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Overhaul syntax::fold::Folder. #58061

Merged
merged 10 commits into from
Feb 6, 2019
Merged

Overhaul syntax::fold::Folder. #58061

merged 10 commits into from
Feb 6, 2019

Commits on Feb 5, 2019

  1. Remove some unnecessary ast:: and fold:: qualifiers.

    @nnethercote
    nnethercote committed Feb 5, 2019
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  2. Streamline Folder. 

    Specifically:

- Remove dead methods: fold_usize, fold_meta_items, fold_opt_bounds.

- Remove useless methods: fold_global_asm, fold_range_end.

- Inline and remove unnecessary methods: fold_item_simple,
  fold_foreign_item_simple.
    @nnethercote
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  3. Fold some overlooked spans.

    @nnethercote
    nnethercote committed Feb 5, 2019
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  4. Simplify fold_attribute. 

    It doesn't need to return an `Option`.
    @nnethercote
    nnethercote committed Feb 5, 2019
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  5. Change fold_qpath to fold_qself. 

    It's simpler that way.
    @nnethercote
    nnethercote committed Feb 5, 2019
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  6. Neaten up fold_crate.

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  7. Streamline Folder some more. 

    By eliminating some unnecessary methods, and moving/renaming some
functions that look like they might be methods but aren't.
    @nnethercote
    nnethercote committed Feb 5, 2019
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  8. Various improvements in Folder impls.

    @nnethercote
    nnethercote committed Feb 5, 2019
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  9. Overhaul syntax::fold::Folder. 

    This commit changes `syntax::fold::Folder` from a functional style
(where most methods take a `T` and produce a new `T`) to a more
imperative style (where most methods take and modify a `&mut T`), and
renames it `syntax::mut_visit::MutVisitor`.

The first benefit is speed. The functional style does not require any
reallocations, due to the use of `P::map` and
`MoveMap::move_{,flat_}map`. However, every field in the AST must be
overwritten; even those fields that are unchanged are overwritten with
the same value. This causes a lot of unnecessary memory writes. The
imperative style reduces instruction counts by 1--3% across a wide range
of workloads, particularly incremental workloads.

The second benefit is conciseness; the imperative style is usually more
concise. E.g. compare the old functional style:
```
fn fold_abc(&mut self, abc: ABC) {
    ABC {
        a: fold_a(abc.a),
        b: fold_b(abc.b),
        c: abc.c,
    }
}
```
with the imperative style:
```
fn visit_abc(&mut self, ABC { a, b, c: _ }: &mut ABC) {
    visit_a(a);
    visit_b(b);
}
```
(The reductions get larger in more complex examples.)

Overall, the patch removes over 200 lines of code -- even though the new
code has more comments -- and a lot of the remaining lines have fewer
characters.

Some notes:

- The old style used methods called `fold_*`. The new style mostly uses
  methods called `visit_*`, but there are a few methods that map a `T`
  to something other than a `T`, which are called `flat_map_*` (`T` maps
  to multiple `T`s) or `filter_map_*` (`T` maps to 0 or 1 `T`s).

- `move_map.rs`/`MoveMap`/`move_map`/`move_flat_map` are renamed
  `map_in_place.rs`/`MapInPlace`/`map_in_place`/`flat_map_in_place` to
  reflect their slightly changed signatures.

- Although this commit renames the `fold` module as `mut_visit`, it
  keeps it in the `fold.rs` file, so as not to confuse git. The next
  commit will rename the file.
    @nnethercote
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  10. Rename fold.rs as mut_visit.rs.

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