docs(mirrors): the -py install fetches a backend

mirrors/py/pre-commit-hooks.yaml

@@ -6,7 +6,10 @@
 #
 # `pre-commit` installs a `language: python` hook with `pip install .` against
 # the clone, and the package declares no dependencies, so these two ids resolve
-# from this tree alone and reach no registry.
+# their own code from this tree rather than from a registry. The build backend
+# is the one thing fetched: `pip install .` builds in isolation and takes
+# `hatchling` from an index, so the install needs one even though the run does
+# not.
 #
 # Generated. Edits belong upstream: this tree is replaced wholesale, never
 # merged.

mirrors/py/pyproject.toml.in

@@ -18,8 +18,11 @@ authors = [{ name = 'Michael I Chen' }]
 # 3.10 tracks what `pre-commit` itself supports, and a hook cannot impose an
 # interpreter on the repositories that install it.
 requires-python = '>=3.10'
-# Empty, and load-bearing: it is what lets a hook environment build from this
-# clone with no network at all.
+# Empty, and load-bearing: a hook environment installs this package and then
+# fetches nothing else in order to run it. The build is the exception, and it
+# is worth stating because the absence of dependencies invites the stronger
+# reading: `pip install .` builds in isolation, so `hatchling` is installed
+# from an index first, and an install therefore still needs one.
 dependencies = []
 
 [project.urls]

mirrors/rs/pre-commit-hooks.yaml

@@ -6,8 +6,9 @@
 #
 # `pre-commit` installs a `language: rust` hook with
 # `cargo install --bins --root <envdir> --path .`, so it builds the wrapper
-# crate in this tree, which pulls the implementation from crates.io. That one
-# fetch is the difference from the `-py` mirror, which needs no network.
+# crate in this tree, which pulls the implementation from crates.io. That fetch
+# and the toolchain it needs are what separate the two mirrors, rather than
+# network against no network: the `-py` install fetches its build backend.
 #
 # Generated. Edits belong upstream: this tree is replaced wholesale, never
 # merged.