Improve diagnostics and code for exhaustiveness of empty matches
There was a completely separate check and diagnostics for the case of an empty match. This led to slightly different error messages and duplicated code.
This improves code reuse and generally clarifies what happens for empty matches. This also clarifies the action of the `exhaustive_patterns` feature, and ensures that this feature doesn't change diagnostics in places it doesn't need to.
Conditionals and loops now have unstable features, and `feature_err` has
its own error code. I think that `feature_err` should take an error code
as a parameter, but don't have the energy to make this change throughout
the codebase. Also, the error code system may be torn out entirely.
Remove the `DelimSpan` from `NamedMatch::MatchedSeq`.
Because it's unused. This then allows the removal of
`MatcherPos::sp_open`. It's a tiny perf win, reducing instruction counts
by 0.1% - 0.2% on a few benchmarks.
r? @Centril
LinkedList: drop remaining items when drop panics
https://github.com/rust-lang/rust/pull/67235, but for `LinkedList`, which has the same issue.
I've also copied over the other drop-related tests from `VecDeque` since AFAICT `LinkedList` didn't have any.
Match `VecDeque::extend` to `Vec::extend_desugared`
Currently, `VecDeque::extend` [does not reserve at all](https://github.com/rust-lang/rust/pull/65069#discussion_r333166522). This implementation still runs a check every iteration of the loop, but should reallocate at most once for the common cases where the `size_hint` lower bound is exact. Further optimizations in the future could improve this for some common cases, but given the complexity of the `Vec::extend` implementation it's not immediately clear that this would be worthwhile.
This commit implements the `write_vectored` method of the `LineWriter`
type. First discovered in bytecodealliance/wasmtime#629 the
`write_vectored` method of `Stdout` bottoms out here but only ends up
writing the first buffer due to the default implementation of
`write_vectored`.
Like `BufWriter`, however, `LineWriter` can have a non-default
implementation of `write_vectored` which tries to preserve the
vectored-ness as much as possible. Namely we can have a vectored write
for everything before the newline and everything after the newline if
all the stars align well.
Also like `BufWriter`, though, special care is taken to ensure that
whenever bytes are written we're sure to signal success since that
represents a "commit" of writing bytes.
