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81996f1bcc
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