src/tools/clippy/clippy_lints/src/methods/map_flatten.rs

   1 use crate::utils::{is_type_diagnostic_item, match_trait_method, paths, snippet, span_lint_and_sugg};
   2 use rustc_errors::Applicability;
   3 use rustc_hir as hir;
   4 use rustc_lint::LateContext;
   5 use rustc_middle::ty;
   6 use rustc_span::symbol::sym;
   7
   8 use super::MAP_FLATTEN;
   9
  10 /// lint use of `map().flatten()` for `Iterators` and 'Options'
  11 pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, expr: &'tcx hir::Expr<'_>, map_args: &'tcx [hir::Expr<'_>]) {
  12     // lint if caller of `.map().flatten()` is an Iterator
  13     if match_trait_method(cx, expr, &paths::ITERATOR) {
  14         let map_closure_ty = cx.typeck_results().expr_ty(&map_args[1]);
  15         let is_map_to_option = match map_closure_ty.kind() {
  16             ty::Closure(_, _) | ty::FnDef(_, _) | ty::FnPtr(_) => {
  17                 let map_closure_sig = match map_closure_ty.kind() {
  18                     ty::Closure(_, substs) => substs.as_closure().sig(),
  19                     _ => map_closure_ty.fn_sig(cx.tcx),
  20                 };
  21                 let map_closure_return_ty = cx.tcx.erase_late_bound_regions(map_closure_sig.output());
  22                 is_type_diagnostic_item(cx, map_closure_return_ty, sym::option_type)
  23             },
  24             _ => false,
  25         };
  26
  27         let method_to_use = if is_map_to_option {
  28             // `(...).map(...)` has type `impl Iterator<Item=Option<...>>
  29             "filter_map"
  30         } else {
  31             // `(...).map(...)` has type `impl Iterator<Item=impl Iterator<...>>
  32             "flat_map"
  33         };
  34         let func_snippet = snippet(cx, map_args[1].span, "..");
  35         let hint = format!(".{0}({1})", method_to_use, func_snippet);
  36         span_lint_and_sugg(
  37             cx,
  38             MAP_FLATTEN,
  39             expr.span.with_lo(map_args[0].span.hi()),
  40             "called `map(..).flatten()` on an `Iterator`",
  41             &format!("try using `{}` instead", method_to_use),
  42             hint,
  43             Applicability::MachineApplicable,
  44         );
  45     }
  46
  47     // lint if caller of `.map().flatten()` is an Option
  48     if is_type_diagnostic_item(cx, cx.typeck_results().expr_ty(&map_args[0]), sym::option_type) {
  49         let func_snippet = snippet(cx, map_args[1].span, "..");
  50         let hint = format!(".and_then({})", func_snippet);
  51         span_lint_and_sugg(
  52             cx,
  53             MAP_FLATTEN,
  54             expr.span.with_lo(map_args[0].span.hi()),
  55             "called `map(..).flatten()` on an `Option`",
  56             "try using `and_then` instead",
  57             hint,
  58             Applicability::MachineApplicable,
  59         );
  60     }
  61 }