rust: add workaround for potential compiler bug

Apply workaround for a potential miscompilation bug:
See
- rust-embedded/cortex-m#503
- rust-lang/rust#118867
We have not observed any abnormal behavior even though we fulfil all the criteria to be affected
(opt-level='z', thumbv7em-none-eabi target, Rust toolchain 1.74.0 being >= 1.73.0), but we apply
the workaround just in case.
This increases the binary size of the `make firmware` (Multi) build by 11568 at the time of
adding this workaround. This can be removed again once the issue above is fixed and we have
updated to a Rust toolchain that contains the fix.

This workaround is one of three suggested alternatives. The other two
are:
- Downgrade the Rust toolchain - dismissed as it is harder to do, as
  it involves re-building the Docker image
- Switch from opt-level='z' to opt-level='s' - this increases the
  binary size by 74416 bytes for the Multi, which is much more than the
  workaround in this commit.

Author: benma

src/CMakeLists.txt

@@ -217,6 +217,19 @@ endif()
 # See https://github.com/rust-bitcoin/rust-secp256k1/tree/7c8270a8506e31731e540fab7ee1abde1f48314e/secp256k1-sys#linking-to-external-symbols
 set(RUSTFLAGS "${RUSTFLAGS} --cfg=rust_secp_no_symbol_renaming")
 
+# Apply workaround for a potential miscompilation bug:
+# See
+# - https://github.com/rust-embedded/cortex-m/discussions/503
+# - https://github.com/rust-lang/rust/issues/118867
+# We have not observed any abnormal behavior even though we fulfil all the criteria to be affected
+# (opt-level='z', thumbv7em-none-eabi target, Rust toolchain 1.74.0 being >= 1.73.0), but we apply
+# the workaround just in case.
+#
+# This increases the binary size of the `make firmware` (Multi) build by 11568 at the time of
+# adding this workaround. This can be removed again once the issue above is fixed and we have
+# updated to a Rust toolchain that contains the fix.
+set(RUSTFLAGS "${RUSTFLAGS} -Cllvm-args=--enable-machine-outliner=never")
+
 if(CMAKE_CROSSCOMPILING)
   set(RUST_TARGET_ARCH thumbv7em-none-eabi)
   set(RUST_TARGET_ARCH_DIR ${RUST_TARGET_ARCH})