Auto merge of #11817 - y21:ptr_arg_mut_ref, r=Alexendoo

[`ptr_arg`]: recognize methods that also exist on slices

Fixes #11816

Not a new lint, just a very small improvement to the existing `ptr_arg` lint which would have caught the linked issue.

The problem was that the lint checks if a `Vec`-specific method was called, that is, if the receiver is `Vec<_>`.
This is the case for `len` and `is_empty`, however these methods also exist on slices so we can still lint there.
This logic exists in a different lint, so we can just reuse that here.

Interestingly, there was even a comment up top that explained what it should have been doing, but the logic for it just wasn't there?

changelog: [`ptr_arg`]: recognize methods that also exist on slices

<sub>Also, this is my 100th PR to clippy 🎉 </sub>

clippy_lints/src/ptr.rs

@@ -28,6 +28,8 @@ use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt as _;
 use std::{fmt, iter};
 
+use crate::vec::is_allowed_vec_method;
+
 declare_clippy_lint! {
     /// ### What it does
     /// This lint checks for function arguments of type `&String`, `&Vec`,
@@ -660,7 +662,7 @@ fn check_ptr_arg_usage<'tcx>(cx: &LateContext<'tcx>, body: &'tcx Body<'_>, args:
                             },
                             // If the types match check for methods which exist on both types. e.g. `Vec::len` and
                             // `slice::len`
-                            ty::Adt(def, _) if def.did() == args.ty_did => {
+                            ty::Adt(def, _) if def.did() == args.ty_did && !is_allowed_vec_method(self.cx, e) => {
                                 set_skip_flag();
                             },
                             _ => (),

clippy_lints/src/vec.rs

@@ -57,7 +57,7 @@ fn adjusts_to_slice(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
 /// Checks if the given expression is a method call to a `Vec` method
 /// that also exists on slices. If this returns true, it means that
 /// this expression does not actually require a `Vec` and could just work with an array.
-fn is_allowed_vec_method(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
+pub fn is_allowed_vec_method(cx: &LateContext<'_>, e: &Expr<'_>) -> bool {
     const ALLOWED_METHOD_NAMES: &[&str] = &["len", "as_ptr", "is_empty"];
 
     if let ExprKind::MethodCall(path, ..) = e.kind {