Implements [flake8-commas](https://github.com/PyCQA/flake8-commas). Fixes#1058.
The plugin is mostly redundant with Black (and also deprecated upstream), but very useful for projects which can't/won't use an auto-formatter.
This linter works on tokens. Before porting to Rust, I cleaned up the Python code ([link](https://gist.github.com/bluetech/7c5dcbdec4a73dd5a74d4bc09c72b8b9)) and made sure the tests pass. In the Rust version I tried to add explanatory comments, to the best of my understanding of the original logic.
Some changes I did make:
- Got rid of rule C814 - "missing trailing comma in Python 2". Ruff doesn't support Python 2.
- Merged rules C815 - "missing trailing comma in Python 3.5+" and C816 - "missing trailing comma in Python 3.6+" into C812 - "missing trailing comma". These Python versions are outdated, didn't think it was worth the complication.
- Added autofixes for C812 and C819.
Autofix is missing for C818 - "trailing comma on bare tuple prohibited". It needs to turn e.g. `x = 1,` into `x = (1, )`, it's a bit difficult to do with tokens only, so I skipped it for now.
I ran the rules on cpython/Lib and on a big internal code base and it works as intended (though I only sampled the diffs).
define_rule_mapping! was previously implemented as a declarative macro,
which was however partially relying on an origin_by_code! proc macro
because declarative macros cannot match on substrings of identifiers.
Currently all define_rule_mapping! lines look like the following:
TID251 => violations::BannedApi,
TID252 => violations::BannedRelativeImport,
We want to break up violations.rs, moving the violation definitions to
the respective rule modules. To do this we want to change the previous
lines to:
TID251 => rules::flake8_tidy_imports::banned_api::BannedApi,
TID252 => rules::flake8_tidy_imports::relative_imports::RelativeImport,
This however doesn't work because the define_rule_mapping! macro is
currently defined as:
($($code:ident => $mod:ident::$name:ident,)+) => { ... }
That is it only supported $module::$name but not longer paths with
multiple modules. While we could define `=> $path:path`[1] then we
could no longer access the last path segment, which we need because
we use it for the DiagnosticKind variant names. And
`$path:path::$last:ident` doesn't work either because it would be
ambiguous (Rust wouldn't know where the path ends ... so path fragments
have to be followed by some punctuation/keyword that may not be part of
paths). And we also cannot just introduce a procedural macro like
path_basename!(...) because the following is not valid Rust code:
enum Foo { foo!(...), }
(macros cannot be called in the place where you define variants.)
So we have to convert define_rule_mapping! into a proc macro in order to
support paths of arbitrary length and this commit implements that.
[1]: https://doc.rust-lang.org/reference/macros-by-example.html#metavariables
We don't have any doctests, but `cargo test --all` spends more than half
the time on doctests? A little confusing, but this brings the test time
from > 4s to < 2s on my machine.
We can reverse this later if it really becomes necessary, but I expect
safe Rust to be sufficient for all our needs.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
