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Sep 17, 2021
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ty::Unevaluated: dealing with unused substs #1190

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unused ct substs
lcnr Aug 18, 2021
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lcnr Aug 18, 2021
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lcnr Aug 30, 2021
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Update src/constants.md
nikomatsakis Sep 17, 2021
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lcnr committed Aug 30, 2021
commit f39e22712981a34db87f50e79984c4f467af6ccb
27 changes: 16 additions & 11 deletions src/constants.md
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Expand Up @@ -34,24 +34,29 @@ this would cause a query cycle.

### 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`.
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`.

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.
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.
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 without being able to first compute the generic parameters
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 without being able to first compute 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.
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
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