fix(build): rust-version names a toolchain that can actually build the tree

[tato123]

Refs #1852 (does not close it — see Scope).

[workspace.package] rust-version claimed 1.85. The tree uses 15 let-chains across 10 files, and let-chains stabilized in 1.88, so no 1.85 toolchain has ever been able to build this workspace. The number was never chosen against the code.

Finding the floor

The ticket asks for the floor empirically rather than reasoned from the feature, since let-chains need not be the only thing past 1.85. Both runs local, this branch:

toolchain	command	result
1.87	cargo check --locked -p streamlib-macros	exit 101 — E0658: let expressions in this position are unstable, 3 errors (codegen.rs:1217, grammar.rs:239, grammar.rs:240)
1.88	cargo check --locked --workspace --all-targets	exit 0, 0 errors

So 1.88 is the floor exactly — not merely a version that happens to work.

Scope — why this is one line, and what stays open

The field is inherited by zero of the 24 workspace members. Nothing declares rust-version.workspace = true, and cargo metadata --no-deps reports no rust_version for any package. So the field is inert today, whatever number it holds.

That makes the ticket's second question — should an MSRV be a promise this project makes at all? — a real fork, and the owner's to settle:

• If yes: the published crates need rust-version.workspace = true, and CI needs a job pinned to the floor, or it drifts again.
• If no: the honest move is to delete the field.

Either path is a decision, not a fix, so neither is in this diff. #1852 stays open for it. This change only stops the field from stating something false — which is decision-free and true under both outcomes.

Worth noting given this week: a declared-but-unenforced invariant silently drifting is exactly the shape of the release wedge fixed in #1955. The enforcement half is the part that would prevent a repeat, and it is the half that needs the owner.

Verification

• cargo run -p xtask -- check-all-source-gates → all 11 pass
• cargo fmt --all --check clean
• cargo metadata --locked succeeds (lockfile untouched)
• rust-version is not a field release-please manages — its extra-files jsonpaths are $.workspace.package.version and $.project.version only, so this does not desync the release baseline

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores
  • Raised the minimum supported Rust version to 1.88.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: f7cf9cca-590d-4896-8252-63f1a51f150d

📥 Commits

Reviewing files that changed from the base of the PR and between 1027b23 and 32463c1.

📒 Files selected for processing (1)

• Cargo.toml

Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review.

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📝 Walkthrough

Walkthrough

The workspace minimum supported Rust version was raised from 1.85 to 1.88 in Cargo.toml.

Changes

Rust Version Requirement Update

Layer / File(s)	Summary
Update workspace Rust version Cargo.toml	The workspace rust-version requirement changed from 1.85 to 1.88.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to 32463

This change updates the declared Rust version to a toolchain that can build the repository without changing runtime behavior; no actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: correcting the declared Rust toolchain version so it can build the workspace.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/msrv-names-a-buildable-toolchain

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