From 20a1b8bde942e69d8bf44b32c8fffbd514d58a86 Mon Sep 17 00:00:00 2001
From: Johnathan Van Why <jrvanwhy@google.com>
Date: Tue, 22 Aug 2023 15:39:49 -0700
Subject: [PATCH] Update the Rust toolchain to `nightly-2023-08-22`.

The semantics of int-to-pointer casts under strict provenance has changed. Ideally, we would update our `From<usize> for Register` implementation to use the `sptr` crate, but `sptr` does not currently have a suitable license. There is a [PR](https://github.com/Gankra/sptr/pull/14) to change that, but it hasn't gotten a response in 7 months, so I don't think we can use it. Instead, this copies the implementation of `core::ptr::invalid`.

This also bumps our `thiserror` dependency, which forces Cargo to pick a newer version of `proc-macro2` that is compatible with recently nightly toolchains.
---
 platform/src/register.rs | 17 ++++++++++++++---
 rust-toolchain           |  2 +-
 unittest/Cargo.toml      |  2 +-
 3 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/platform/src/register.rs b/platform/src/register.rs
index 976653a53..0afde2f15 100644
--- a/platform/src/register.rs
+++ b/platform/src/register.rs
@@ -1,3 +1,5 @@
+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`
@@ -16,19 +18,28 @@ pub struct Register(pub *mut ());
 
 impl From<crate::ErrorCode> for Register {
     fn from(value: crate::ErrorCode) -> Register {
-        Register(value as u16 as *mut ())
+        (value as usize).into()
     }
 }
 
 impl From<u32> for Register {
     fn from(value: u32) -> Register {
-        Register(value as *mut ())
+        (value as usize).into()
     }
 }
 
 impl From<usize> for Register {
     fn from(value: usize) -> Register {
-        Register(value as *mut ())
+        // 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) })
     }
 }
 
diff --git a/rust-toolchain b/rust-toolchain
index 4c36818b2..873e2ff65 100644
--- a/rust-toolchain
+++ b/rust-toolchain
@@ -1,7 +1,7 @@
 [toolchain]
 # See https://rust-lang.github.io/rustup-components-history/ for a list of
 # recently nightlies and what components are available for them.
-channel = "nightly-2022-06-10"
+channel = "nightly-2023-08-22"
 components = ["clippy", "miri", "rustfmt"]
 targets = ["thumbv6m-none-eabi",
            "thumbv7em-none-eabi",
diff --git a/unittest/Cargo.toml b/unittest/Cargo.toml
index 3a7eb5f08..094f09152 100644
--- a/unittest/Cargo.toml
+++ b/unittest/Cargo.toml
@@ -10,4 +10,4 @@ version = "0.1.0"
 
 [dependencies]
 libtock_platform = { path = "../platform" }
-thiserror = "1.0"
+thiserror = "1.0.44"