clippy_lints/src/get_first.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::source::snippet_with_applicability;
   3 use clippy_utils::{is_slice_of_primitives, match_def_path, paths};
   4 use if_chain::if_chain;
   5 use rustc_ast::LitKind;
   6 use rustc_errors::Applicability;
   7 use rustc_hir as hir;
   8 use rustc_lint::{LateContext, LateLintPass};
   9 use rustc_session::{declare_lint_pass, declare_tool_lint};
  10 use rustc_span::source_map::Spanned;
  11
  12 declare_clippy_lint! {
  13     /// ### What it does
  14     /// Checks for using `x.get(0)` instead of
  15     /// `x.first()`.
  16     ///
  17     /// ### Why is this bad?
  18     /// Using `x.first()` is easier to read and has the same
  19     /// result.
  20     ///
  21     /// ### Example
  22     /// ```rust
  23     /// // Bad
  24     /// let x = vec![2, 3, 5];
  25     /// let first_element = x.get(0);
  26     /// ```
  27     /// Use instead:
  28     /// ```rust
  29     /// // Good
  30     /// let x = vec![2, 3, 5];
  31     /// let first_element = x.first();
  32     /// ```
  33     #[clippy::version = "1.63.0"]
  34     pub GET_FIRST,
  35     style,
  36     "Using `x.get(0)` when `x.first()` is simpler"
  37 }
  38 declare_lint_pass!(GetFirst => [GET_FIRST]);
  39
  40 impl<'tcx> LateLintPass<'tcx> for GetFirst {
  41     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>) {
  42         if_chain! {
  43             if let hir::ExprKind::MethodCall(_, [struct_calling_on, method_arg], _) = &expr.kind;
  44             if let Some(expr_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
  45             if match_def_path(cx, expr_def_id, &paths::SLICE_GET);
  46
  47             if let Some(_) = is_slice_of_primitives(cx, struct_calling_on);
  48             if let hir::ExprKind::Lit(Spanned { node: LitKind::Int(0, _), .. }) = method_arg.kind;
  49
  50             then {
  51                 let mut applicability = Applicability::MachineApplicable;
  52                 let slice_name = snippet_with_applicability(
  53                     cx,
  54                     struct_calling_on.span, "..",
  55                     &mut applicability,
  56                 );
  57                 span_lint_and_sugg(
  58                     cx,
  59                     GET_FIRST,
  60                     expr.span,
  61                     &format!("accessing first element with `{0}.get(0)`", slice_name),
  62                     "try",
  63                     format!("{}.first()", slice_name),
  64                     applicability,
  65                 );
  66             }
  67         }
  68     }
  69 }