Auto merge of #77616 - jyn514:no-normalize, r=lcnr

Don't run `resolve_vars_if_possible` in `normalize_erasing_regions`

Neither `@eddyb` nor I could figure out what this was for. I changed it to `assert_eq!(normalized_value, infcx.resolve_vars_if_possible(&normalized_value));` and it passed the UI test suite.

<details><summary>

Outdated, I figured out the issue - `needs_infer()` needs to come _after_ erasing the lifetimes

</summary>

Strangely, if I change it to `assert!(!normalized_value.needs_infer())` it panics almost immediately:

```
query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `<str::IsWhitespace as str::pattern::Pattern>::Searcher`
#1 [needs_drop_raw] computing whether `str::iter::Split<str::IsWhitespace>` needs drop
#2 [mir_built] building MIR for `str::<impl str>::split_whitespace`
#3 [unsafety_check_result] unsafety-checking `str::<impl str>::split_whitespace`
#4 [mir_const] processing MIR for `str::<impl str>::split_whitespace`
#5 [mir_promoted] processing `str::<impl str>::split_whitespace`
#6 [mir_borrowck] borrow-checking `str::<impl str>::split_whitespace`
#7 [analysis] running analysis passes on this crate
end of query stack
```

I'm not entirely sure what's going on - maybe the two disagree?

</details>

For context, this came up while reviewing https://github.com/rust-lang/rust/pull/77467/ (cc `@lcnr).`

Possibly this needs a crater run?

r? `@nikomatsakis`
cc `@matthewjasper`

1 files changed, 9 insertions, 3 deletions

diff --git a/compiler/rustc_traits/src/normalize_erasing_regions.rs b/compiler/rustc_traits/src/normalize_erasing_regions.rs
index 750a0922be4..4841e4286a9 100644
--- a/compiler/rustc_traits/src/normalize_erasing_regions.rs
+++ b/compiler/rustc_traits/src/normalize_erasing_regions.rs
@@ -2,7 +2,7 @@ use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::subst::GenericArg;
-use rustc_middle::ty::{self, ParamEnvAnd, TyCtxt};
+use rustc_middle::ty::{self, ParamEnvAnd, TyCtxt, TypeFoldable};
 use rustc_trait_selection::traits::query::normalize::AtExt;
 use rustc_trait_selection::traits::{Normalized, ObligationCause};
 use std::sync::atomic::Ordering;
@@ -31,8 +31,14 @@ fn normalize_generic_arg_after_erasing_regions<'tcx>(
                     None,
                 );
 
-                let normalized_value = infcx.resolve_vars_if_possible(normalized_value);
-                infcx.tcx.erase_regions(normalized_value)
+                let resolved_value = infcx.resolve_vars_if_possible(normalized_value);
+                // It's unclear when `resolve_vars` would have an effect in a
+                // fresh `InferCtxt`. If this assert does trigger, it will give
+                // us a test case.
+                debug_assert_eq!(normalized_value, resolved_value);
+                let erased = infcx.tcx.erase_regions(resolved_value);
+                debug_assert!(!erased.needs_infer(), "{:?}", erased);
+                erased
             }
             Err(NoSolution) => bug!("could not fully normalize `{:?}`", value),
         }