fix(cargo-anvil): make stable toolchains deterministic

[martin-kolinek]

🤖 Fixes runner-dependent stable Rust selection in cargo-anvil and adds explicit MSRV testing.

What changed

• resolve stable checks from RUSTUP_TOOLCHAIN, then a selecting root rust-toolchain/rust-toolchain.toml, then the root MSRV
• fail instead of inheriting a runner image's moving stable toolchain
• preserve full toolchain-file behavior, including targets, components, and profile
• validate that MSRV fallback is uniform across workspace packages
• run affected-package unit and integration tests under every declared root MSRV with all features and default features
• execute MSRV tests across the same four GitHub and two ADO platform legs as ordinary PR tests so cfg-gated configurations are covered
• allow internal builds to map the public MSRV to a provisioned toolchain through ANVIL_MSRV_TOOLCHAIN
• apply deterministic selection before compiler/cache setup in GitHub Actions, Azure Pipelines, and container builds
• update design docs, generated artifacts, snapshots, and resolver regression coverage

Adopter migration

Ambient runner stable is no longer accepted. Adopters must provide one of:

• a root [workspace.package].rust-version or [package].rust-version shared by every workspace package
• a selecting root rust-toolchain or rust-toolchain.toml
• a preconfigured RUSTUP_TOOLCHAIN

Workspaces with missing or heterogeneous package MSRVs cannot use MSRV fallback and must select a catalog toolchain explicitly. Internal builds can override public selection with RUSTUP_TOOLCHAIN and map the public MSRV test run to a provisioned compiler with ANVIL_MSRV_TOOLCHAIN.

Motivation

Oxidizer PR #698 passed the legacy Rust 1.96.1 job but failed Anvil on a Windows ARM runner that had inherited Rust 1.98.0 and its new Clippy lints. Stable checks should not change behavior based on runner image rollout timing.

Coverage executes the ordinary test suite on the catalog nightly. The dedicated pr-msrv group runs in parallel and ensures that the same affected unit and integration tests continue to compile and pass on the minimum supported compiler across every supported host configuration.

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.7%. Comparing base (ff4aa04) to head (264c51a).

❌ Your project status has failed because the head coverage (97.7%) is below the target coverage (100.0%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #109   +/-   ##
=====================================
  Coverage   97.7%   97.7%           
=====================================
  Files        286     286           
  Lines      62438   62441    +3     
=====================================
+ Hits       61009   61015    +6     
+ Misses      1429    1426    -3     

Flag	Coverage Δ
linux	97.6% <100.0%> (?)
linux-arm	97.6% <100.0%> (?)
windows	97.9% <100.0%> (?)

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[Copilot]

Pull request overview

Copilot reviewed 93 out of 94 changed files in this pull request and generated 2 comments.

Suppressed comments (2)

Previously missed (1) — in code that hasn't changed since the last review.

justfiles/anvil/tools.just:483

• _check-tool runs cargo {{_anvil_stable_toolchain_args}} install --list during validate-prereqs. When the stable selector falls back to +<MSRV>, this can cause rustup to auto-install the MSRV toolchain (or fail in a confusing way) just to list installed cargo tools, which makes validation non-read-only and hides the intended "install stable toolchain" guidance.

justfiles/anvil/tools.just:254

• Test-ToolchainInstalled doesn't recognize installed stable-version toolchains when rustup normalizes them to include a patch (e.g. 1.95.0-x86_64-...). The current regex only matches ^<toolchain>(-|$), so 1.95 will not match 1.95.0-..., causing false "not installed" errors and redundant install attempts.

[Copilot]

Pull request overview

Copilot reviewed 93 out of 94 changed files in this pull request and generated 1 comment.

Suppressed comments (1)

Previously missed (1) — in code that hasn't changed since the last review.

justfiles/anvil/tools.just:272

• cargo metadata writes JSON to stdout and warnings/errors to stderr. Capturing 2>&1 into $metadataText can corrupt the JSON even when cargo metadata succeeds with warnings, causing ConvertFrom-Json to throw a misleading parse error. Capture stdout only for the success path, and only merge stderr when re-running for diagnostics on failure.

[Copilot]

Pull request overview

Copilot reviewed 93 out of 94 changed files in this pull request and generated no new comments.

Suppressed comments (4)

Previously missed (2) — in code that hasn't changed since the last review.

justfiles/anvil/tools.just:271

• cargo metadata output is captured with 2>&1 and then piped to ConvertFrom-Json. Any warning text written to stderr (even when the command succeeds) will corrupt the JSON and make MSRV compatibility validation fail non-deterministically.

