Detect struct construction with private field in field with default

[estebank]

When trying to construct a struct that has a public field of a private type, suggest using .. if that field has a default value.

error[E0603]: struct `Priv1` is private
  --> $DIR/non-exhaustive-ctor.rs:25:39
   |
LL |     let _ = S { field: (), field1: m::Priv1 {} };
   |                            ------     ^^^^^ private struct
   |                            |
   |                            while setting this field
   |
note: the struct `Priv1` is defined here
  --> $DIR/non-exhaustive-ctor.rs:14:4
   |
LL |    struct Priv1 {}
   |    ^^^^^^^^^^^^
help: the field `field1` you're trying to set has a default value, you can use `..` to use it
   |
LL |     let _ = S { field: (), .. };
   |                            ~~

[rustbot]

r? @fmease

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[fmease]

Preliminary review.

[fmease]

Slightly butchered suggestion (double comma).

[estebank]

I need to spend some time building some machinery to properly handle all possible cases of removal of elements from the HIR. We have too many copies of the same error-prone logic that needs to account for potential trailing commas or commas between the current element being removed and the next one. I'll do that as a separate PR.

[fmease]

Using Symbol instead Ident raises a few warning flags in my head because I'm pretty sure that this means anything involving field_defaults doesn't handle macro hygiene correctly. This is only relevant to decl macros 2.0 (feature decl_macro).

It might be a non-issue, I haven't thought that hard yet. I'll need to read the rest of this PR first.

[fmease]

Checking perf due to additional query feeding in the happy path (if I saw that correctly).

@bors try @rust-timer queue

[bors]

⌛ Trying commit 7e07e4f with merge 5e87c8e...

[bors]

☀️ Try build successful - checks-actions
Build commit: 5e87c8e (5e87c8e1a28c02bbe637ed03b08b5c0e63f85b11)

[rust-timer]

Finished benchmarking commit (5e87c8e): comparison URL.

Overall result: ❌✅ regressions and improvements - 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 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)	0.8%	[0.8%, 0.8%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.2%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.1%, secondary -0.1%)

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)	2.1%	[0.8%, 3.4%]	2
Regressions ❌ (secondary)	1.2%	[1.2%, 1.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.6%	[-0.6%, -0.6%]	3
All ❌✅ (primary)	2.1%	[0.8%, 3.4%]	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: 770.579s -> 772.7s (0.28%)
Artifact size: 325.79 MiB -> 325.79 MiB (-0.00%)