Auto merge of rust-lang#103957 - JakobDegen:drop-retag, r=RalfJung

Retag as FnEntry on `drop_in_place`

This commit changes the mir drop shim to always retag its argument as if it were a `&mut`.

cc rust-lang/unsafe-code-guidelines#373

Author: bors

tests/fail/stacked_borrows/drop_in_place_protector.rs

@@ -0,0 +1,27 @@
+//! Test that drop_in_place retags the entire place,
+//! invalidating all aliases to it.
+
+// A zero-sized drop type -- the retagging of `fn drop` itself won't
+// do anything (since it is zero-sized); we are entirely relying on the retagging
+// in `drop_in_place` here.
+#[repr(transparent)]
+struct HasDrop;
+impl Drop for HasDrop {
+    fn drop(&mut self) {
+        unsafe {
+            let _val = *P;
+            //~^ ERROR: /not granting access .* because that would remove .* which is strongly protected/
+        }
+    }
+}
+
+static mut P: *mut u8 = core::ptr::null_mut();
+
+fn main() {
+    unsafe {
+        let mut x = (HasDrop, 0u8);
+        let x = core::ptr::addr_of_mut!(x);
+        P = x.cast();
+        core::ptr::drop_in_place(x);
+    }
+}

tests/fail/stacked_borrows/drop_in_place_protector.stderr

@@ -0,0 +1,33 @@
+error: Undefined Behavior: not granting access to tag <TAG> because that would remove [Unique for <TAG>] which is strongly protected because it is an argument of call ID
+  --> $DIR/drop_in_place_protector.rs:LL:CC
+   |
+LL |             let _val = *P;
+   |                        ^^ not granting access to tag <TAG> because that would remove [Unique for <TAG>] which is strongly protected because it is an argument of call ID
+   |
+   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
+   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
+help: <TAG> was created by a SharedReadWrite retag at offsets [0x0..0x1]
+  --> $DIR/drop_in_place_protector.rs:LL:CC
+   |
+LL |         let x = core::ptr::addr_of_mut!(x);
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: <TAG> is this argument
+  --> $DIR/drop_in_place_protector.rs:LL:CC
+   |
+LL |         core::ptr::drop_in_place(x);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: BACKTRACE:
+   = note: inside `<HasDrop as std::ops::Drop>::drop` at $DIR/drop_in_place_protector.rs:LL:CC
+   = note: inside `std::ptr::drop_in_place::<HasDrop> - shim(Some(HasDrop))` at RUSTLIB/core/src/ptr/mod.rs:LL:CC
+   = note: inside `std::ptr::drop_in_place::<(HasDrop, u8)> - shim(Some((HasDrop, u8)))` at RUSTLIB/core/src/ptr/mod.rs:LL:CC
+note: inside `main`
+  --> $DIR/drop_in_place_protector.rs:LL:CC
+   |
+LL |         core::ptr::drop_in_place(x);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: this error originates in the macro `core::ptr::addr_of_mut` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+

tests/fail/stacked_borrows/drop_in_place_retag.rs

@@ -0,0 +1,12 @@
+//! Test that drop_in_place mutably retags the entire place, even for a type that does not need
+//! dropping, ensuring among other things that it is writeable
+
+//@error-pattern: /retag .* for Unique permission .* only grants SharedReadOnly permission/
+
+fn main() {
+    unsafe {
+        let x = 0u8;
+        let x = core::ptr::addr_of!(x);
+        core::ptr::drop_in_place(x.cast_mut());
+    }
+}

tests/fail/stacked_borrows/drop_in_place_retag.stderr

@@ -0,0 +1,29 @@
+error: Undefined Behavior: trying to retag from <TAG> for Unique permission at ALLOC[0x0], but that tag only grants SharedReadOnly permission for this location
+  --> RUSTLIB/core/src/ptr/mod.rs:LL:CC
+   |
+LL | pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   | |
+   | trying to retag from <TAG> for Unique permission at ALLOC[0x0], but that tag only grants SharedReadOnly permission for this location
+   | this error occurs as part of retag at ALLOC[0x0..0x1]
+   |
+   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
+   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
+help: <TAG> was created by a SharedReadOnly retag at offsets [0x0..0x1]
+  --> $DIR/drop_in_place_retag.rs:LL:CC
+   |
+LL |         let x = core::ptr::addr_of!(x);
+   |                 ^^^^^^^^^^^^^^^^^^^^^^
+   = note: BACKTRACE:
+   = note: inside `std::ptr::drop_in_place::<u8> - shim(None)` at RUSTLIB/core/src/ptr/mod.rs:LL:CC
+note: inside `main`
+  --> $DIR/drop_in_place_retag.rs:LL:CC
+   |
+LL |         core::ptr::drop_in_place(x.cast_mut());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: this error originates in the macro `core::ptr::addr_of` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+

tests/fail/unaligned_pointers/drop_in_place.rs

@@ -0,0 +1,25 @@
+#[repr(transparent)]
+struct HasDrop(u8);
+
+impl Drop for HasDrop {
+    fn drop(&mut self) {}
+}
+
+#[repr(C, align(2))]
+struct PartialDrop {
+    a: HasDrop,
+    b: u8,
+}
+
+//@error-pattern: /alignment 2 is required/
+fn main() {
+    unsafe {
+        // Create an unaligned pointer
+        let mut x = [0_u16; 2];
+        let p = core::ptr::addr_of_mut!(x).cast::<u8>();
+        let p = p.add(1);
+        let p = p.cast::<PartialDrop>();
+
+        core::ptr::drop_in_place(p);
+    }
+}

tests/fail/unaligned_pointers/drop_in_place.stderr

@@ -0,0 +1,20 @@
+error: Undefined Behavior: accessing memory with alignment ALIGN, but alignment ALIGN is required
+  --> RUSTLIB/core/src/ptr/mod.rs:LL:CC
+   |
+LL | pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ accessing memory with alignment ALIGN, but alignment ALIGN is required
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `std::ptr::drop_in_place::<PartialDrop> - shim(Some(PartialDrop))` at RUSTLIB/core/src/ptr/mod.rs:LL:CC
+note: inside `main`
+  --> $DIR/drop_in_place.rs:LL:CC
+   |
+LL |         core::ptr::drop_in_place(p);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+