This issue also appears on line 489 of the same file.
crates/cargo-anvil/templates/justfiles/anvil/tools.just:271

• cargo metadata output is captured with 2>&1 and then piped to ConvertFrom-Json. Any warning text written to stderr (even when the command succeeds) will corrupt the JSON and make MSRV compatibility validation fail non-deterministically.

This issue also appears on line 489 of the same file.

justfiles/anvil/tools.just:491

• If cargo install --list fails (for example because RUSTUP_AUTO_INSTALL=0 prevents auto-provisioning of the selected toolchain), this code silently treats the tool as missing and prints an incorrect install hint. It should fail with a message that the selected stable toolchain/cargo invocation is unavailable.
  crates/cargo-anvil/templates/justfiles/anvil/tools.just:491
• If cargo install --list fails (for example because RUSTUP_AUTO_INSTALL=0 prevents auto-provisioning of the selected toolchain), this code silently treats the tool as missing and prints an incorrect install hint. It should fail with a message that the selected stable toolchain/cargo invocation is unavailable.

[Copilot]

Pull request overview

Copilot reviewed 93 out of 94 changed files in this pull request and generated no new comments.

Suppressed comments (4)

Previously missed (2) — in code that hasn't changed since the last review.

justfiles/anvil/tools.just:633

• When the stable selection is provided via RUSTUP_TOOLCHAIN or a repository rust-toolchain(.toml), this branch runs rustup component add <component> without --toolchain. rustup component add targets rustup's default toolchain, not the toolchain selected by RUSTUP_TOOLCHAIN or by a toolchain file, so clippy/rustfmt/etc. can be installed onto the wrong toolchain and the subsequent cargo clippy/cargo fmt checks can still fail.

This issue also appears on line 715 of the same file.
crates/cargo-anvil/templates/justfiles/anvil/tools.just:633

• When the stable selection is provided via RUSTUP_TOOLCHAIN or a repository rust-toolchain(.toml), this branch runs rustup component add <component> without --toolchain. rustup component add targets rustup's default toolchain, not the toolchain selected by RUSTUP_TOOLCHAIN or by a toolchain file, so clippy/rustfmt/etc. can be installed onto the wrong toolchain and the subsequent cargo clippy/cargo fmt checks can still fail.

This issue also appears on line 715 of the same file.

justfiles/anvil/tools.just:719

• The remediation hint for missing components suggests rustup component add <component> for the default toolchain when the stable selection comes from RUSTUP_TOOLCHAIN or a repository rust-toolchain(.toml). That command installs onto rustup's default toolchain, which may not be the toolchain Anvil is actually using, so the suggested fix can be ineffective.
  crates/cargo-anvil/templates/justfiles/anvil/tools.just:719
• The remediation hint for missing components suggests rustup component add <component> for the default toolchain when the stable selection comes from RUSTUP_TOOLCHAIN or a repository rust-toolchain(.toml). That command installs onto rustup's default toolchain, which may not be the toolchain Anvil is actually using, so the suggested fix can be ineffective.

[Copilot]

Pull request overview

Copilot reviewed 93 out of 94 changed files in this pull request and generated no new comments.

Suppressed comments (2)

justfiles/anvil/tools.just:254

• Test-ToolchainInstalled() only matches toolchain list entries that are exactly '-' or '' at the start of the line. For MSRV values like "1.95" (this repo uses rust-version = "1.95"), rustup typically lists the installed toolchain as "1.95.0-", so this check will incorrectly report the toolchain as not installed and can break validate-workspace-msrv / re-run installs repeatedly. Consider also matching a dot after the requested prefix so "1.95" matches "1.95.0-…".
  crates/cargo-anvil/templates/justfiles/anvil/tools.just:254
• Same as the emitted justfile: Test-ToolchainInstalled() only matches '-' or '' at line start. For MSRV values like "1.95" rustup typically lists the installed toolchain as "1.95.0-", so this check can incorrectly report the MSRV toolchain as missing and break validate-workspace-msrv / repeat installs. Match a dot as well so "1.95" matches "1.95.0-…".

[Copilot]

Pull request overview

Copilot reviewed 93 out of 94 changed files in this pull request and generated 2 comments.

Suppressed comments (1)

justfiles/anvil/tools.just:695

• In _check-component, when toolchain="default" and stable selection resolves to an explicit MSRV (via _anvil_stable_toolchain_args), failures in the rustc ... --print sysroot probe report toolchain 'default' not installed and suggest rustup toolchain install default, which is not actionable ("default" isn't a rustup toolchain name).