detects redundant imports that can be eliminated.
for #117772 :
In order to facilitate review and modification, split the checking code and
removing redundant imports code into two PR.
Instead of updating global state to mark attributes as used,
we now explicitly emit a warning when an attribute is used in
an unsupported position. As a side effect, we are to emit more
detailed warning messages (instead of just a generic "unused" message).
`Session.check_name` is removed, since its only purpose was to mark
the attribute as used. All of the callers are modified to use
`Attribute.has_name`
Additionally, `AttributeType::AssumedUsed` is removed - an 'assumed
used' attribute is implemented by simply not performing any checks
in `CheckAttrVisitor` for a particular attribute.
We no longer emit unused attribute warnings for the `#[rustc_dummy]`
attribute - it's an internal attribute used for tests, so it doesn't
mark sense to treat it as 'unused'.
With this commit, a large source of global untracked state is removed.
The `large_assignments` lints detects moves over specified limit. The
limit is configured through `move_size_limit = "N"` attribute placed at
the root of a crate. When attribute is absent, the lint is disabled.
Make it possible to enable the lint without making any changes to the
source code, through a new flag `-Zmove-size-limit=N`. For example, to
detect moves exceeding 1023 bytes in a cargo crate, including all
dependencies one could use:
```
$ env RUSTFLAGS=-Zmove-size-limit=1024 cargo build -vv
```
