Detect struct construction with private field in field with default #135846

estebank · 2025-01-21T21:35:41Z

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-2.rs:19:39
   |
LL |     let _ = S { field: (), field1: m::Priv1 {} };
   |                            ------     ^^^^^ private struct
   |                            |
   |                            while setting this field
   |
note: the struct `Priv1` is defined here
  --> $DIR/non-exhaustive-ctor-2.rs:14:4
   |
LL |    struct Priv1 {}
   |    ^^^^^^^^^^^^
help: the type `Priv1` of field `field1` is private, but you can construct the default value defined for it in `S` using `..` in the struct initializer expression
   |
LL |     let _ = S { field: (), .. };
   |                            ~~

rustbot · 2025-01-21T21:35:50Z

r? @fmease

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tests/ui/structs/default-field-values/non-exhaustive-ctor-2.stderr

fmease

Preliminary review.

tests/ui/structs/default-field-values/non-exhaustive-ctor-2.stderr

fmease · 2025-01-25T18:30:57Z

compiler/rustc_resolve/src/lib.rs

@@ -1022,6 +1023,7 @@ pub struct Resolver<'ra, 'tcx> {

    /// N.B., this is used only for better diagnostics, not name resolution itself.
    field_names: LocalDefIdMap<Vec<Ident>>,
+    field_defaults: LocalDefIdMap<Vec<Symbol>>,


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.

Worst case it's just wrong diagnostics in macro edge cases

compiler/rustc_middle/src/query/mod.rs

fmease · 2025-01-25T18:40:30Z

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

@bors try @rust-timer queue

…r=<try> Detect struct construction with private field in field with default 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: (), .. }; | ~~ ```

rust-timer · 2025-01-25T23:10:36Z

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%)

bors · 2025-03-11T21:57:07Z

☔ The latest upstream changes (presumably #128440) made this pull request unmergeable. Please resolve the merge conflicts.

wesleywiser · 2025-04-17T14:30:17Z

Hi @fmease, I think this is just waiting on some feedback from you whether this is an issue or not and then a rebase from the author since perf looks neutral. Thanks!

wesleywiser · 2025-06-05T14:25:18Z

r? rust-lang/compiler

BoxyUwU · 2025-06-10T04:44:50Z

compiler/rustc_middle/src/query/mod.rs

+    query default_field(def_id: DefId) -> Option<DefId> {
+        desc { |tcx| "looking up the `const` corresponding to the default for `{}`", tcx.def_path_str(def_id) }
+        separate_provide_extern
+        feedable


Why is this feedable? Also, why does this query exist at all, do we not already have a way of determining if a field has a default? I would have thought we need some way of doing that in order for type checking/mir building/codegen/that whole set of things to know whether Struct { .. } is valid.

Why is this feedable?

Best guess: must have been a copy-paste mistake.

Also, why does this query exist at all, do we not already have a way of determining if a field has a default?

I didn't see a better way to do cross-crate checking of this in a reasonable way, but if there is I'm more than happy to use that instead.

compiler/rustc_resolve/src/diagnostics.rs

bors · 2025-07-11T04:35:26Z

☔ The latest upstream changes (presumably #143762) made this pull request unmergeable. Please resolve the merge conflicts.

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 assigned fmease Jan 21, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jan 21, 2025

estebank added F-default_field_values `#![feature(default_field_values)]` and removed T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 21, 2025

estebank mentioned this pull request Jan 21, 2025

Tracking issue for RFC 3681: Default field values #132162

Open

17 tasks

estebank added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Jan 21, 2025

estebank mentioned this pull request Jan 21, 2025

Detect missing fields with default values and suggest .. #135794

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estebank added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. A-diagnostics Area: Messages for errors, warnings, and lints labels Jan 22, 2025

jieyouxu reviewed Jan 22, 2025

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estebank force-pushed the non-exhaustive-dfv-ctor-2 branch 2 times, most recently from 5f24487 to 7e07e4f Compare January 23, 2025 05:35

fmease reviewed Jan 25, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 25, 2025

estebank force-pushed the non-exhaustive-dfv-ctor-2 branch from 7e07e4f to 38f8f98 Compare January 28, 2025 07:35

estebank force-pushed the non-exhaustive-dfv-ctor-2 branch from 38f8f98 to 7d576f6 Compare February 12, 2025 00:18

rustbot assigned BoxyUwU and unassigned fmease Jun 5, 2025

BoxyUwU reviewed Jun 10, 2025

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BoxyUwU added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 10, 2025

estebank force-pushed the non-exhaustive-dfv-ctor-2 branch 2 times, most recently from d3a309b to 9bcda61 Compare June 27, 2025 22:40

estebank added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 3, 2025

estebank added 2 commits July 18, 2025 16:39

review comments

c41fee3

estebank force-pushed the non-exhaustive-dfv-ctor-2 branch from 9bcda61 to c41fee3 Compare July 18, 2025 16:39

Detect struct construction with private field in field with default #135846

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Detect struct construction with private field in field with default #135846

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rust-timer commented Jan 25, 2025

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

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bors commented Mar 11, 2025

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