Refactor type visitor walking

[oli-obk]

r? @petrochenkov

pulling out the uncontroversial parts of #113671

[petrochenkov]

Outdated comment, this function is no longer just about opaques.
(And one more similar comment below.)

[petrochenkov]

r=me with the comments updated.
@rustbot author

[oli-obk]

@bors r=petrochenkov

[bors]

📌 Commit 3121576 has been approved by petrochenkov

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3121576 with merge c2ef351...

[bors]

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing c2ef351 to master...

[rust-timer]

Finished benchmarking commit (c2ef351): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.1%]	11
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.2%	[-0.3%, -0.1%]	11

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: missing data
Artifact size: 304.44 MiB -> 304.40 MiB (-0.01%)

[xxchan]

Hi, this PR caused a regression for us. Don't know if it's expected. 👀

---

searched nightlies: from nightly-2023-10-25 to nightly-2023-10-27
regressed nightly: nightly-2023-10-26
searched commit range: df871fb...ab5c841
regressed commit: c2ef351

bisected with cargo-bisect-rustc v0.6.7

Host triple: aarch64-apple-darwin
Reproduce with:

cargo bisect-rustc --start 2023-10-25 --end 2023-10-27 -- check -p risingwave_storage

---

There's a lot of "non-defining opaque type use in defining scope" errors.
https://github.com/risingwavelabs/risingwave/actions/runs/6673382874/job/18138864486 (Sorry for not minimized)

error[E0792]: non-defining opaque type use in defining scope
   --> src/storage/src/mem_table.rs:444:9
    |
444 | /         async move {
445 | |             let stream = self
446 | |                 .inner
447 | |                 .iter(key_range.clone(), self.epoch(), read_options)
...   |
454 | |             ))
455 | |         }
    | |_________^ argument `'_` is not a generic parameter
    |
note: for this opaque type
   --> src/storage/src/mem_table.rs:304:1
    |
304 | #[try_stream(ok = StateStoreIterItem, error = StorageError)]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[oli-obk]

Nope, no regression was expected. I'll try to find a minimization on Monday. You're using a lot of unstable features and proc macros, so i'll try to come up with an example by looking at this PRs diff instead of minimizing your crate

[oli-obk]

Somewhat minimized

// edition: 2021

#![feature(impl_trait_in_assoc_type)]

use std::future::Future;

pub struct MemtableLocalStateStore {
    mem_table: MemTable,
}

impl LocalStateStore for MemtableLocalStateStore {
    type IterStream<'a> = impl StateStoreIterItemStream + 'a where Self: 'a;

    fn iter(&self) -> impl Future<Output = Self::IterStream<'_>> + '_ {
        async move { merge_stream(self.mem_table.iter()) }
    }
}

trait LocalStateStore {
    type IterStream<'a>
    where
        Self: 'a;

    fn iter(&self) -> impl Future<Output = Self::IterStream<'_>> + '_;
}
trait StateStoreIterItemStream {}

impl<T> StateStoreIterItemStream for T {}

struct MemTable;

impl MemTable {
    fn iter<'a>(&'a self) -> impl Iterator<Item = &'a ()> {
        std::iter::empty()
    }
}

pub(crate) async fn merge_stream<'a>(mem_table_iter: impl Iterator<Item = &'a ()>) {}

fn main() {}

[oli-obk]

#117371 silences the non-defining opaque type use error on the iter return type. the merge_stream return type error persists

[oli-obk]

#117396 fixes the regression correctly