Make sure to instantiate placeholders correctly in old solver

[compiler-errors]

When creating the query substitution guess for an input placeholder type like !1_T (in universe 1), we were guessing the response substitution with something like !0_T. This failed to unify with !1_T, causing an ICE.

This PR reworks the query substitution guess code to work a bit more like the new solver. I'm pretty sure this is correct, though I'd really appreciate some scrutiny from someone (cough lcnr) who knows a bit more about query instantiation :)

Fixes #119941

r? lcnr

[compiler-errors]

I'm somewhat concerned that this doesn't fail if we, for example, replace with this for<'a> <&'a () as Id>::Output, which should have this same issue? 🤔

[lcnr]

I think this is also an issue for regions, but we may never canonicalize placeholders from a non-root universe?

in MIR typeck we instantiate the binder in the query, so it doesn't contain placeholders.

Method probing also only has root variables 🤔

Where else do we use the old canonicalizer?

[lcnr]

@bors try @rust-timer queue

r=me after perf

[bors]

⌛ Trying commit 985e1e0 with merge 4d4360e...

[bors]

☀️ Try build successful - checks-actions
Build commit: 4d4360e (4d4360e59012da0d069cde10bc04cedf525bdd48)

[rust-timer]

Finished benchmarking commit (4d4360e): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

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

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.0%	[2.4%, 3.7%]	2
Regressions ❌ (secondary)	1.6%	[1.6%, 1.6%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.0%	[2.4%, 3.7%]	2

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: 666.737s -> 667.157s (0.06%)
Artifact size: 308.19 MiB -> 308.14 MiB (-0.02%)

[compiler-errors]

uwu

@bors r=lcnr

[bors]

📌 Commit 985e1e0 has been approved by lcnr

It is now in the queue for this repository.

[bors]

⌛ Testing commit 985e1e0 with merge 533cfde...

[bors]

☀️ Test successful - checks-actions
Approved by: lcnr
Pushing 533cfde to master...

[rust-timer]

Finished benchmarking commit (533cfde): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-1.3%	[-1.3%, -1.3%]	1
All ❌✅ (primary)	-2.6%	[-2.6%, -2.6%]	1

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: 664.881s -> 664.25s (-0.09%)
Artifact size: 308.33 MiB -> 308.27 MiB (-0.02%)