Introduce `default_field_values` feature #129514

estebank · 2024-08-24T17:25:50Z

Initial implementation of #[feature(default_field_values], proposed in rust-lang/rfcs#3681.

We now parse const expressions after a = in a field definition, to specify a struct field default value.

We now allow Struct { field, .. } where there's no base after ...

#[derive(Default)] now uses the default value if present, continuing to use Default::default() if not.

#[derive(Debug)]
pub struct S;

#[derive(Debug, Default)]
pub struct Foo {
    pub bar: S = S,
    pub baz: i32 = 42 + 3,
}

fn main () {
    let x = Foo { .. };
    let y = Foo::default();
    let z = Foo { baz: 1, .. };

    assert_eq!(45, x.baz);
    assert_eq!(45, y.baz);
    assert_eq!(1, z.baz);
}

rustbot · 2024-08-24T17:25:58Z

r? @cjgillot

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compiler/rustc_mir_build/src/build/expr/into.rs

estebank · 2024-08-24T19:07:09Z

It just occurred to me to try how the following fails, and it Just Works™️ 😄:

pub struct Foo {
    pub baz: i32 = Self::X,
}

impl Foo {
    const X: i32 = 1;
}

cjgillot

I made a first pass. I don't have anything substantial to say, that's a great pr.

Just an avalanche of nits. An additional one: could you prefer matching on Rest now there are >2 cases?

compiler/rustc_ast/src/ast.rs

compiler/rustc_builtin_macros/src/deriving/default.rs

compiler/rustc_hir/src/intravisit.rs

compiler/rustc_hir/src/hir.rs

compiler/rustc_hir/src/intravisit.rs

compiler/rustc_middle/src/ty/mod.rs

compiler/rustc_mir_build/src/build/custom/parse/instruction.rs

compiler/rustc_mir_build/src/build/expr/into.rs

tests/ui/structs/default-field-values-support.rs

tests/ui/structs/default-field-values-failures.stderr

estebank · 2024-08-25T19:01:18Z

compiler/rustc_builtin_macros/src/deriving/default.rs

+                                    // We use `Default::default()`.
+                                    None => default_call(cx, field.span),


Given this, I filed rust-lang/rfcs#3683 as this bit is independent of the default values RFC.

compiler-errors · 2024-12-06T23:19:30Z

Could you also squash all of those line 2-line commits into the original commit?

estebank · 2024-12-07T04:50:50Z

Could you also squash all of those line 2-line commits into the original commit?

I'll do a squash-rebase soon, once I've addressed all comments. Trying to keep things tidy enough for both review and trying to avoid accidentally squashing different things in the same commit.

Edit: squashed the commits that didn't need splitting up into their "parents". The rewordings span changes from multiple commits so I left them as is. I believe I addressed all the review comments so far. (I can push a branch with the commits before the squash rebase, if that'll be helpful.)

compiler-errors · 2024-12-09T18:30:20Z

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -1104,6 +1104,23 @@ fn check_type_defn<'tcx>(
        for variant in variants.iter() {
            // All field types must be well-formed.
            for field in &variant.fields {
+                if let Some(def_id) = field.value


For the record, erroring eagerly even if we don't instantiate the default is consistent with how defaults in struct const generics works:

struct Foo<const N: u8 = {130 + 130}> {}

which errors with:

error[E0080]: evaluation of constant value failed --> src/lib.rs:1:27 | 1 | struct Foo<const N: u8 = {130 + 130}> {} | ^^^^^^^^^ attempt to compute `130_u8 + 130_u8`, which would overflow For more information about this error, try `rustc --explain E0080`. error: could not compile `playground` (lib) due to 1 previous error

Not totally certain why the RFC specified that it would only be linted against, but this seems to me to be the correct behavior.

compiler/rustc_hir_analysis/src/check/wfcheck.rs

compiler-errors · 2024-12-09T18:33:00Z

Please address nits and then please clean up the history a bit (that zip_eq commit could be squashed into whatever commit, and bbdd6ba too -- have you ever heard of git absorb? that might be useful to automate this type of cleanup).

Then r=me

Initial implementation of `#[feature(default_field_values]`, proposed in rust-lang/rfcs#3681. Support default fields in enum struct variant Allow default values in an enum struct variant definition: ```rust pub enum Bar { Foo { bar: S = S, baz: i32 = 42 + 3, } } ``` Allow using `..` without a base on an enum struct variant ```rust Bar::Foo { .. } ``` `#[derive(Default)]` doesn't account for these as it is still gating `#[default]` only being allowed on unit variants. Support `#[derive(Default)]` on enum struct variants with all defaulted fields ```rust pub enum Bar { #[default] Foo { bar: S = S, baz: i32 = 42 + 3, } } ``` Check for missing fields in typeck instead of mir_build. Expand test with `const` param case (needs `generic_const_exprs` enabled). Properly instantiate MIR const The following works: ```rust struct S<A> { a: Vec<A> = Vec::new(), } S::<i32> { .. } ``` Add lint for default fields that will always fail const-eval We *allow* this to happen for API writers that might want to rely on users' getting a compile error when using the default field, different to the error that they would get when the field isn't default. We could change this to *always* error instead of being a lint, if we wanted. This will *not* catch errors for partially evaluated consts, like when the expression relies on a const parameter. Suggestions when encountering `Foo { .. }` without `#[feature(default_field_values)]`: - Suggest adding a base expression if there are missing fields. - Suggest enabling the feature if all the missing fields have optional values. - Suggest removing `..` if there are no missing fields.

Emit a specific error for unsupported default field value syntax in tuple structs.

People might extrapolate from `Struct { .. }` that `Struct(..)` would work, but it doesn't.

…errors Emit E0080 always on struct definition with default fields that have unconditional const errors and remove `default_field_always_invalid_const` lint.

compiler-errors · 2024-12-09T22:15:08Z

@bors r+ rollup=never

bors · 2024-12-09T22:15:11Z

📌 Commit fa331f4 has been approved by compiler-errors

It is now in the queue for this repository.

fmease · 2024-12-09T22:35:33Z

@bors p=10 bitrotty

bors · 2024-12-10T00:07:03Z

⌛ Testing commit fa331f4 with merge 974ccc1...

bors · 2024-12-10T02:55:22Z

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing 974ccc1 to master...

rust-timer · 2024-12-10T04:27:43Z

Finished benchmarking commit (974ccc1): 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)	0.7%	[0.3%, 2.3%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.7%	[-0.7%, -0.7%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -0.0%, secondary 2.4%)

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)	1.3%	[1.1%, 1.5%]	2
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-1.4%	[-2.2%, -0.7%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-2.2%, 1.5%]	4

Cycles

Results (secondary 2.2%)

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)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.1%, secondary 0.0%)

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)	0.1%	[0.0%, 0.2%]	53
Regressions ❌ (secondary)	0.0%	[0.0%, 0.1%]	13
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.2%]	53

Bootstrap: 767.262s -> 767.421s (0.02%)
Artifact size: 330.86 MiB -> 331.00 MiB (0.04%)

rustbot assigned cjgillot Aug 24, 2024

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. T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. labels Aug 24, 2024

estebank commented Aug 24, 2024

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