Retire the unnamed_fields feature for now

[compiler-errors]

#![feature(unnamed_fields)] was implemented in part in #115131 and #115367, however work on that feature has (afaict) stalled and in the mean time there have been some concerns raised (e.g.¹²) about whether unnamed_fields is worthwhile to have in the language, especially in its current desugaring. Because it represents a compiler implementation burden including a new kind of anonymous ADT and additional complication to field selection, and is quite prone to bugs today, I'm choosing to remove the feature.

However, since I'm not one to really write a bunch of words, I'm specifically not going to de-RFC this feature. This PR essentially rolls back the state of this feature to "RFC accepted but not yet implemented"; however if anyone wants to formally unapprove the RFC from the t-lang side, then please be my guest. I'm just not totally willing to summarize the various language-facing reasons for why this feature is or is not worthwhile, since I'm coming from the compiler side mostly.

Fixes #117942
Fixes #121161
Fixes #121263
Fixes #121299
Fixes #121722
Fixes #121799
Fixes #126969
Fixes #131041

Tracking:

• Tracking issue for RFC 2102, "Unnamed fields of struct and union type" #49804

Footnotes

1. https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Unnamed.20struct.2Funion.20fields ↩

2. https://github.com/rust-lang/rust/issues/49804#issuecomment-1972619108 ↩

[rustbot]

r? @fee1-dead

rustbot has assigned @fee1-dead.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

[compiler-errors]

I'm nominating this for T-lang somewhat out of courtesy; my understanding is that whether or not an RFC-approved unstable implementation of a feature moves forwards or backwards really is a T-compiler judgement call, but I would like to hear T-lang's take on this.

I generally think that "let's go back to the drawing board" is the right call here, and in the mean time, I'd like to remove this from the compiler since there's a fair chance that a future design could be implemented some other way, and it's always easier to add things back into the compiler -- and I would be glad to either mentor or implement that work myself when that time comes, of course.

@rustbot label: +I-lang-nominated

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[fee1-dead]

currently capacity deficient, so

r? compiler

[scottmcm]

Especially given that the tracking issue for this is marked S-tracking-design-concerns and S-tracking-impl-incomplete, I agree that T-compiler has free rein to remove an implementation with which they're not happy.

[traviscross]

@rustbot labels -I-lang-nominated

We discussed this in the meeting today, and we appreciated the courtesy, and agree that it's OK as far as lang is concerned to rip this out.

[michaelwoerister]

r? compiler (cc rust-lang/team#1565)

[wesleywiser]

I agree, removing this implementation is well motivated. Thanks @compiler-errors for doing this and also notifying T-lang of the removal.

@bors r+

[bors]

📌 Commit 6628bba has been approved by wesleywiser

It is now in the queue for this repository.

[bors]

⌛ Testing commit 6628bba with merge 80325f2...

[bors]

⌛ Testing commit 6628bba with merge f496659...

[bors]

☀️ Test successful - checks-actions
Approved by: wesleywiser
Pushing f496659 to master...

[rust-timer]

Finished benchmarking commit (f496659): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.0%	[3.0%, 3.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary 0.6%)

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)	3.0%	[3.0%, 3.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.8%	[-1.8%, -1.8%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.0%, secondary 3.5%)

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)	3.5%	[3.5%, 3.5%]	1
Improvements ✅ (primary)	-2.0%	[-2.0%, -2.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.0%	[-2.0%, -2.0%]	1

Binary size

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

Bootstrap: 780.481s -> 780.255s (-0.03%)
Artifact size: 332.12 MiB -> 332.07 MiB (-0.01%)