Nightly sync mode

[flip1995]

Clippy wants to pin its version in the repo to a nightly, so that contributors don't need any special tooling in order to compile it and rustup will automatically take care of toolchain management.

This requires some extra logic for syncing:

• read and modify the rust-toolchain.toml file -> toml_edit
• get the latest nightly -> chrono -> date -> nightly-{date}
• get the commit from the current/next nightly -> rustc +nightly-{date} -vV

This PR moves a bunch of code around to add the new nightly sync-mode as just a new helper function. More details of the separate steps in the commit messages.

WIP, as I'm still testing it with this config:

org = "rust-lang"
repo = "rust-clippy"
filter = ":~(history=\"keep-trivial-merges,no-splice\")[:rev(<=7c06e7c42a8b56f9c1ac20e9ce34cadca0fce100:prefix=src/tools/clippy,<=da5114692c9ebe46b869488c5f34f92eb10b98c1:SQUASH)]:/src/tools/clippy"
filter-version = 2
base-commit = "Nightly"
rust-version-path = "rust-toolchain.toml"
post-pull = [ { cmd = [ "cargo dev sync update_nightly" ], commit-message = "Update nightly in clippy-utils README.md"} ]

[Kobzol]

Hi, thanks for the PR. Fiddling with the TOML adds some complexity, but it makes sense.

I wonder, instead of using dates manually, maybe we can download https://static.rust-lang.org/manifests.txt and read the latest entry?

[bjorn3]

manifests.txt in the past on occasion not been getting updated for several days in a row.

[Kobzol]

I think that only happens if we forget to update the CI job which needs to be poked once every 6 months I think 😆 But I realized that adding a dependency on a network client is perhaps a bit of a heavy hammer for this functionality.

Using the local nightly rustc to figure out its commit hash actually seems quite elegant. It could even work on CI, if we do something like rustup update nightly, and then read both the date and the commit hash from it. That might be the simplest solution, after all.

This PR already reads the hash from the nightly rustc, so maybe it could also read its date from it? Or you could pass the nightly verison explicitly. It seems better to me than guessing the date, because the nightly dates are always offset-by-one and it gets confusing.

[RalfJung]

base-commit = "Nightly"
rust-version-path = "rust-toolchain.toml"

These only really make sense in combination, right? IMO they should be controlled by a single flag.

[bjorn3]

It could even work on CI, if we do something like rustup update nightly, and then read both the date and the commit hash from it.

That would mean you did have to download rustc twice, right? Once for the nightly toolchain and once for nightly-$(date). I did rather either fetch both date and commit from the nightly manifest: https://static.rust-lang.org/dist/channel-rust-nightly.toml or take the current date and then download the corresponding nightly toolchain and get the commit from it.

It seems better to me than guessing the date, because the nightly dates are always offset-by-one and it gets confusing.

The date reported by rustc is off by one, but the date in the toolchain name matches the current day.

[Kobzol]

Ok, fair enough. Since we would anyway be dealing with parsing TOML files, downloading from the manifest seems reasonable, as it is the source of truth for Rustup too, and contains both the date and the commit.