remove StructuralEq trait

[RalfJung]

The documentation given for the trait is outdated: all function pointers implement PartialEq and Eq these days. So the StructuralEq trait doesn't really seem to have any reason to exist any more.

One side-effect of this PR is that we allow matching on some consts that do not implement Eq. However, we already allowed matching on floats and consts containing floats, so this is not new, it is just allowed in more cases now. IMO it makes no sense at all to allow float matching but also sometimes require an Eq instance. If we want to require Eq we should adjust #115893 to check for Eq, and rule out float matching for good.

Fixes #115881

[rustbot]

r? @cjgillot

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[rustbot]

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo

Changes to the code generated for builtin derived traits.

cc @nnethercote

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[RalfJung]

@compiler-errors you said you'd have opinions/questions about this one. :)

Cc @oli-obk @lcnr

[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo

Changes to the code generated for builtin derived traits.

cc @nnethercote

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[lcnr]

I agree that StructuralEq being necessary to match on custom adts while still allowing float in matches does not make sense.

I am a bit unsure whether this makes sense as a transition step. If we allow all types implementing PartialEq in matches, and require types to implement some trait StructuralMatch to opt-in to exhaustiveness checking, then I would expect the StructuralMatch trait to have an Eq super trait bound (we currently don't allow exhaustive matches on floats afaict.

Given that we currently try to use constants of all allowed types for exhaustiveness checking, allowing more types by removing the "derives Eq" requirement feels wrong.

[RalfJung]

Given that we currently try to use constants of all allowed types for exhaustiveness checking, allowing more types by removing the "derives Eq" requirement feels wrong.

Hm, that's fair.

We could do what the comment on StructuralEq says we want to do -- have the constant participate in exhaustiveness checking only if it implements Eq. However I think that would be a breaking change: currently a constant const C: Result<i32, f32> = Ok(0) would participate in exhaustiveness checking I think; under the proposed alternative it would not.

[RalfJung]

Marking as blocked on rust-lang/lang-team#220.

[bors]

☔ The latest upstream changes (presumably #116260) made this pull request unmergeable. Please resolve the merge conflicts.

[RalfJung]

rust-lang/rfcs#3535 motivates why we don't need the StructuralEq trait any more.

[bors]

☔ The latest upstream changes (presumably #119088) made this pull request unmergeable. Please resolve the merge conflicts.

[RalfJung]

@lcnr @cjgillot with rust-lang/rfcs#3535 having been accepted, I think this should be ready to go?

[lcnr]

r? @lcnr

@bors r+ rollup=never

[bors]

📌 Commit 0df7810 has been approved by lcnr

It is now in the queue for this repository.

[bors]

💥 Test timed out

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

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[RalfJung]

@bors retry timeout

[bors]

⌛ Testing commit 0df7810 with merge e89fdc4...

[bors]

💥 Test timed out

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

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[RalfJung]

@bors retry timeout
we are seeing quite a few of these recently...

[bors]

⌛ Testing commit 0df7810 with merge dd2559e...

[bors]

☀️ Test successful - checks-actions
Approved by: lcnr
Pushing dd2559e to master...

[rust-timer]

Finished benchmarking commit (dd2559e): 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)	0.3%	[0.3%, 0.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.6%, -0.2%]	48
Improvements ✅ (secondary)	-1.3%	[-3.0%, -0.3%]	23
All ❌✅ (primary)	-0.4%	[-0.6%, 0.3%]	49

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)	4.2%	[4.2%, 4.2%]	1
Improvements ✅ (primary)	-2.9%	[-5.2%, -0.7%]	2
Improvements ✅ (secondary)	-2.7%	[-4.0%, -1.3%]	8
All ❌✅ (primary)	-2.9%	[-5.2%, -0.7%]	2

Cycles

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)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-3.3%, -1.3%]	8
All ❌✅ (primary)	-	-	0

Binary size

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)	-0.1%	[-0.2%, -0.0%]	30
Improvements ✅ (secondary)	-1.3%	[-2.3%, -0.0%]	10
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	30

Bootstrap: 662.586s -> 663.451s (0.13%)
Artifact size: 308.16 MiB -> 308.15 MiB (-0.00%)