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Rollup merge of #69651 - Mark-Simulacrum:black-box-marker, r=eddyb

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Try to ensure usize marker does not get merged

This follows up on [this conversation](https://github.com/rust-lang/rust/pull/69209#discussion_r379911282). However, I'm not confident this is quite correct, so feedback is appreciated, as always.
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Mazdak Farrokhzad 2020-03-08 16:53:38 +01:00 committed by GitHub
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src/libcore/fmt/mod.rs
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@ -264,8 +264,18 @@ pub struct ArgumentV1<'a> {
// could have been miscompiled. In practice, we never call as_usize on non-usize // could have been miscompiled. In practice, we never call as_usize on non-usize
// containing data (as a matter of static generation of the formatting // containing data (as a matter of static generation of the formatting
// arguments), so this is merely an additional check. // arguments), so this is merely an additional check.
//
// We primarily want to ensure that the function pointer at `USIZE_MARKER` has
// an address corresponding *only* to functions that also take `&usize` as their
// first argument. The read_volatile here ensures that we can safely ready out a
// usize from the passed reference and that this address does not point at a
// non-usize taking function.
#[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")] #[unstable(feature = "fmt_internals", reason = "internal to format_args!", issue = "none")]
static USIZE_MARKER: fn(&usize, &mut Formatter<'_>) -> Result = |_, _| loop {}; static USIZE_MARKER: fn(&usize, &mut Formatter<'_>) -> Result = |ptr, _| {
// SAFETY: ptr is a reference
let _v: usize = unsafe { crate::ptr::read_volatile(ptr) };
loop {}
};
impl<'a> ArgumentV1<'a> { impl<'a> ArgumentV1<'a> {
#[doc(hidden)] #[doc(hidden)]