# Experiment for missing normalizations

For all these experiments, we use https://github.com/rust-lang/rust/pull/103985 as a base to then check where there are missing normalization calls. Don't forget to replace:

if let Some(identity_def_id) = tcx.lang_items().identity_type() {

with:

if let Some(identity_def_id) = tcx.lang_items().identity_type() && std::env::var("WHATEVER").is_ok() {

So then you can try it out with "WHATEVER=1" in your environment to enable it.

### Associated type of projection as const generic

The following code should work:

pub trait Identity {
    type Identity: ?Sized;
}

impl<T: ?Sized> Identity for T {
    type Identity = Self;
}

pub fn foo<const X: <i32 as Identity>::Identity>() {
    // ^ error here
    assert!(X == 12);
}

fn main() {
    foo::<12>();
}

To make it work, here are the changes to be done:

In `rustc_middle::ty::consts::from_opt_const_arg_anon_const`:

This [`ty`](https://github.com/rust-lang/rust/blob/14ca83a04b00433a8caf3b805d5ea08cb2691e1b/compiler/rustc_middle/src/ty/consts.rs#L74) should be normalized:

let param_env = tcx.param_env(def.did);
let ty = tcx.normalize_erasing_regions(param_env, ty);

In `rustc_hir_analysis::check::wfcheck::check_param_wf`:

This [`Ty`](https://github.com/rust-lang/rust/blob/14ca83a04b00433a8caf3b805d5ea08cb2691e1b/compiler/rustc_hir_analysis/src/check/wfcheck.rs#L863) should be normalized using:

// `owner_id` must be added as an argument to this function
enter_wf_checking_ctxt(tcx, param.span, owner_id, |wfcx| {
    let ty = tcx.type_of(param.def_id);    
    let ty = wfcx.normalize(hir_ty.span, None, ty);
    // ...
});

However, these changes create a lot of cycle errors (as can be seen in [#104443](https://github.com/rust-lang/rust/pull/104443)).

### Associated type of projection as union field's type

The following code should work:

trait Identity {
    type Identity;
}
trait NotImplemented {}

impl<T: NotImplemented> Identity for T {
    type Identity = Self;
}

type Foo = u8;

union Bar {
    a: <Foo as Identity>::Identity, // Error here
    b: u8,
}

Basically, it was missing a normalization before checking the if the field's type was compatible with a union in `compiler/rustc_hir_analysis/src/check/check.rs`.

It was fixed in https://github.com/rust-lang/rust/pull/106938.

### Impl block on a projection

The following code should work:

pub trait Identity {
    type Identity: ?Sized;
}

impl<T: ?Sized> Identity for T {
    type Identity = Self;
}

pub struct S;

impl <S as Identity>::Identity { // error here
    pub fn foo(&self) {}
}

It was missing a normalization before the check for the `self` type of the impl block. It was added in https://github.com/rust-lang/rust/pull/107263.

Side-note: the privacy check was wrong for the projection so it needed a fix as well.

However, it cannot work because:

pub trait Identity {
    type Identity: ?Sized;
}

impl<T: ?Sized> Identity for T {
    type Identity = Self;
}

pub struct I8<const F: i8>;

impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
    pub fn foo(&self) {}
}

Gives the following error:

error[E0391]: cycle detected when finding all inherent impls defined in crate
   |
   = note: ...which requires normalizing `<I8<{i8::MIN}> as Identity>::Identity`...
note: ...which requires evaluating type-level constant...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
note: ...which requires const-evaluating + checking `<impl at bar.rs:13:1: 13:43>::{constant#0}`...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
note: ...which requires const-evaluating + checking `<impl at bar.rs:13:1: 13:43>::{constant#0}`...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
note: ...which requires caching MIR for CTFE of the const argument `<impl at bar.rs:13:1: 13:43>::{constant#0}`...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
note: ...which requires elaborating drops for `<impl at bar.rs:13:1: 13:43>::{constant#0}`...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
note: ...which requires borrow-checking the const argument`<impl at bar.rs:13:1: 13:43>::{constant#0}`...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
note: ...which requires processing MIR for the const argument `<impl at bar.rs:13:1: 13:43>::{constant#0}`...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
note: ...which requires const checking the const argument `<impl at bar.rs:13:1: 13:43>::{constant#0}`...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
note: ...which requires preparing the const argument `<impl at bar.rs:13:1: 13:43>::{constant#0}` for borrow checking...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
note: ...which requires unsafety-checking the const argument `<impl at bar.rs:13:1: 13:43>::{constant#0}`...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
note: ...which requires building MIR for `<impl at bar.rs:13:1: 13:43>::{constant#0}`...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
note: ...which requires building THIR for `<impl at bar.rs:13:1: 13:43>::{constant#0}`...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
note: ...which requires type-checking the const argument `<impl at bar.rs:13:1: 13:43>::{constant#0}`...
  --> bar.rs:13:10
   |
13 | impl <I8<{i8::MIN}> as Identity>::Identity { //~ ERROR
   |          ^^^^^^^^^
   = note: ...which requires collecting all inherent impls for `IntSimplifiedType(I8)`...
   = note: ...which requires collecting all impls for a type in a crate...
   = note: ...which again requires finding all inherent impls defined in crate, completing the cycle
   = note: cycle used when running analysis passes on this crate

error: aborting due to previous error

For more information about this error, try `rustc --explain E0391`.

Because of the normalization in `compiler/rustc_hir_analysis/src/coherence/inherent_impls.rs`.