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This post is auto-generated from RSS feed The Rust Programming Language Forum - Latest topics. Source: [Nightly] Traits generic const expressions and slice size

Hello everyone,

We are dealing with generic traits and generic const expressions and the interchange among different implementations of that trait with different generics.

There is a Happy trait that is implemented to a zst crate::Origin and a module abroad with abroad::Origin; in abroad module besides Origin there is the Foreign struct that do also implement Happy<abroad::Origin>. We are to implement at crate scope

impl <A: Happy<abroad::Origin>> Happy<crate::Origin> for A

Here is the code with intended behavior commented out (66-67):

By uncommenting leads to:

   Compiling playground v0.0.1 (/playground)
error: unconstrained generic constant
  --> src/main.rs:66:73
   |
66 |         let recall = <Self as crate::Happy<abroad::Origin>>::tale_bytes(&self);
   |                      -------------------------------------------------- ^^^^^
   |                      |
   |                      required by a bound introduced by this call
   |
note: required by a bound in `Happy::tale_bytes`
  --> src/main.rs:8:34
   |
 8 |     fn tale_bytes(&self) -> [u8; <Self as Happy<Origin>>::TALE_SIZE] {
   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Happy::tale_bytes`
help: try adding a `where` bound
   |
64 |     fn tale_bytes(&self) -> [u8; <Self as crate::Happy<Origin>>::TALE_SIZE] where [(); <Self as Happy<Origin>>::TALE_SIZE]: {
   |                                                                             +++++++++++++++++++++++++++++++++++++++++++++++

error: unconstrained generic constant
  --> src/main.rs:66:22
   |
66 |         let recall = <Self as crate::Happy<abroad::Origin>>::tale_bytes(&self);
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
help: try adding a `where` bound
   |
64 |     fn tale_bytes(&self) -> [u8; <Self as crate::Happy<Origin>>::TALE_SIZE] where [(); <Self as Happy<Origin>>::TALE_SIZE]: {
   |                                                                             +++++++++++++++++++++++++++++++++++++++++++++++

error: could not compile `playground` (bin "playground") due to 2 previous errors

by following the compile it leads me to:

   Compiling playground v0.0.1 (/playground)
error[E0391]: cycle detected when building an abstract representation for `<impl at src/main.rs:58:1: 60:55>::{constant#0}`
  --> src/main.rs:60:11
   |
60 |      [(); <Self as crate::Happy<Origin>>::TALE_SIZE]:,
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: ...which requires building THIR for `<impl at src/main.rs:58:1: 60:55>::{constant#0}`...
  --> src/main.rs:60:11
   |
60 |      [(); <Self as crate::Happy<Origin>>::TALE_SIZE]:,
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...which requires type-checking `<impl at src/main.rs:58:1: 60:55>::{constant#0}`...
  --> src/main.rs:60:11
   |
60 |      [(); <Self as crate::Happy<Origin>>::TALE_SIZE]:,
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: ...which again requires building an abstract representation for `<impl at src/main.rs:58:1: 60:55>::{constant#0}`, completing the cycle
note: cycle used when type-checking `<impl at src/main.rs:58:1: 60:55>::tale_bytes::{constant#0}`
  --> src/main.rs:64:34
   |
64 |     fn tale_bytes(&self) -> [u8; <Self as crate::Happy<Origin>>::TALE_SIZE] {
   |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

For more information about this error, try `rustc --explain E0391`.
error: could not compile `playground` (bin "playground") due to 1 previous error

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