Fix inlining with -Zalways-encode-mir

[tmiasko]

Only inline functions that are considered eligible for inlining
by the reachability pass.

This constraint was previously indirectly enforced by only exporting MIR
of eligible functions, but that approach doesn't work with
-Zalways-encode-mir enabled.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[tmiasko]

r? compiler

[saethlin]

This change is coupling the MIR inliner even more to our dubious reachability analysis, and I think that's the wrong way to improve things.

[tmiasko]

How does it increase coupling to reachability analysis?

[saethlin]

MIR inlining needs to agree with the logic used to generate MIR for items and also with whether statics are considered reachable. If it doesn't, we end up needing MIR for a function that we don't have, or we end up requiring visibility for a static that was previously determined to be internal.

This change as written currently is restricting MIR inlining to match the behavior of this function:

rust/compiler/rustc_passes/src/reachable.rs

Line 24 in 22d41ae

fn item_might_be_inlined(tcx: TyCtxt<'_>, item: &hir::Item<'_>, attrs: &CodegenFnAttrs) -> bool {

[tmiasko]

Yes, of course it needs to agree with definition in reachability pass. It was also the case before, so I am quite confused by your comment about increased coupling.

[compiler-errors]

Yeah, the coupling was always there.

@bors r+

[bors]

📌 Commit fe3cd2d has been approved by compiler-errors

It is now in the queue for this repository.

[tmiasko]

Ups, pushed to a wrong branch.

@bors r=compiler-errors

[bors]

📌 Commit fe3cd2d has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit fe3cd2d with merge b1b244d...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing b1b244d to master...

[rust-timer]

Finished benchmarking commit (b1b244d): comparison URL.

Overall result: ❌ regressions - 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)	0.5%	[0.5%, 0.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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)	1.2%	[0.6%, 1.8%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.4%	[-1.4%, -1.4%]	1
Improvements ✅ (secondary)	-3.8%	[-3.8%, -3.8%]	1
All ❌✅ (primary)	0.7%	[-1.4%, 1.8%]	5

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: 633.863s -> 631.655s (-0.35%)
Artifact size: 316.71 MiB -> 316.64 MiB (-0.02%)