[jdx]

https://entire.io/gh/jdx/mise/trails/262

Since 2026.8.6, rust = "nightly" installs a dated toolchain such as nightly-2026-09-26 so installs are reproducible. As a side effect, rustup no longer had a toolchain named nightly, so cargo +nightly broke (reported in #4737):

$ mise use -q rust@nightly
$ RUSTUP_AUTO_INSTALL=0 cargo +nightly check
error: toolchain 'nightly-x86_64-unknown-linux-gnu' is not installed

With rustup's auto-install left on, the same command quietly downloaded a second, unpinned nightly instead.

Now, after installing the dated nightly for a rolling nightly request, mise also gives rustup a nightly-<host> toolchain made from it, so cargo +nightly runs the same pinned nightly with no extra download:

$ mise use -q rust@nightly
$ RUSTUP_AUTO_INSTALL=0 cargo +nightly -V
cargo 1.101.0-nightly (3d7cf6e93 2026-09-25)

Behavior

• mise only writes nightly-<host> when it is missing, older than the nightly just installed, or the same nightly without all of its components and targets (for example after adding components = "rust-src"). A rustup nightly that is already newer, for example after rustup update nightly, is left alone, and so is one with components or targets that the dated toolchain lacks (for example rustup target add wasm32-unknown-unknown --toolchain nightly), since replacing it would remove them. mise records in a .mise-seeded file inside the copy when rustup's components file was written; while rustup hasn't changed any component since, everything in the copy came from mise and may be dropped, so a switch to a smaller profile still refreshes it. If rustup is mid-change when mise goes to swap, mise re-checks and backs off.
• Replacing an older copy swaps the directories atomically on Linux (renameat2(RENAME_EXCHANGE)) and macOS (renamex_np(RENAME_SWAP)), so a concurrent cargo +nightly never sees it missing. Elsewhere it falls back to two renames and restores the old copy if the second fails; if even that restore fails, the old copy is kept on disk and its path is included in the warning instead of being deleted.
• Explicitly dated requests such as rust = "nightly-2026-08-13" never touch rustup's nightly.
• Once written, the toolchain belongs to rustup. rustup update nightly, rustup component add --toolchain nightly, and rustup toolchain uninstall nightly work normally and never change the dated toolchain mise installed. mise does not remove it when uninstalling the dated nightly.
• mise deletes rustup's update-hashes/nightly-<host> when it replaces the toolchain, so a later rustup update nightly can't wrongly report it as unchanged.
• If this step fails, install still succeeds and prints a warning.
• Existing installs pick this up on their next nightly install or upgrade, or right away with mise install -f rust.

Why a copy instead of an alias

• rustup toolchain link nightly … is rejected: rustup won't allow a custom toolchain to use a channel name.
• A symlink would let rustup update nightly write through it into mise's dated toolchain.

The copy costs almost no disk space: a nightly with the default profile is about 1.5 GB, and here the files share their contents with the dated toolchain.

• Filesystems with copy-on-write clones (APFS, btrfs, XFS, ReFS/Dev Drive): files are reflinked, with clonefile cloning the whole tree in one call on macOS.
• Other filesystems (ext4, NTFS): files are hardlinked. The exception is rustup's metadata directly under lib/rustlib/, which is copied. I tested this against a real toolchain: rustup replaces component files by deleting and recreating them, which leaves hardlinked originals intact, but it rewrites multirust-channel-manifest.toml in place.

This adds reflink-copy as a direct dependency; it was already in the lockfile through aube.

Validation

• Unit tests cover the hardlink fallback (component files share inodes, metadata files don't), in-place metadata writes not reaching the dated toolchain with and without reflinks, symlinks being preserved, the directory swap, and the refresh rules, including a same-date copy missing components an older copy with rustup-added targets, a profile change that drops components mise seeded, a component removed and re-added through rustup, and rustup changing the copy after the decision.
• A new e2e test, e2e/core/test_rust_nightly_channel_toolchain, uses a fake rustup whose cargo proxy resolves +<toolchain> like rustup's (no auto-install). It runs mise exec -- cargo +nightly build to cover creating the copy, refreshing it on upgrade (including clearing the update hash), keeping a newer rustup nightly, and leaving explicitly dated pins alone.
• Ran by hand on macOS/APFS with real rustup 1.29.1. mise install of rust = { version = "nightly", profile = "minimal" } into a fresh rustup home produced a working cargo +nightly -V / rustc +nightly -V with RUSTUP_AUTO_INSTALL=0.
• Earlier I tested the hardlink layout by hand against real rustup: component add, update nightly (2026-07-04 → 2026-09-26), and toolchain uninstall nightly all left every file in the dated toolchain byte-identical.
• Not tested on Windows.

