8733: Simplify code

Following the suggestions of @y21 in rust-lang#11987.

clippy_lints/src/splitting_strings_at_newlines.rs

@@ -1,14 +1,11 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
 use clippy_utils::source::snippet;
-use clippy_utils::ty::implements_trait;
 use rustc_ast::ast::LitKind;
 use rustc_errors::Applicability;
 use rustc_hir::{Expr, ExprKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
 
-use rustc_span::symbol::sym;
-
 declare_clippy_lint! {
     /// ### What it does
     ///
@@ -34,40 +31,25 @@ declare_clippy_lint! {
 
 declare_lint_pass!(SplittingStringsAtNewlines => [SPLITTING_STRINGS_AT_NEWLINES]);
 
-// Check if an expr is of a type that implements `Deref` with `Target = str`
-fn derefs_into_str(cx: &LateContext<'_>, expr: &'_ Expr<'_>) -> bool {
-    matches!(
-        cx.tcx.get_diagnostic_item(sym::Deref),
-        Some(def_id) if (
-            implements_trait(cx, cx.typeck_results().expr_ty(expr), def_id, &[])
-            && matches!(
-                cx.get_associated_type(cx.typeck_results().expr_ty_adjusted(expr), def_id, "Target"),
-                Some(target) if target.is_str()
-            )
-        )
-    )
-}
-
 impl LateLintPass<'_> for SplittingStringsAtNewlines {
     fn check_expr(&mut self, cx: &LateContext<'_>, expr: &'_ Expr<'_>) {
-        // We're looking for `A.trim().split(B)`, where the type of `A` implements `Deref` with `Target =
-        // str` (e.g. a `str` literal or an expression returning `String`), and `B` is a `Pattern`
-        // that hard-codes a newline (either `"\n"` or `"\r\n"`). There are a lot of ways to specify a
-        // pattern, and this lint only checks the most basic ones: a `'\n'`, `"\n"`, and `"\r\n"`.
+        // We're looking for `A.trim().split(B)`, where the adjusted type of `A` is `&str` (e.g. a `str`
+        // literal or an expression returning `String`), and `B` is a `Pattern` that hard-codes a
+        // newline (either `"\n"` or `"\r\n"`). There are a lot of ways to specify a pattern, and
+        // this lint only checks the most basic ones: a `'\n'`, `"\n"`, and `"\r\n"`.
 
         if expr.span.from_expansion() {
             // Don't generate lints in code expanded from macros
             return;
         }
 
-        if let ExprKind::MethodCall(split_method_name, split_receiver, split_args, _) = expr.kind
+        if let ExprKind::MethodCall(split_method_name, split_receiver, [split_arg], _) = expr.kind
             && split_method_name.ident.as_str() == "split"
-            && split_args.len() == 1
             && let ExprKind::MethodCall(trim_method_name, trim_receiver, trim_args, _) = split_receiver.kind
             && trim_method_name.ident.as_str() == "trim"
             && trim_args.is_empty()
-            && derefs_into_str(cx, trim_receiver)
-            && let ExprKind::Lit(split_lit) = split_args[0].kind
+            && cx.typeck_results().expr_ty_adjusted(trim_receiver).peel_refs().is_str()
+            && let ExprKind::Lit(split_lit) = split_arg.kind
         {
             let char_n = matches!(split_lit.node, LitKind::Char('\n'));
             let str_n_or_rn =