Register: document provenance, fix conversion implementations.

Now that the presence of provenance in Rust has been stabilized, we should document the effect that `Register` has on pointer provenance. Note that the current implementation sometimes attaches provenance when we don't want it to, which we can only fix when strict provenance is stabilized (which should happen in Rust 1.84).

This also fixes the implementation of `From<usize> for Register`, which relies on transmuting a usize to a pointer, which [is not a stable guarantee](https://github.com/rust-lang/rust/blob/81eef2d362a6f03db6f8928f82d94298d31eb81b/library/core/src/ptr/mod.rs#L640).

platform/src/register.rs

@@ -1,11 +1,16 @@
-use core::mem::transmute;
-
-/// In order to work with Miri's `-Zmiri-track-raw-pointers` flag, we cannot
-/// pass pointers to the kernel through `usize` values (as casting to and from
-/// `usize` drops the pointer`s tag). Instead, `RawSyscalls` uses the `Register`
-/// type. `Register` wraps a raw pointer type that keeps that tags around. User
-/// code should not depend on the particular type of pointer that `Register`
-/// wraps, but instead use the conversion functions in this module.
+/// `Register` represents the value of a register used in Tock's syscall ABI. It
+/// can contain integer values as well as pointer values.
+///
+/// `Register` currently wraps a raw pointer, but that is not a stable guarantee
+/// and users should not rely on it. However, `Register` does guarantee that the
+/// type it wraps is a valid operand type for inline assembly.
+///
+/// If a pointer is converted to a `Register`, that `Register` has that
+/// pointer's provenance. The provenance is not exposed. If an integer is
+/// converted to a `Register`, that `Register` has no provenance. When a
+/// `Register` with provenance is converted into a pointer, that pointer carries
+/// the `Register`'s provenance. When a `Register` without provenance is
+/// converted into a pointer, that pointer has no provenance.
 // Register is repr(transparent) so that an upcall's application data can be
 // soundly passed as a Register.
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
@@ -36,22 +41,28 @@ impl From<i32> for Register {
 
 impl From<usize> for Register {
     fn from(value: usize) -> Register {
-        // Note: clippy is wrong here; transmute has different semantics than
-        // `as` casts under strict provenance.
-        #[allow(clippy::useless_transmute)]
-        // We want to convert using the same semantics as core::ptr::invalid:
-        // convert the usize into a pointer with that address without attaching
-        // provenance to it. However, core::ptr::invalid is a nightly-only
-        // function. In order to build on stable, we copy its implementation.
-        // Safety: Raw pointers do not have any validity invariants that usize
-        // does not have; a raw pointer can point to any address.
-        Register(unsafe { transmute(value) })
+        // TODO(Rust 1.84): We want to convert using the same semantics as
+        // core::ptr::without_provenance. However, until our MSRV is >= 1.84, we
+        // have to use an `as` cast instead. This may result in this Register
+        // converting into a pointer with provenance later on, but that
+        // shouldn't break any users of Register in practice.
+        #[cfg(not(miri))]
+        {
+            Register(value as *mut ())
+        }
+        // However, on Miri, we cannot do the conversion using an `as` cast.
+        // Fortunately, since Miri runs on nightly Rust, we can use
+        // `without_provenance_mut`.
+        #[cfg(miri)]
+        {
+            Register(core::ptr::without_provenance_mut(value))
+        }
     }
 }
 
 impl<T> From<*mut T> for Register {
     fn from(value: *mut T) -> Register {
-        Register(value as *mut ())
+        Register(value.cast())
     }
 }
 
@@ -89,13 +100,17 @@ impl Register {
 
 impl From<Register> for usize {
     fn from(register: Register) -> usize {
+        // TODO(Rust 1.84): We want to convert using the same semantics as
+        // .addr(). Until our MSRV is >= 1.84, we have to convert using an `as`
+        // cast instead. This exposes the provenance of the pointer, which is
+        // not correct but shouldn't break any users in practice.
         register.0 as usize
     }
 }
 
 impl<T> From<Register> for *mut T {
     fn from(register: Register) -> *mut T {
-        register.0 as *mut T
+        register.0.cast()
     }
 }
 
@@ -116,6 +131,6 @@ impl TryFrom<Register> for u32 {
     type Error = core::num::TryFromIntError;
 
     fn try_from(register: Register) -> Result<u32, core::num::TryFromIntError> {
-        (register.0 as usize).try_into()
+        usize::from(register).try_into()
     }
 }