Cannot infer type with a default const parameter

[ejpcmac]

I tried this code:

#[derive(Default)]
pub struct TestStruct<const N: usize = 0>;

pub fn main() {
    let _ = TestStruct::default();
}

I expected this to compile properly, since without specifying the const parameter of the TestStruct<N> type, it should be inferred as TestStruct<0>.

Instead, this happened:

error[E0282]: type annotations needed for `TestStruct<N>`
 --> test.rs:5:13
  |
5 |     let _ = TestStruct::default();
  |         -   ^^^^^^^^^^^^^^^^^^^^^ cannot infer type for struct `TestStruct<{_: usize}>`
  |         |
  |         consider giving this pattern a type

error: aborting due to previous error

For more information about this error, try `rustc --explain E0282`.

If I replace my code with this, it compiles properly:

#[derive(Default)]
pub struct TestStruct<const N: usize = 0>;

pub fn main() {
    // Note the absence of the N parameter.
    let _: TestStruct = TestStruct::default();
}

Meta

rustc --version --verbose:

rustc 1.59.0 (9d1b2106e 2022-02-23)
binary: rustc
commit-hash: 9d1b2106e23b1abd32fce1f17267604a5102f57a
commit-date: 2022-02-23
host: x86_64-unknown-linux-gnu
release: 1.59.0
LLVM version: 13.0.0

Also tested with:

rustc +nightly --version --verbose:

rustc 1.61.0-nightly (9c06e1ba4 2022-03-29)
binary: rustc
commit-hash: 9c06e1ba47e1077725a950e0b5d1870a89c8b536
commit-date: 2022-03-29
host: x86_64-unknown-linux-gnu
release: 1.61.0-nightly
LLVM version: 14.0.0

[frank-king]

@rustbot claim

[frank-king]

I'm afraid this is by design, since there is a similar ui test

rust/src/test/ui/const-generics/defaults/doesnt_infer.rs

Lines 1 to 13 in 1b7c3bc

	// test that defaulted const params are not used to help type inference
	struct Foo<const N: u32 = 2>;
	impl<const N: u32> Foo<N> {
	fn foo() -> Self { loop {} }
	}
	fn main() {
	let foo = Foo::<1>::foo();
	let foo = Foo::foo();
	//~^ error: type annotations needed for `Foo<N>`
	}

@rustbot release-assignment
@rustbot label: +A-const-generics +F-const_generics_defaults +F-const_generics +T-compiler
cc @rust-lang/project-const-generics

[ejpcmac]

Oh :( And what would be the rationale behind this decision? Because I feel this is kind a redundant to have to provide a type without the defaults. It does not look like usual Rust code.

[ejpcmac]

Oh, apparently this is due to a tricky design issue: #27336. As it is known, I’m closing this one.

[wyfo]

As I've just posted in #27336, you can,also use let _ = <TestStruct>::default().