Add `try_canonicalize` to `rustc_fs_util` and use it over `fs::canonicalize`
This adds `try_canonicalize` which tries to call `fs::canonicalize`, but falls back to `std::path::absolute` if it fails. Existing `canonicalize` calls are replaced with it. `fs::canonicalize` is not guaranteed to work on Windows.
This commit implements both the native linking modifiers infrastructure
as well as an initial attempt at the individual modifiers from the RFC.
It also introduces a feature flag for the general syntax along with
individual feature flags for each modifier.
This PR modifies the macro expansion infrastructure to handle attributes
in a fully token-based manner. As a result:
* Derives macros no longer lose spans when their input is modified
by eager cfg-expansion. This is accomplished by performing eager
cfg-expansion on the token stream that we pass to the derive
proc-macro
* Inner attributes now preserve spans in all cases, including when we
have multiple inner attributes in a row.
This is accomplished through the following changes:
* New structs `AttrAnnotatedTokenStream` and `AttrAnnotatedTokenTree` are introduced.
These are very similar to a normal `TokenTree`, but they also track
the position of attributes and attribute targets within the stream.
They are built when we collect tokens during parsing.
An `AttrAnnotatedTokenStream` is converted to a regular `TokenStream` when
we invoke a macro.
* Token capturing and `LazyTokenStream` are modified to work with
`AttrAnnotatedTokenStream`. A new `ReplaceRange` type is introduced, which
is created during the parsing of a nested AST node to make the 'outer'
AST node aware of the attributes and attribute target stored deeper in the token stream.
* When we need to perform eager cfg-expansion (either due to `#[derive]` or `#[cfg_eval]`),
we tokenize and reparse our target, capturing additional information about the locations of
`#[cfg]` and `#[cfg_attr]` attributes at any depth within the target.
This is a performance optimization, allowing us to perform less work
in the typical case where captured tokens never have eager cfg-expansion run.
