This is a copy of #4296 with these changes:
* file is not reopened again to find if the file is generated
* first five lines are scanned for `@generated` marker instead of one
* no attempt is made to only search for marker in comments
`@generated` marker is used by certain tools to understand that the
file is generated, so it should be treated differently than a file
written by a human:
* linters should not be invoked on these files,
* diffs in these files are less important,
* and these files should not be reformatted.
This PR proposes builtin support for `@generated` marker.
I have not found a standard for a generated file marker, but:
* Facebook [uses `@generated` marker](https://tinyurl.com/fb-generated)
* Phabricator tool which was spawned from Facebook internal tool
[also understands `@generated` marker](https://git.io/JnVHa)
* Cargo inserts `@generated` marker into [generated Cargo.lock files](https://git.io/JnVHP)
My personal story is that rust-protobuf project which I maintain
was broken twice because of incompatibilities/bugs in rustfmt marker
handling: [one](https://github.com/stepancheg/rust-protobuf/issues/493),
[two](https://github.com/stepancheg/rust-protobuf/issues/551).
(Also, rust-protobuf started generating `@generated` marker
[6 years ago](https://git.io/JnV5h)).
While rustfmt AST markers are useful to apply to a certain AST
elements, disable whole-file-at-once all-tools-at-once text level
marker might be easier to use and more reliable for generated code.
Added test covering this. Chose to treat the code block
as rust if and only if all of the comma-separated attributes
are rust-valid. Chose to allow/preserve whitespace around commas
Fixes#3158
In the event a pattern starts with a leading pipe
the pattern span will contain, and begin with, the pipe.
This updates the process to see if a match arm contains
a leading pipe by leveraging this recent(ish) change to
the patterns in the AST, and avoids an indexing bug that
occurs when a pattern starts with a non-ascii char in the
old implementation.
This renames the existing `true`/`false` options to `Crate`/`Never`, then adds a
new `Module` option which causes imports to be grouped together by their
originating module.
