We do not coerce &mut &mut T -> *mut mut T

tests/ui/coercion/mut-mut-wont-coerce.rs

@@ -0,0 +1,43 @@
+// Documents that Rust currently does not permit the coercion &mut &mut T -> *mut *mut T
+// Making this compile was a feature request in rust-lang/rust#34117 but this is currently
+// "working as intended". Allowing "deep pointer coercion" seems footgun-prone, and would
+// require proceeding carefully.
+use std::ops::{Deref, DerefMut};
+
+struct Foo(i32);
+
+struct SmartPtr<T>(*mut T);
+
+impl<T> SmartPtr<T> {
+ fn get_addr(&mut self) -> &mut *mut T {
+ &mut self.0
+ }
+}
+
+impl<T> Deref for SmartPtr<T> {
+ type Target = T;
+ fn deref(&self) -> &T {
+ unsafe { &*self.0 }
+ }
+}
+impl<T> DerefMut for SmartPtr<T> {
+ fn deref_mut(&mut self) -> &mut T {
+ unsafe { &mut *self.0 }
+ }
+}
+
+/// Puts a Foo into the pointer provided by the caller
+fn make_foo(_: *mut *mut Foo) {
+ unimplemented!()
+}
+
+fn main() {
+ let mut result: SmartPtr<Foo> = SmartPtr(std::ptr::null_mut());
+ make_foo(&mut &mut *result); //~ mismatched types
+ //~^ expected `*mut *mut Foo`, found `&mut &mut Foo`
+ make_foo(out(&mut result)); // this is okay, but makes one wonder why the above can't have type inference
+}
+
+fn out<T>(ptr: &mut SmartPtr<T>) -> &mut *mut T {
+ ptr.get_addr()
+}

tests/ui/coercion/mut-mut-wont-coerce.stderr

@@ -0,0 +1,19 @@
+error[E0308]: mismatched types
+ --> $DIR/mut-mut-wont-coerce.rs:36:14
+ |
+LL | make_foo(&mut &mut *result);
+ | -------- ^^^^^^^^^^^^^^^^^ expected `*mut *mut Foo`, found `&mut &mut Foo`
+ | |
+ | arguments to this function are incorrect
+ |
+ = note: expected raw pointer `*mut *mut Foo`
+ found mutable reference `&mut &mut Foo`
+note: function defined here
+ --> $DIR/mut-mut-wont-coerce.rs:30:4
+ |
+LL | fn make_foo(_: *mut *mut Foo) {
+ | ^^^^^^^^ ----------------
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.