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---

Note

Medium Risk
The change performs concurrent filesystem swaps and hardlinks/reflinks under the user’s rustup home; mistakes could corrupt toolchains, though guards limit scope to rolling nightly installs and failures are non-fatal warnings.

Overview
Rolling rust = "nightly" still resolves to a dated toolchain for reproducible locks, but after install mise now mirrors that toolchain into rustup’s nightly-<host> slot so cargo +nightly works without auto-installing a second nightly. Explicit pins like nightly-2026-08-13 never touch rustup’s nightly.

The new nightly_channel logic reflinks or hardlinks the dated tree (full reflink-copy dependency), copies rustup metadata under lib/rustlib/ so in-place updates don’t corrupt mise’s install, and refreshes the alias only when it’s missing, older, or missing components—while preserving newer nightlies and user-added components/targets. Swaps use atomic directory exchange on Linux/macOS when possible; failures during linking warn only and don’t fail install.

Docs explain the alias behavior; e2e/core/test_rust_nightly_channel_toolchain and unit tests cover refresh rules and cloning.

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Summary by CodeRabbit

• New Features
  • Installing a dated Rust nightly through the rolling nightly channel now makes it available to rustup as nightly when the existing alias is missing, older, or lacks components or targets. Newer aliases and rustup-added components or targets are preserved.
  • The rustup copy can be updated independently with rustup update nightly, without changing mise’s dated toolchain.
• Documentation
  • Updated Rust nightly documentation to explain alias behavior and how the copy is created.

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📝 Walkthrough

Walkthrough

Rust installation now checks whether a dated nightly toolchain can refresh rustup’s rolling nightly alias. The refresh logic compares manifests and components, clones toolchain files, and replaces the alias when needed. Tests and documentation cover the behavior.

Changes

Rust nightly alias

Layer / File(s)	Summary
Clone toolchain files Cargo.toml, src/plugins/core/rust/nightly_channel.rs	Adds the reflink-copy dependency and cloning that uses reflinks when available, falls back to hardlinks, copies direct files under lib/rustlib/, and preserves symlinks. Tests cover cloning and metadata isolation.
Refresh the rustup nightly alias src/plugins/core/rust/nightly_channel.rs	Compares manifests and components, stages a clone, guards against concurrent changes, and replaces the alias when needed. It removes the alias update hash after success. Tests cover refresh decisions and replacement.
Connect alias refresh to Rust installation src/plugins/core/rust.rs, e2e/core/test_rust_nightly_channel_toolchain, docs/lang/rust.md	Rust installation checks the toolchain sysroot before refreshing the alias. End-to-end tests cover rolling and explicitly dated nightly requests. The documentation describes alias behavior and how rustup can advance its nightly toolchain.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~40 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant RustPlugin
  participant rustc
  participant NightlyChannel
  participant RustupToolchains
  RustPlugin->>rustc: Query installed toolchain sysroot
  rustc-->>RustPlugin: Return sysroot path
  RustPlugin->>NightlyChannel: Refresh alias for matching rustup sysroot
  NightlyChannel->>RustupToolchains: Read dated and alias manifests
  RustupToolchains-->>NightlyChannel: Return manifest dates and components
  NightlyChannel->>RustupToolchains: Stage clone and replace alias when needed
  NightlyChannel->>RustupToolchains: Remove alias update hash after success

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Merge Risk: 🔵 Low · up to a2355

On a filesystem with coarse timestamps, refreshing nightly could remove a component added through rustup. The narrow trigger and recoverable impact make this mergeable with owner awareness or follow-up.

Security Architecture Review

Security architecture risk: 🟡 Moderate · up to a2355

The normal path protects newer toolchains and user-added components, but certain timestamp and interruption cases could remove those protections or leave the nightly alias unavailable. The impact is limited to the affected user’s Rust toolchains.

