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Make Encodable and Encoder infallible. #94732

Merged
merged 5 commits into from
Jun 8, 2022
Merged

Make Encodable and Encoder infallible. #94732

merged 5 commits into from
Jun 8, 2022

Commits on Jun 7, 2022

  1. Remove an unnecessary encoder operation.

    @nnethercote
    nnethercote committed Jun 7, 2022
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  2. Don't pass in a vector to Encoder::new. 

    It's not necessary.
    @nnethercote
    nnethercote committed Jun 7, 2022
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  3. Use delayed error handling for Encodable and Encoder infallible. 

    There are two impls of the `Encoder` trait: `opaque::Encoder` and
`opaque::FileEncoder`. The former encodes into memory and is infallible, the
latter writes to file and is fallible.

Currently, standard `Result`/`?`/`unwrap` error handling is used, but this is a
bit verbose and has non-trivial cost, which is annoying given how rare failures
are (especially in the infallible `opaque::Encoder` case).

This commit changes how `Encoder` fallibility is handled. All the `emit_*`
methods are now infallible. `opaque::Encoder` requires no great changes for
this. `opaque::FileEncoder` now implements a delayed error handling strategy.
If a failure occurs, it records this via the `res` field, and all subsequent
encoding operations are skipped if `res` indicates an error has occurred. Once
encoding is complete, the new `finish` method is called, which returns a
`Result`. In other words, there is now a single `Result`-producing method
instead of many of them.

This has very little effect on how any file errors are reported if
`opaque::FileEncoder` has any failures.

Much of this commit is boring mechanical changes, removing `Result` return
values and `?` or `unwrap` from expressions. The more interesting parts are as
follows.
- serialize.rs: The `Encoder` trait gains an `Ok` associated type. The
  `into_inner` method is changed into `finish`, which returns
  `Result<Vec<u8>, !>`.
- opaque.rs: The `FileEncoder` adopts the delayed error handling
  strategy. Its `Ok` type is a `usize`, returning the number of bytes
  written, replacing previous uses of `FileEncoder::position`.
- Various methods that take an encoder now consume it, rather than being
  passed a mutable reference, e.g. `serialize_query_result_cache`.
    @nnethercote
    nnethercote committed Jun 7, 2022
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  4. Move finish out of the Encoder trait. 

    This simplifies things, but requires making `CacheEncoder` non-generic.
    @nnethercote
    nnethercote committed Jun 7, 2022
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  5. Rename rustc_serialize::opaque::Encoder as MemEncoder. 

    This avoids the name clash with `rustc_serialize::Encoder` (a trait),
and allows lots qualifiers to be removed and imports to be simplified
(e.g. fewer `as` imports).
    @nnethercote
    nnethercote committed Jun 7, 2022
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