## Summary
Historically, we've stored "qualified names" on our
`BindingKind::Import`, `BindingKind::SubmoduleImport`, and
`BindingKind::ImportFrom` structs. In Ruff, a "qualified name" is a
dot-separated path to a symbol. For example, given `import foo.bar`, the
"qualified name" would be `"foo.bar"`; and given `from foo.bar import
baz`, the "qualified name" would be `foo.bar.baz`.
This PR modifies the `BindingKind` structs to instead store _call paths_
rather than qualified names. So in the examples above, we'd store
`["foo", "bar"]` and `["foo", "bar", "baz"]`. It turns out that this
more efficient given our data access patterns. Namely, we frequently
need to convert the qualified name to a call path (whenever we call
`resolve_call_path`), and it turns out that we do this operation enough
that those conversations show up on benchmarks.
There are a few other advantages to using call paths, rather than
qualified names:
1. The size of `BindingKind` is reduced from 32 to 24 bytes, since we no
longer need to store a `String` (only a boxed slice).
2. All three import types are more consistent, since they now all store
a boxed slice, rather than some storing an `&str` and some storing a
`String` (for `BindingKind::ImportFrom`, we needed to allocate a
`String` to create the qualified name, but the call path is a slice of
static elements that don't require that allocation).
3. A lot of code gets simpler, in part because we now do call path
resolution "earlier". Most notably, for relative imports (`from .foo
import bar`), we store the _resolved_ call path rather than the relative
call path, so the semantic model doesn't have to deal with that
resolution. (See that `resolve_call_path` is simpler, fewer branches,
etc.)
In my testing, this change improves the all-rules benchmark by another
4-5% on top of the improvements mentioned in #6047.
## Summary
This PR just naively unrolls `collect_call_path` to handle attribute
resolutions of up to eight segments. In profiling via Instruments, it
seems to be about 4x faster for a very hot code path (4% of total
execution time on `main`, 1% here).
Profiling by running `RAYON_NUM_THREADS=1 cargo instruments -t time
--profile release-debug --time-limit 10000 -p ruff_cli -o
FromSlice.trace -- check crates/ruff/resources/test/cpython --silent -e
--no-cache --select ALL`, and modifying the linter to loop infinitely up
to the specified time (10 seconds) to increase sample size.
Before:
<img width="1792" alt="Screen Shot 2023-07-15 at 5 13 34 PM"
src="4a8b0b45-8b67-43e9-af5e-65b326928a8e">
After:
<img width="1792" alt="Screen Shot 2023-07-15 at 8 38 51 PM"
src="d8829159-2c79-4a49-ab3c-9e4e86f5b2b1">
