remove blanket-Freeze for PhantomData

Author: RalfJung

compiler/rustc_codegen_cranelift/example/mini_core.rs

@@ -95,7 +95,6 @@ unsafe impl Sync for [u8; 16] {}
 #[lang = "freeze"]
 unsafe auto trait Freeze {}
 
-unsafe impl<T: ?Sized> Freeze for PhantomData<T> {}
 unsafe impl<T: ?Sized> Freeze for *const T {}
 unsafe impl<T: ?Sized> Freeze for *mut T {}
 unsafe impl<T: ?Sized> Freeze for &T {}

compiler/rustc_codegen_gcc/example/mini_core.rs

@@ -91,7 +91,6 @@ unsafe impl Sync for [u8; 16] {}
 #[lang = "freeze"]
 unsafe auto trait Freeze {}
 
-unsafe impl<T: ?Sized> Freeze for PhantomData<T> {}
 unsafe impl<T: ?Sized> Freeze for *const T {}
 unsafe impl<T: ?Sized> Freeze for *mut T {}
 unsafe impl<T: ?Sized> Freeze for &T {}

library/core/src/marker.rs

@@ -872,7 +872,6 @@ pub(crate) unsafe auto trait Freeze {}
 impl<T: ?Sized> !Freeze for UnsafeCell<T> {}
 marker_impls! {
     unsafe Freeze for
-        {T: ?Sized} PhantomData<T>,
         {T: ?Sized} *const T,
         {T: ?Sized} *mut T,
         {T: ?Sized} &T,

tests/ui/unsafe-cell/phantom-data-not-freeze.rs

@@ -0,0 +1,15 @@
+use std::cell::UnsafeCell;
+use std::marker::PhantomData;
+
+// This checks that `PhantomData` does not entirely mask interior mutability.
+
+trait Trait {
+    const C: (u32, PhantomData<UnsafeCell<u32>>);
+}
+
+fn bar<T: Trait>() {
+    let x: &'static (u32, PhantomData<UnsafeCell<u32>>) = &T::C;
+    //~^ ERROR: dropped while borrowed
+}
+
+fn main() {}

tests/ui/unsafe-cell/phantom-data-not-freeze.stderr

@@ -0,0 +1,14 @@
+error[E0716]: temporary value dropped while borrowed
+  --> $DIR/phantom-data-not-freeze.rs:11:60
+   |
+LL |     let x: &'static (u32, PhantomData<UnsafeCell<u32>>) = &T::C;
+   |            --------------------------------------------    ^^^^ creates a temporary value which is freed while still in use
+   |            |
+   |            type annotation requires that borrow lasts for `'static`
+LL |
+LL | }
+   | - temporary value is freed at the end of this statement
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0716`.