Add as_ptr_cast_mut lint

This lint detects calls to a `&self`-taking `as_ptr` method, where
the result is then immediately cast to a `*mut T`. Code like this
is probably invalid, as that pointer will not have write permissions,
and `*mut T` is usually used to write through.

tests/ui/as_ptr_cast_mut.rs

@@ -0,0 +1,37 @@
+#![allow(unused)]
+#![warn(clippy::as_ptr_cast_mut)]
+#![allow(clippy::wrong_self_convention)]
+
+struct MutPtrWrapper(Vec<u8>);
+impl MutPtrWrapper {
+    fn as_ptr(&mut self) -> *const u8 {
+        self.0.as_mut_ptr() as *const u8
+    }
+}
+
+struct Covariant<T>(*const T);
+impl<T> Covariant<T> {
+    fn as_ptr(self) -> *const T {
+        self.0
+    }
+}
+
+fn main() {
+    let mut string = String::new();
+    let _ = string.as_ptr() as *mut u8;
+    let _: *mut i8 = string.as_ptr() as *mut _;
+    let _ = string.as_ptr() as *const i8;
+    let _ = string.as_mut_ptr();
+    let _ = string.as_mut_ptr() as *mut u8;
+    let _ = string.as_mut_ptr() as *const u8;
+
+    let nn = std::ptr::NonNull::new(4 as *mut u8).unwrap();
+    let _ = nn.as_ptr() as *mut u8;
+
+    let mut wrap = MutPtrWrapper(Vec::new());
+    let _ = wrap.as_ptr() as *mut u8;
+
+    let mut local = 4;
+    let ref_with_write_perm = Covariant(std::ptr::addr_of_mut!(local) as *const _);
+    let _ = ref_with_write_perm.as_ptr() as *mut u8;
+}

tests/ui/as_ptr_cast_mut.stderr

@@ -0,0 +1,16 @@
+error: casting the result of `as_ptr` to *mut u8
+  --> $DIR/as_ptr_cast_mut.rs:21:13
+   |
+LL |     let _ = string.as_ptr() as *mut u8;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `string.as_mut_ptr()`
+   |
+   = note: `-D clippy::as-ptr-cast-mut` implied by `-D warnings`
+
+error: casting the result of `as_ptr` to *mut i8
+  --> $DIR/as_ptr_cast_mut.rs:22:22
+   |
+LL |     let _: *mut i8 = string.as_ptr() as *mut _;
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace with: `string.as_mut_ptr()`
+
+error: aborting due to 2 previous errors
+