Rollup merge of #100473 - compiler-errors:normalize-the-fn-def-sig-plz, r=lcnr

Attempt to normalize `FnDef` signature in `InferCtxt::cmp`

Stashes a normalization callback in `InferCtxt` so that the signature we get from `tcx.fn_sig(..).subst(..)` in `InferCtxt::cmp` can be properly normalized, since we cannot expect for it to have normalized types since it comes straight from astconv.

This is kind of a hack, but I will say that `@jyn514` found the fact that we present unnormalized types to be very confusing in real life code, and I agree with that feeling. Though altogether I am still a bit unsure about whether this PR is worth the effort, so I'm open to alternatives and/or just closing it outright.

On the other hand, this isn't a ridiculously heavy implementation anyways -- it's less than a hundred lines of changes, and half of that is just miscellaneous cleanup.

This is stacked onto #100471 which is basically unrelated, and it can be rebased off of that when that lands or if needed.

---

The code:
```rust
trait Foo { type Bar; }

impl<T> Foo for T {
    type Bar = i32;
}

fn foo<T>(_: <T as Foo>::Bar) {}

fn needs_i32_ref_fn(f: fn(&'static i32)) {}

fn main() {
    needs_i32_ref_fn(foo::<()>);
}
```

Before:
```
   = note: expected fn pointer `fn(&'static i32)`
                 found fn item `fn(<() as Foo>::Bar) {foo::<()>}`
```

After:
```
   = note: expected fn pointer `fn(&'static i32)`
                 found fn item `fn(i32) {foo::<()>}`
```

2 files changed, 30 insertions, 12 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/engine.rs b/compiler/rustc_trait_selection/src/traits/engine.rs
index 72533a42d80..dba4d4f69da 100644
--- a/compiler/rustc_trait_selection/src/traits/engine.rs
+++ b/compiler/rustc_trait_selection/src/traits/engine.rs
@@ -17,6 +17,7 @@ use rustc_span::Span;
 
 pub trait TraitEngineExt<'tcx> {
     fn new(tcx: TyCtxt<'tcx>) -> Box<Self>;
+    fn new_in_snapshot(tcx: TyCtxt<'tcx>) -> Box<Self>;
 }
 
 impl<'tcx> TraitEngineExt<'tcx> for dyn TraitEngine<'tcx> {
@@ -27,6 +28,14 @@ impl<'tcx> TraitEngineExt<'tcx> for dyn TraitEngine<'tcx> {
             Box::new(FulfillmentContext::new())
         }
     }
+
+    fn new_in_snapshot(tcx: TyCtxt<'tcx>) -> Box<Self> {
+        if tcx.sess.opts.unstable_opts.chalk {
+            Box::new(ChalkFulfillmentContext::new())
+        } else {
+            Box::new(FulfillmentContext::new_in_snapshot())
+        }
+    }
 }
 
 /// Used if you want to have pleasant experience when dealing
@@ -41,6 +50,10 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
         Self { infcx, engine: RefCell::new(<dyn TraitEngine<'_>>::new(infcx.tcx)) }
     }
 
+    pub fn new_in_snapshot(infcx: &'a InferCtxt<'a, 'tcx>) -> Self {
+        Self { infcx, engine: RefCell::new(<dyn TraitEngine<'_>>::new_in_snapshot(infcx.tcx)) }
+    }
+
     pub fn register_obligation(&self, obligation: PredicateObligation<'tcx>) {
         self.engine.borrow_mut().register_predicate_obligation(self.infcx, obligation);
     }
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index 54f01577c5e..02adae5bde1 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -20,7 +20,7 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::{AsyncGeneratorKind, GeneratorKind, Node};
-use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_middle::hir::map;
 use rustc_middle::ty::{
     self, suggest_arbitrary_trait_bound, suggest_constraining_type_param, AdtKind, DefIdTree,
@@ -1589,32 +1589,38 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         expected: ty::PolyTraitRef<'tcx>,
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         pub(crate) fn build_fn_sig_ty<'tcx>(
-            tcx: TyCtxt<'tcx>,
+            infcx: &InferCtxt<'_, 'tcx>,
             trait_ref: ty::PolyTraitRef<'tcx>,
         ) -> Ty<'tcx> {
             let inputs = trait_ref.skip_binder().substs.type_at(1);
             let sig = match inputs.kind() {
                 ty::Tuple(inputs)
-                    if tcx.fn_trait_kind_from_lang_item(trait_ref.def_id()).is_some() =>
+                    if infcx.tcx.fn_trait_kind_from_lang_item(trait_ref.def_id()).is_some() =>
                 {
-                    tcx.mk_fn_sig(
+                    infcx.tcx.mk_fn_sig(
                         inputs.iter(),
-                        tcx.mk_ty_infer(ty::TyVar(ty::TyVid::from_u32(0))),
+                        infcx.next_ty_var(TypeVariableOrigin {
+                            span: DUMMY_SP,
+                            kind: TypeVariableOriginKind::MiscVariable,
+                        }),
                         false,
                         hir::Unsafety::Normal,
                         abi::Abi::Rust,
                     )
                 }
-                _ => tcx.mk_fn_sig(
+                _ => infcx.tcx.mk_fn_sig(
                     std::iter::once(inputs),
-                    tcx.mk_ty_infer(ty::TyVar(ty::TyVid::from_u32(0))),
+                    infcx.next_ty_var(TypeVariableOrigin {
+                        span: DUMMY_SP,
+                        kind: TypeVariableOriginKind::MiscVariable,
+                    }),
                     false,
                     hir::Unsafety::Normal,
                     abi::Abi::Rust,
                 ),
             };
 
-            tcx.mk_fn_ptr(trait_ref.rebind(sig))
+            infcx.tcx.mk_fn_ptr(trait_ref.rebind(sig))
         }
 
         let argument_kind = match expected.skip_binder().self_ty().kind() {
@@ -1634,11 +1640,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         let found_span = found_span.unwrap_or(span);
         err.span_label(found_span, "found signature defined here");
 
-        let expected = build_fn_sig_ty(self.tcx, expected);
-        let found = build_fn_sig_ty(self.tcx, found);
+        let expected = build_fn_sig_ty(self, expected);
+        let found = build_fn_sig_ty(self, found);
 
-        let (expected_str, found_str) =
-            self.tcx.infer_ctxt().enter(|infcx| infcx.cmp(expected, found));
+        let (expected_str, found_str) = self.cmp(expected, found);
 
         let signature_kind = format!("{argument_kind} signature");
         err.note_expected_found(&signature_kind, expected_str, &signature_kind, found_str);