, &paths::ITER_EMPTY)
  41         {
  42             let mut applicability = Applicability::MachineApplicable;
  43             let sugg = make_sugg(cx, ty_path, &mut applicability);
  44             span_lint_and_sugg(
  45                 cx,
  46                 DEFAULT_INSTEAD_OF_ITER_EMPTY,
  47                 expr.span,
  48                 "`std::iter::empty()` is the more idiomatic way",
  49                 "try",
  50                 sugg,
  51                 applicability,
  52             );
  53         }
  54     }
  55 }
  56
  57 fn make_sugg(cx: &LateContext<'_>, ty_path: &rustc_hir::QPath<'_>, applicability: &mut Applicability) -> String {
  58     if let Some(last) = last_path_segment(ty_path).args
  59         && let Some(iter_ty) = last.args.iter().find_map(|arg| match arg {
  60             GenericArg::Type(ty) => Some(ty),
  61             _ => None,
  62         })
  63     {
  64         format!("std::iter::empty::<{}>()", snippet_with_applicability(cx, iter_ty.span, "..", applicability))
  65     } else {
  66         "std::iter::empty()".to_owned()
  67     }
  68 }