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update section for type system constants (#1329) 

* update section for type system constants

* Update src/constants.md
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@ -16,6 +16,33 @@ or a generic parameter, e.g. `[u8; N]`, converting a constant to its [`ty::Const
returns an unevaluated constant. Even fully concrete constants which do not depend on
generic parameters are not evaluated right away.
Anonymous constants are typechecked separately from their containing item, e.g.
```rust
fn foo<const N: usize>() -> [u8; N + 1] {
[0; N + 1]
}
```
is treated as
```rust
const ANON_CONST_1<const N: usize> = N + 1;
const ANON_CONST_2<const N: usize> = N + 1;
fn foo<const N: usize>() -> [u8; ANON_CONST_1::<N>] {
[0; ANON_CONST_2::<N>]
}
```
### Unifying constants
For the compiler, `ANON_CONST_1` and `ANON_CONST_2` are completely different, so
we have to somehow look into unevaluated constants to check whether they should
unify.
For this we use [InferCtxt::try_unify_abstract_consts](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_infer/infer/struct.InferCtxt.html#method.try_unify_abstract_consts).
This builds a custom AST for the two inputs from their THIR. This is then used for
the actual comparison.
### Lazy normalization for constants
We do not eagerly evaluate constant as they can be used in the `where`-clauses of their
parent item, for example:
@ -32,7 +59,7 @@ its parents caller bounds, but is also part of another bound itself.
If we were to eagerly evaluate this constant while computing its parents bounds
this would cause a query cycle.
### Generic arguments of anonymous constants
### Unused generic arguments of anonymous constants
Anonymous constants inherit the generic parameters of their parent, which is
why the array length in `foo<const N: usize>() -> [u8; N + 1]` can use `N`.
@ -41,25 +68,7 @@ Without any manual adjustments, this causes us to include parameters even if
the constant doesn't use them in any way. This can cause
[some interesting errors][pcg-unused-substs] and breaks some already stable code.
To deal with this, we intend to look at the generic parameters explicitly mentioned
by the constants and then search the predicates of its parents to figure out which
of the other generic parameters are reachable by our constant.
**TODO**: Expand this section once the parameter filtering is implemented.
As constants can be part of their parents `where`-clauses, we mention unevaluated
constants in their parents predicates. It is therefore necessary to mention unevaluated
constants before we have computed the generic parameters
available to these constants.
To do this unevaluated constants start out with [`substs_`] being `None` while assuming
that their generic arguments could be arbitrary generic parameters.
When first accessing the generic arguments of an unevaluated constants, we then replace
`substs_` with the actual default arguments of a constants, which are the generic parameters
of their parent we assume to be used by this constant.
[`ty::Const`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Const.html
[`ty::ConstKind`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.ConstKind.html
[`ty::TyKind`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/enum.TyKind.html
[pcg-unused-substs]: https://github.com/rust-lang/project-const-generics/blob/master/design-docs/anon-const-substs.md#unused-substs
[`substs_`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/consts/kind/struct.Unevaluated.html#structfield.substs_
[pcg-unused-substs]: https://github.com/rust-lang/project-const-generics/blob/master/design-docs/anon-const-substs.md#unused-substs