Box `DiagnosticBuilder`.
It's a large type -- 176 bytes on 64-bit. And it's passed around and
returned from a lot of functions, including within `PResult`.
This commit boxes it, which reduces memory traffic. In particular,
`PResult` shrinks to 16 bytes in the best case; this reduces instruction
counts by up to 2% on various workloads. The commit touches a lot of
lines but it's almost all trivial plumbing changes.
The syntax crate contains those things concerned purely with syntax
– that is, the AST ("abstract syntax tree"), parser, pretty-printer,
lexer, macro expander, and utilities for traversing ASTs.
For more information about how these things work in rustc, see the
rustc guide:
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