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Auto merge of #86580 - BoxyUwU:cgd-subst-ice, r=nikomatsakis

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dont provide fwd declared params to cg defaults

Fixes #83938

```rust
#![feature(const_evaluatable_checked, const_generics, const_generics_defaults)]
#![allow(incomplete_features)]

pub struct Bar<const N: usize, const M: usize = { N + 1 }>;
pub fn foo<const N1: usize>() -> Bar<N1> { loop {} }

fn main() {}
```
This PR makes this code no longer ICE, it was ICE'ing previously because when building substs for `Bar<N1>` we would subst the anon ct: `ConstKind::Unevaluated({N + 1}, substs: [N, M])` with substs of `[N1]`. the anon const has forward declared params supplied though so we end up trying to substitute the provided `M` param which causes the ICE.

This PR doesn't handle the predicates of the const so
```rust
trait Foo<const N: usize> { const Assoc: usize; }
pub struct Bar<const N: usize = { <()>::Assoc }> where (): Foo<N>;
```
Resolves to `<() as Foo<N>>::Assoc` which can allow for using fwd declared params indirectly.

```rust
trait Foo<const N: usize> {}
struct Bar<const N: usize = { 2 + 3 }> where (): Foo<N>;
```
This code also ICEs under this PR because instantiating the default's predicates causes an ICE as predicates_of contains predicates with fwd declared params

PR was briefly discussed [in this zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/evil.20preds.20in.20param.20env.20.2386580)
This commit is contained in:
bors 2021-07-24 20:01:51 +00:00
commit d9aa287672
10 changed files with 201 additions and 1 deletions
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13
compiler/rustc_middle/src/hir/map/mod.rs
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@ -919,6 +919,19 @@ impl<'hir> Map<'hir> {
pub fn node_to_string(&self, id: HirId) -> String {
hir_id_to_string(self, id)
}
/// Returns the HirId of `N` in `struct Foo<const N: usize = { ... }>` when
/// called with the HirId for the `{ ... }` anon const
pub fn opt_const_param_default_param_hir_id(&self, anon_const: HirId) -> Option<HirId> {
match self.get(self.get_parent_node(anon_const)) {
Node::GenericParam(GenericParam {
hir_id: param_id,
kind: GenericParamKind::Const { .. },
..
}) => Some(*param_id),
_ => None,
}
}
}
impl<'hir> intravisit::Map<'hir> for Map<'hir> {