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Make Decodable and Decoder infallible. #93066

Merged
merged 3 commits into from
Jan 23, 2022
Merged

Make Decodable and Decoder infallible. #93066

merged 3 commits into from
Jan 23, 2022

Commits on Jan 21, 2022

  1. Rename Decoder::read_nil and read_unit. 

    Because `()` is called "unit" and it makes it match
`Encoder::emit_unit`.
    @nnethercote
    nnethercote committed Jan 21, 2022
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  2. Make Decodable and Decoder infallible. 

    `Decoder` has two impls:
- opaque: this impl is already partly infallible, i.e. in some places it
  currently panics on failure (e.g. if the input is too short, or on a
  bad `Result` discriminant), and in some places it returns an error
  (e.g. on a bad `Option` discriminant). The number of places where
  either happens is surprisingly small, just because the binary
  representation has very little redundancy and a lot of input reading
  can occur even on malformed data.
- json: this impl is fully fallible, but it's only used (a) for the
  `.rlink` file production, and there's a `FIXME` comment suggesting it
  should change to a binary format, and (b) in a few tests in
  non-fundamental ways. Indeed rust-lang#85993 is open to remove it entirely.

And the top-level places in the compiler that call into decoding just
abort on error anyway. So the fallibility is providing little value, and
getting rid of it leads to some non-trivial performance improvements.

Much of this commit is pretty boring and mechanical. Some notes about
a few interesting parts:
- The commit removes `Decoder::{Error,error}`.
- `InternIteratorElement::intern_with`: the impl for `T` now has the same
  optimization for small counts that the impl for `Result<T, E>` has,
  because it's now much hotter.
- Decodable impls for SmallVec, LinkedList, VecDeque now all use
  `collect`, which is nice; the one for `Vec` uses unsafe code, because
  that gave better perf on some benchmarks.
    @nnethercote
    nnethercote committed Jan 21, 2022
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  3. Address review comments.

    @nnethercote
    nnethercote committed Jan 21, 2022
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