tyencode: Make sure that projection bounds are handled in stable order.

[michaelwoerister]

Fixes #34796.

r? @alexcrichton

[michaelwoerister]

cc @eddyb

[eddyb]

@bors r+

[bors]

📌 Commit 5ad5072 has been approved by eddyb

[alexcrichton]

triage: beta-nominated

[aravind-pg]

Newbie question (and total nit): why couldn't this basically be the following (or something close to it)?

let mut projection_bounds = bs.projection_bounds
                              .clone()
                              .sort_by_key(|&b| b.item_name().as_str());

Because then it seems we'd be able to keep the next line just for tp in &projection_bounds { ... }, avoiding the extra call to map.

And even if we didn't do this the next line still seems like it could stay for tp in &projection_bounds with s/tp.0/tp.1.0 in the loop body.

[michaelwoerister]

Since sort_by_key() returns nothing, projection_bounds with be the unit value (). It would have to look at least like:

let mut projection_bounds = bs.projection_bounds.clone();
projection_bounds.sort_by_key(|&b| b.item_name().as_str());

And why didn't I do it like the above? In order to avoid that b.item_name().as_str() gets called more than once per item. Although, I somehow mistakenly assumed that b.item_name().as_str() would need to hash the string each time when looking it up in the interner, which it doesn't, so this "optimization" is even more premature than I thought :)

So, your version, when fixed as indicated, would actually be nicer than mine.

[aravind-pg]

Ahh, because sort_by_key would make basically O(n log n) calls to its closure, i.e. to b.item_name().as_str()? Makes much more sense now (notwithstanding the question of whether it's a premature optimization :). And yeah I overlooked the fact that sort_by_key returns unit, so your fix makes sense. Thanks for the explanation! Up to you what changes you want to make, if any.

[alexcrichton]

@bors: p=1

(fixing a regression)

[bors]

⌛ Testing commit 5ad5072 with merge 3c85f41...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild
Approved by: eddyb
Pushing 3c85f41 to master...