Suggest replacing `const` with `let` on "can't use generic parameters from outer item" (outer fn specifically) (if appropriate)

[sm-Fifteen]

I tried this code, assuming the result of size_of::<T>() would be considered a constant value since it would be known at function monomorphization:

const fn size_plus_one<T:Sized>() -> usize {
    const size: usize = ::core::mem::size_of::<T>();

    size + 1
}

This resulted in the following compilation error:

error[E0401]: can't use generic parameters from outer function
 --> src/lib.rs:2:48
  |
1 | const fn size_plus_one<T:Sized>() -> usize {
  |                        - type parameter from outer function
2 |     const size: usize = ::core::mem::size_of::<T>();
  |                                                ^ use of generic parameter from outer function

As it turns out, the error message here is incorrect, and the issue can be solved by using let instead, something the error message was rather unhelpful with.

const fn size_plus_one<T:Sized>() -> usize {
    let size: usize = ::core::mem::size_of::<T>();

    size + 1
}

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rustc 1.40.0 (73528e339 2019-12-16)
binary: rustc
commit-hash: 73528e339aae0f17a15ffa49a8ac608f50c6cf14
commit-date: 2019-12-16
host: x86_64-unknown-linux-gnu
release: 1.40.0
LLVM version: 9.0

[jonas-schievink]

Note that the message is technically correct – const defines an item, and items (other than impl and trait items) don't inherit generic parameters from the thing they're defined in.

[sm-Fifteen]

Yes, I realize that now. It makes sense why const was innapropriate in that context (I had the wrong impression on how const would work inside const fns), it's just that the error message wasn't very helpful, and looking it up didn't provide any information about my case.

[estebank]

Current ouptut:

error[E0401]: can't use generic parameters from outer item
 --> src/lib.rs:2:48
  |
1 | const fn size_plus_one<T:Sized>() -> usize {
  |                        - type parameter from outer item
2 |     const size: usize = ::core::mem::size_of::<T>();
  |           ----                                 ^ use of generic parameter from outer item
  |           |
  |           generic parameter used in this inner constant item
  |
  = note: nested items are independent from their parent item for everything except for privacy and name resolution
  = note: a `const` is a separate item from the item that contains it

Still no suggestion. It should be possible to add.