THRIFT-6288: Clarify the message-size TODOs in the Rust protocols

[Jens-G]

THRIFT-6288

Comment-only change: the three message-size TODOs in the Rust protocols now say what the code does today and why the more precise check is missing.

Background

The ticket quotes a TODO that is not the text in the tree. The actual comment in check_container_size, there since THRIFT-5871 (42d0b71), reads:

// TODO: When Rust trait specialization stabilizes, we can add more precise checks
// for transports that track exact remaining bytes. For now, we use the message
// size limit as a best-effort check.

That comment names a real obstacle. The THRIFT-5871 commit message says the same: full message-size tracking was left out because trait specialization is not stable.

• TReadTransport is implemented for every io::Read by one blanket impl (transport/mod.rs). So no transport can supply its own remaining-bytes count without specialization or a change to the trait.
• The other bindings keep that count in their transports, for example C++ TTransport and Java TEndpointTransport.

What the comment did not say is what the check compares against today, which is the whole max_message_size. It also did not say that up-front allocation does not depend on the check.

What changes

• protocol/mod.rs, check_container_size: the TODO says that min_bytes_needed is held to the whole max_message_size rather than to what is left of the current message, and why. It also notes that generated code reserves list capacity through prealloc_size, which caps it; sets and maps are BTreeSet/BTreeMap and reserve nothing.
• protocol/binary.rs and protocol/compact.rs, read_message_begin: the matching "call the message size tracking here" TODOs now say what would start there, and point to check_container_size.

No behaviour changes, so there is no test to add.

Verification

Rust 1.85.1, as in CI, in lib/rs:

• cargo fmt --all -- --check, cargo clippy --all -- -D warnings and cargo clippy --all --all-features -- -D warnings are clean.
• cargo check --no-default-features --features rustls passes.
• cargo test and cargo test --all-features: 177 of 178 pass.
  • The one failure is transport::socket::tests::split_halves_must_not_clobber_each_others_timeout. It fails the same way on unmodified master on this machine: Some(252ms) vs Some(250ms).
  • The host kernel runs with CONFIG_HZ=250, and Linux stores socket timeouts in jiffies, so 250 ms comes back as 252 ms. The test passes in CI, so the runners' kernels keep 250 ms exact. The failure is unrelated to this change.
• Merges cleanly with the open Rust PR THRIFT-6290: Rust: skip string/binary fields without heap-allocating #3854.

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The change was written with AI assistance (Claude Opus 5), then reviewed and checked as described above.

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