simplify_type improvements and cursed docs

[lcnr]

the existing TreatParams enum pretty much mixes everything up. Not sure why this looked right to me in #94057

This also includes two changes which impact perf:

• ty::Projection with inference vars shouldn't be treated as a rigid type, even if fully normalized
• ty::Placeholder only unifies with itself, so actually return Some for them

r? @nikomatsakis

[lcnr]

I feel like SimplifiedType isn't a good name here. Maybe something like RigidTyConstructor or something like that would be better 🤔

edit: after talking to @BoxyUwU, InjectiveTyConstructor might be a pretty good name ✨

[lcnr]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit db19e2b with merge c067287...

[nikomatsakis]

ah yes, I am equally to blame :) it's really just universal (placeholders) vs existential (infer)

[nikomatsakis]

@bors r+

[bors]

📌 Commit db19e2b has been approved by nikomatsakis

[bors]

☀️ Test successful - checks-actions
Approved by: nikomatsakis
Pushing c067287 to master...

[lcnr]

oh, we've used the @bors try from before the r+ to actually merge this PR? Idk if that's an issue? 🤔 cc @rust-lang/infra

[Mark-Simulacrum]

Gah. rust-lang/homu#47 is the issue for this, we'll need to back this out from master I think, I'm not sure whether a force push dropping it is the best idea.

[rust-timer]

Finished benchmarking commit (c067287): comparison url.

Instruction count

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

Max RSS (memory usage)

Results

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: 🎉 relevant improvements found

	Regressions 😿 (primary)	Regressions 😿 (secondary)	Improvements 🎉 (primary)	Improvements 🎉 (secondary)	All 😿 🎉 (primary)
count1	0	0	0	3	0
mean2	N/A	N/A	N/A	-1.6%	N/A
max	N/A	N/A	N/A	-1.8%	N/A

Cycles

Results

• Primary benchmarks: 🎉 relevant improvements found
• Secondary benchmarks: mixed results

	Regressions 😿 (primary)	Regressions 😿 (secondary)	Improvements 🎉 (primary)	Improvements 🎉 (secondary)	All 😿 🎉 (primary)
count1	0	2	3	1	3
mean2	N/A	1.7%	-1.4%	-2.2%	-1.4%
max	N/A	2.0%	-1.4%	-2.2%	-1.4%

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

Footnotes

1. number of relevant changes ↩ ↩²

2. the arithmetic mean of the percent change ↩ ↩²

[Mark-Simulacrum]

To follow up on the merge -- we manually ran CI on the merged commit, which thankfully passed. See https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/try-based.20merge.20into.20master for some discussion.