realize that a test case is incorrect

`Mutex<*const _>` doesn't make a lot of sense (there can be no
contention over a read-only reference), but `AtomicPtr::new(*const _)`
straight up doesn't compile

clippy_lints/src/mutex_atomic.rs

@@ -5,6 +5,7 @@ use clippy_utils::ty::ty_from_hir_ty;
 use rustc_errors::{Applicability, Diag};
 use rustc_hir::{Expr, ExprKind, Item, ItemKind, LetStmt, QPath};
 use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::mir::Mutability;
 use rustc_middle::ty::{self, IntTy, Ty, UintTy};
 use rustc_session::declare_lint_pass;
 use rustc_span::sym;
@@ -169,7 +170,8 @@ fn get_atomic_name(ty: Ty<'_>) -> Option<&'static str> {
                 IntTy::I128 => None,
             }
         },
-        ty::RawPtr(_, _) => Some("AtomicPtr"),
+        // `AtomicPtr` only accepts `*mut T`
+        ty::RawPtr(_, Mutability::Mut) => Some("AtomicPtr"),
         _ => None,
     }
 }

tests/ui/mutex_atomic.rs

@@ -16,8 +16,8 @@ fn main() {
     //~^ mutex_atomic
 
     let mut x = 4u32;
+    // `AtomicPtr` only accepts `*mut T`, so this should not lint
     let _ = Mutex::new(&x as *const u32);
-    //~^ mutex_atomic
 
     let _ = Mutex::new(&mut x as *mut u32);
     //~^ mutex_atomic

tests/ui/mutex_atomic.stderr

@@ -24,14 +24,6 @@ LL |     let _ = Mutex::new(9isize);
    |
    = help: if you just want the locking behavior and not the internal type, consider using `Mutex<()>`
 
-error: using a `Mutex` where an atomic would do
-  --> tests/ui/mutex_atomic.rs:19:13
-   |
-LL |     let _ = Mutex::new(&x as *const u32);
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `std::sync::atomic::AtomicPtr::new(&x as *const u32)`
-   |
-   = help: if you just want the locking behavior and not the internal type, consider using `Mutex<()>`
-
 error: using a `Mutex` where an atomic would do
   --> tests/ui/mutex_atomic.rs:22:13
    |
@@ -115,5 +107,5 @@ LL |     let reassigned = mtx;
    = help: consider using an `AtomicI32` instead
    = help: if you just want the locking behavior and not the internal type, consider using `Mutex<()>`
 
-error: aborting due to 14 previous errors
+error: aborting due to 13 previous errors