build from source fails with "let" expression errors.

What version of Codex is running?

0.25.0

Which model were you using?

gpt-5

What platform is your computer?

Darwin 24.5.0 arm64 arm

What steps can reproduce the bug?

follow the documentation to build from source:

# Clone the repository and navigate to the root of the Cargo workspace.
git clone https://github.com/openai/codex.git
cd codex/codex-rs

# Install the Rust toolchain, if necessary.
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
source "$HOME/.cargo/env"
rustup component add rustfmt
rustup component add clippy

# Build Codex.
cargo build

What is the expected behavior?

it runs correctly.

What do you see instead?

   Compiling rustls-pki-types v1.12.0
   Compiling darling_core v0.20.11
   Compiling num-bigint v0.4.6
error[E0658]: `let` expressions in this position are unstable
   --> protocol/src/protocol.rs:340:24
    |
340 |                     && let Some(tmpdir) = std::env::var_os("TMPDIR")
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information

   Compiling rand v0.8.5
   Compiling hyper-tls v0.6.0
...

   Compiling darling_macro v0.20.11
   Compiling tiny_http v0.12.0
For more information about this error, try `rustc --explain E0658`.
error: could not compile `codex-protocol` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...