This commit is an implementation of [RFC 1513] which allows applications to
alter the behavior of panics at compile time. A new compiler flag, `-C panic`,
is added and accepts the values `unwind` or `panic`, with the default being
`unwind`. This model affects how code is generated for the local crate, skipping
generation of landing pads with `-C panic=abort`.
[RFC 1513]: https://github.com/rust-lang/rfcs/blob/master/text/1513-less-unwinding.md
Panic implementations are then provided by crates tagged with
`#![panic_runtime]` and lazily required by crates with
`#![needs_panic_runtime]`. The panic strategy (`-C panic` value) of the panic
runtime must match the final product, and if the panic strategy is not `abort`
then the entire DAG must have the same panic strategy.
With the `-C panic=abort` strategy, users can expect a stable method to disable
generation of landing pads, improving optimization in niche scenarios,
decreasing compile time, and decreasing output binary size. With the `-C
panic=unwind` strategy users can expect the existing ability to isolate failure
in Rust code from the outside world.
Organizationally, this commit dismantles the `sys_common::unwind` module in
favor of some bits moving part of it to `libpanic_unwind` and the rest into the
`panicking` module in libstd. The custom panic runtime support is pretty similar
to the custom allocator support with the only major difference being how the
panic runtime is injected (takes the `-C panic` flag into account).
Paths are mostly parsed without taking whitespaces into account, e.g. `std :: vec :: Vec :: new ()` parses successfully, however, there are some special cases involving keywords `super`, `self` and `Self`. For example, `self::` is considered a path start only if there are no spaces between `self` and `::`. These restrictions probably made sense when `self` and friends weren't keywords, but now they are unnecessary.
The first two commits remove this special treatment of whitespaces by removing `token::IdentStyle` entirely and therefore fix https://github.com/rust-lang/rust/issues/14109.
This change also affects naked `self` and `super` (which are not tightly followed by `::`, obviously) they can now be parsed as paths, however they are still not resolved correctly in imports (cc @jseyfried, see `compile-fail/use-keyword.rs`), so https://github.com/rust-lang/rust/issues/29036 is not completely fixed.
The third commit also makes `super`, `self`, `Self` and `static` keywords nominally (before this they acted as keywords for all purposes) and removes most of remaining \"special idents\".
The last commit (before tests) contains some small improvements - some qualified paths with type parameters are parsed correctly, `parse_path` is not used for parsing single identifiers, imports are sanity checked for absence of type parameters - such type parameters can be generated by syntax extensions or by macros when https://github.com/rust-lang/rust/issues/10415 is fixed (~~soon!~~already!).
This patch changes some pretty basic things in `libsyntax`, like `token::Token` and the keyword list, so it's a plugin-[breaking-change].
r? @eddyb
nodes in statement position.
Extended #[cfg] folder to allow removal of statements, and
of expressions in optional positions like expression lists and trailing
block expressions.
Extended lint checker to recognize lint levels on expressions and
locals.
Make sure Name, SyntaxContext and Ident are passed by value
Make sure Idents don't serve as keys (or parts of keys) in maps, Ident comparison is not well defined
