Lake will now update a package's `lean-toolchain` file on `lake update`
if it finds the package's direct dependencies use a newer compatible
toolchain. To skip this step, use the `--keep-toolchain` CLI option.
Closes#2582. Closes#2752. Closes#5615.
### Toolchain update details
To determine "newest compatible" toolchain, Lake parses the toolchain
listed in the packages' `lean-toolchain` files into four categories:
release , nightly, PR, and other. For newness, release toolchains are
compared by semantic version (e.g., `"v4.4.0" < "v4.8.0"` and
`"v4.6.0-rc1" < "v4.6.0"`) and nightlies are compared by date (e.g.,
`"nightly-2024-01-10" < "nightly-2014-10-01"`). All other toolchain
types and mixtures are incompatible. If there is not a single newest
toolchain, Lake will print a warning and continue updating without
changing the toolchain.
If Lake does find a new toolchain, Lake updates the workspace's
`lean-toolchain` file accordingly and restarts the update process on the
new Lake. If Elan is detected, it will spawn the new Lake process via
`elan run` with the same arguments Lake was initially run with. If Elan
is missing, it will prompt the user to restart Lake manually and exit
with a special error code (4).
### Other changes
To implement this new logic, various other refactors were needed. Here
are some key highlights:
* Logs emitted during package and workspace loading are now eagerly
printed.
* The Elan executable used by Lake is now configurable by the `ELAN`
environment variable.
* The `--lean` CLI option was removed. Use the `LEAN` environment
variable instead.
* `Package.deps` / `Package.opaqueDeps` have been removed. Use
`findPackage?` with a dependency's name instead.
* The dependency resolver now uses a pure breadth-first traversal to
resolve dependencies. It also resolves dependencies in reverse order,
which is done for consistency with targets. Latter targets shadow
earlier ones and latter dependencies take precedence over earlier ones.
**These changes mean the order of dependencies in a Lake manifest will
change after the first `lake update` on this version of Lake.**
