The original purpose of this PR was to modernize the external parser to
use the new Shape system.
This commit does include some of that change, but a more important
aspect of this change is an improvement to the expansion trace.
Previous commit 6a7c00ea adding trace infrastructure to the syntax coloring
feature. This commit adds tracing to the expander.
The bulk of that work, in addition to the tree builder logic, was an
overhaul of the formatter traits to make them more general purpose, and
more structured.
Some highlights:
- `ToDebug` was split into two traits (`ToDebug` and `DebugFormat`)
because implementations needed to become objects, but a convenience
method on `ToDebug` didn't qualify
- `DebugFormat`'s `fmt_debug` method now takes a `DebugFormatter` rather
than a standard formatter, and `DebugFormatter` has a new (but still
limited) facility for structured formatting.
- Implementations of `ExpandSyntax` need to produce output that
implements `DebugFormat`.
Unlike the highlighter changes, these changes are fairly focused in the
trace output, so these changes aren't behind a flag.
Previously, there was a single parsing rule for "bare words" that
applied to both internal and external commands.
This meant that, because `cargo +nightly` needed to work, we needed to
add `+` as a valid character in bare words.
The number of characters continued to grow, and the situation was
becoming untenable. The current strategy would eventually eat up all
syntax and make it impossible to add syntax like `@foo` to internal
commands.
This patch significantly restricts bare words and introduces a new token
type (`ExternalWord`). An `ExternalWord` expands to an error in the
internal syntax, but expands to a bare word in the external syntax.
`ExternalWords` are highlighted in grey in the shell.