Retained concerns

• Medium · security · inferred: If rustup changes the alias’s components while its reported mtime still matches the mise-seeded marker, refresh treats the alias as mise-owned and can replace it with a copy lacking rustup-added components or targets.
• Medium · reliability · inferred: Where atomic exchange is unavailable, interruption between two renames leaves the public nightly alias absent, with no recovery path shown for the previous toolchain. cargo +nightly may then fail or, with rustup auto-install enabled, obtain an independently selected nightly.
• Low · reliability · inferred: Alias replacement commits before rustup’s update hash is removed. A removal error leaves the new alias and old hash together; the code identifies a matching hash as a reason a later rustup update could skip advancing that alias.

Security review details

Security Blast Radius

• inferred — The direct state affected is the selected user’s rustup nightly alias and update metadata, and commands that use that alias. The examined path does not show a cross-tenant or privileged-service transition.

Security Findings and Attack Paths

• inferred — No independently exploitable attacker path is established. The material risks are loss of rustup-owned additions under an mtime collision and loss of the pinned-nightly alias during an interrupted fallback replacement.

Trust Boundaries and Controls

• observed — The caller limits refresh to rolling dated-nightly installations and checks the reported sysroot shape. Refresh validates manifests and normally preserves newer aliases and rustup additions, but the caller does not explicitly tie the reported toolchains directory to the configured rustup home.

Resilience and Maintainability Implications

• observed — Normal error handling attempts to preserve or restore the prior alias, while the two-rename fallback and post-replacement hash deletion remain separate transitions.

Hardening Proposals

• proposed — Bind seeded ownership to component contents or another collision-resistant state check, and cover an edit that retains the reported mtime.
• proposed — Provide interruption recovery for a displaced alias and a retry or reconciliation path for update-hash cleanup; consider verifying that the reported toolchains directory belongs to the configured rustup home before mutation.

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Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
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Title check	✅ Passed	The title clearly and concisely describes the main change: preserving cargo +nightly support when rust@nightly installs a dated toolchain.

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[greptile-apps]

Confidence Score: 4/5

[High risk] Adds a new Rust toolchain linking mechanism with a dependency.

The PR should not merge until a failed restore cannot replace a concurrently modified rustup nightly with a copy missing its changes.

Findings

1.  Rustup changes become inaccessible ▶

Summary

The PR copies a dated Rust nightly into rustup’s rolling nightly toolchain so cargo +nightly works without installing a second nightly.

• It adds component-aware refresh decisions, filesystem cloning and replacement, documentation, and tests.
• The latest change attempts to keep the alias present when restoring a concurrently modified rustup toolchain fails.

_{Reviews (8) · Last reviewed commit: "fix(rust): keep rustup's nightly present..."}

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Actionable comments posted: 1

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Inline comments:
In @src/plugins/core/rust/nightly_channel.rs:
- Around line 42-83: Update the existing-alias decision in the
`toolchain_nightly` match so an existing rustup `nightly` is left unchanged
rather than replaced based only on its date. Preserve the current installation
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Instruction counts

Nothing was compared, and so nothing was gated. No series appears on both sides: either the base has no measurements recorded, or the two were measured on different runner classes, which are deliberately not comparable — counts shift between machine types by more than a real regression does.

New, nothing to compare against: env on jdx-perf-v1-ubuntu24.04-x64-mise-rust1.97.1-img5263c143, hook-env on jdx-perf-v1-ubuntu24.04-x64-mise-rust1.97.1-img5263c143, ls on jdx-perf-v1-ubuntu24.04-x64-mise-rust1.97.1-img5263c143, registry on jdx-perf-v1-ubuntu24.04-x64-mise-rust1.97.1-img5263c143, startup on jdx-perf-v1-ubuntu24.04-x64-mise-rust1.97.1-img5263c143

_{Only instruction counts gate. Wall clock is shown for context — on identical hardware it moves 4-20% run to run.}

_{Measured by tak — instruction-counted CLI benchmarks, stored in this repository's git notes.}

_{a23557ed80d7 vs 1973f32edf11 · measured on the runner, not pushed to the history.}