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3
src/macro-expansion.md
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@ -2,9 +2,6 @@
<!-- toc --> <!-- toc -->
> N.B. [`rustc_ast`], [`rustc_expand`], and [`rustc_builtin_macros`] are all
> undergoing refactoring, so some of the links in this chapter may be broken.
Rust has a very powerful macro system. In the previous chapter, we saw how Rust has a very powerful macro system. In the previous chapter, we saw how
the parser sets aside macros to be expanded (using temporary [placeholders]). the parser sets aside macros to be expanded (using temporary [placeholders]).
This chapter is about the process of expanding those macros iteratively until This chapter is about the process of expanding those macros iteratively until

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src/traits/unsize.md
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@ -32,21 +32,21 @@ Built-in implementations are provided for:
## Structural implementations ## Structural implementations
There are two implementations of `Unsize` which can be thought of as There is one implementation of `Unsize` which can be thought of as
structural: structural:
* `(A1, A2, .., An): Unsize<(A1, A2, .., U)>` given `An: Unsize<U>`, which
allows the tail field of a tuple to be unsized. This is gated behind the
[`unsized_tuple_coercion`] feature.
* `Struct<.., Pi, .., Pj, ..>: Unsize<Struct<.., Ui, .., Uj, ..>>` given * `Struct<.., Pi, .., Pj, ..>: Unsize<Struct<.., Ui, .., Uj, ..>>` given
`TailField<Pi, .., Pj>: Unsize<Ui, .. Uj>`, which allows the tail field of a `TailField<Pi, .., Pj>: Unsize<Ui, .. Uj>`, which allows the tail field of a
struct to be unsized if it is the only field that mentions generic parameters struct to be unsized if it is the only field that mentions generic parameters
`Pi`, .., `Pj` (which don't need to be contiguous). `Pi`, .., `Pj` (which don't need to be contiguous).
The rules for the latter implementation are slightly complicated, since they The rules for struct unsizing are slightly complicated, since they
may allow more than one parameter to be changed (not necessarily unsized) and may allow more than one parameter to be changed (not necessarily unsized) and
are best stated in terms of the tail field of the struct. are best stated in terms of the tail field of the struct.
[`unsized_tuple_coercion`]: https://doc.rust-lang.org/beta/unstable-book/language-features/unsized-tuple-coercion.html (Tuple unsizing was previously implemented behind the feature gate
`unsized_tuple_coercion`, but the implementation was removed by [#137728].)
[#137728]: https://github.com/rust-lang/rust/pull/137728
## Upcasting implementations ## Upcasting implementations