clippy_lints/src/methods/map_flatten.rs

   1 use clippy_utils::diagnostics::span_lint_and_sugg;
   2 use clippy_utils::is_trait_method;
   3 use clippy_utils::source::snippet;
   4 use clippy_utils::ty::is_type_diagnostic_item;
   5 use rustc_errors::Applicability;
   6 use rustc_hir as hir;
   7 use rustc_lint::LateContext;
   8 use rustc_middle::ty;
   9 use rustc_span::symbol::sym;
  10
  11 use super::MAP_FLATTEN;
  12
  13 /// lint use of `map().flatten()` for `Iterators` and 'Options'
  14 pub(super) fn check<'tcx>(
  15     cx: &LateContext<'tcx>,
  16     expr: &'tcx hir::Expr<'_>,
  17     recv: &'tcx hir::Expr<'_>,
  18     map_arg: &'tcx hir::Expr<'_>,
  19 ) {
  20     // lint if caller of `.map().flatten()` is an Iterator
  21     if is_trait_method(cx, expr, sym::Iterator) {
  22         let map_closure_ty = cx.typeck_results().expr_ty(map_arg);
  23         let is_map_to_option = match map_closure_ty.kind() {
  24             ty::Closure(_, _) | ty::FnDef(_, _) | ty::FnPtr(_) => {
  25                 let map_closure_sig = match map_closure_ty.kind() {
  26                     ty::Closure(_, substs) => substs.as_closure().sig(),
  27                     _ => map_closure_ty.fn_sig(cx.tcx),
  28                 };
  29                 let map_closure_return_ty = cx.tcx.erase_late_bound_regions(map_closure_sig.output());
  30                 is_type_diagnostic_item(cx, map_closure_return_ty, sym::Option)
  31             },
  32             _ => false,
  33         };
  34
  35         let method_to_use = if is_map_to_option {
  36             // `(...).map(...)` has type `impl Iterator<Item=Option<...>>
  37             "filter_map"
  38         } else {
  39             // `(...).map(...)` has type `impl Iterator<Item=impl Iterator<...>>
  40             "flat_map"
  41         };
  42         let func_snippet = snippet(cx, map_arg.span, "..");
  43         let hint = format!(".{0}({1})", method_to_use, func_snippet);
  44         span_lint_and_sugg(
  45             cx,
  46             MAP_FLATTEN,
  47             expr.span.with_lo(recv.span.hi()),
  48             "called `map(..).flatten()` on an `Iterator`",
  49             &format!("try using `{}` instead", method_to_use),
  50             hint,
  51             Applicability::MachineApplicable,
  52         );
  53     }
  54
  55     // lint if caller of `.map().flatten()` is an Option or Result
  56     let caller_type = match cx.typeck_results().expr_ty(recv).kind() {
  57         ty::Adt(adt, _) => {
  58             if cx.tcx.is_diagnostic_item(sym::Option, adt.did) {
  59                 "Option"
  60             } else if cx.tcx.is_diagnostic_item(sym::Result, adt.did) {
  61                 "Result"
  62             } else {
  63                 return;
  64             }
  65         },
  66         _ => {
  67             return;
  68         },
  69     };
  70
  71     let func_snippet = snippet(cx, map_arg.span, "..");
  72     let hint = format!(".and_then({})", func_snippet);
  73     let lint_info = format!("called `map(..).flatten()` on an `{}`", caller_type);
  74     span_lint_and_sugg(
  75         cx,
  76         MAP_FLATTEN,
  77         expr.span.with_lo(recv.span.hi()),
  78         &lint_info,
  79         "try using `and_then` instead",
  80         hint,
  81         Applicability::MachineApplicable,
  82     );
  83 }