Suggested by @lzutao

Fixes rust-lang#67639

`Instance.ty` assumes that we are in a fully monomorphic context (e.g.
codegen), and can therefore use an empty `ParamEnv` when performing
normalization. Howver, the MIR constant evaluator code ends up calling
`Instance.ty` as a result of us attemptign to 'speculatively'
const-evaluate generic functions during const propagation.

As a result,
we may end up with projections involving type parameters
(e.g. <T as MyTrait>::Bar>) in the type we are trying to normalize.
Normalization expects us to have proper predicates in the `ParamEnv` for
such projections, and will ICE if we don't.

This commit adds a new method `Instance.ty_env`, which takes a
`ParamEnv` for use during normalization. The MIR const-evaluator code is
changed to use this method, passing in the proper `ParamEnv` for the
context at hand.

Co-Authored-By: Wesley Wiser <wwiser@gmail.com>

This error may be produced during intermediate failed attempts at evaluation of a generic const, which may nevertheless succeed later.

spotted while reviewing the todo!macro docs

… r=oli-obk

Fix ICE involving calling `Instance.ty` during const evaluation

Fixes rust-lang#67639

`Instance.ty` assumes that we are in a fully monomorphic context (e.g.
codegen), and can therefore use an empty `ParamEnv` when performing
normalization. Howver, the MIR constant evaluator code ends up calling
`Instance.ty` as a result of us attemptign to 'speculatively'
const-evaluate generic functions during const propagation.

As a result,
we may end up with projections involving type parameters
(e.g. <T as MyTrait>::Bar>) in the type we are trying to normalize.
Normalization expects us to have proper predicates in the `ParamEnv` for
such projections, and will ICE if we don't.

This commit adds a new method `Instance.ty_env`, which takes a
`ParamEnv` for use during normalization. The MIR const-evaluator code is
changed to use this method, passing in the proper `ParamEnv` for the
context at hand.

…partial, r=Amanieu

change remove to have a PartialEq bound

Addresses [comment](rust-lang#67727 (comment)).

References rust-lang#40062

r? @Amanieu

Use `as_deref()` to replace `as_ref().map(...)`

Suggested by @lzutao

Silence `TooGeneric` error

This error may be produced during intermediate failed attempts at evaluation of a generic const, which may nevertheless succeed later.

Fixes rust-lang#66962.

r? @eddyb

macros: typo fix

spotted while reviewing the todo!macro docs

Use Self instead of $type

r? @Dylan-DPC