Auto merge of #17916 - ShoyuVanilla:issue-17711, r=Veykril

fix: Wrong BoundVar index when lowering impl trait parameter of parent generics

Fixes #17711

From the following test code;

```rust
//- minicore: deref
use core::ops::Deref;

struct Struct<'a, T>(&'a T);

trait Trait {}

impl<'a, T: Deref<Target = impl Trait>> Struct<'a, T> {
    fn foo(&self) -> &Self { self }

    fn bar(&self) {
        let _ = self.foo();
    }

}
```

when we call `register_obligations_for_call` for `let _ = self.foo();`,

07659783fd/crates/hir-ty/src/infer/expr.rs (L1939-L1952)

we are querying `generic_predicates` and it has `T: Deref<Target = impl Trait>` predicate from the parent `impl Struct`;

07659783fd/crates/hir-ty/src/lower.rs (L375-L399)

but as we can see above, lowering `TypeRef = impl Trait` doesn't take into account the parent generic parameters, so the `BoundVar` index here is `0`, as `fn foo` has no generic args other than parent's,

But this `BoundVar` is pointing at `'a` in `<'a, T: Deref<Target = impl Trait>>`.
So, in the first code reference `register_obligations_for_call`'s L:1948 - `.substitute(Interner, parameters)`, we are substituting `'a` with `Ty`, not `Lifetime` and this makes panic inside the chalk.

This PR fixes this wrong `BoundVar` index in such cases

crates/hir-ty/src/lower.rs

@@ -377,26 +377,25 @@ impl<'a> TyLoweringContext<'a> {
                         // Count the number of `impl Trait` things that appear within our bounds.
                         // Since t hose have been emitted as implicit type args already.
                         counter.set(idx + count_impl_traits(type_ref) as u16);
-                        let (
-                            _parent_params,
-                            self_param,
-                            type_params,
-                            const_params,
-                            _impl_trait_params,
-                            lifetime_params,
-                        ) = self
+                        let kind = self
                             .generics()
                             .expect("variable impl trait lowering must be in a generic def")
-                            .provenance_split();
-                        TyKind::BoundVar(BoundVar::new(
-                            self.in_binders,
-                            idx as usize
-                                + self_param as usize
-                                + type_params
-                                + const_params
-                                + lifetime_params,
-                        ))
-                        .intern(Interner)
+                            .iter()
+                            .enumerate()
+                            .filter_map(|(i, (id, data))| match (id, data) {
+                                (
+                                    GenericParamId::TypeParamId(_),
+                                    GenericParamDataRef::TypeParamData(data),
+                                ) if data.provenance == TypeParamProvenance::ArgumentImplTrait => {
+                                    Some(i)
+                                }
+                                _ => None,
+                            })
+                            .nth(idx as usize)
+                            .map_or(TyKind::Error, |id| {
+                                TyKind::BoundVar(BoundVar { debruijn: self.in_binders, index: id })
+                            });
+                        kind.intern(Interner)
                     }
                     ImplTraitLoweringState::Disallowed => {
                         // FIXME: report error

crates/hir-ty/src/tests/regression.rs

@@ -2162,3 +2162,36 @@ fn main() {
         "#]],
     );
 }
+
+#[test]
+fn issue_17711() {
+    check_infer(
+        r#"
+//- minicore: deref
+use core::ops::Deref;
+
+struct Struct<'a, T>(&'a T);
+
+trait Trait {}
+
+impl<'a, T: Deref<Target = impl Trait>> Struct<'a, T> {
+    fn foo(&self) -> &Self { self }
+
+    fn bar(&self) {
+        let _ = self.foo();
+    }
+
+}
+"#,
+        expect![[r#"
+            137..141 'self': &'? Struct<'a, T>
+            152..160 '{ self }': &'? Struct<'a, T>
+            154..158 'self': &'? Struct<'a, T>
+            174..178 'self': &'? Struct<'a, T>
+            180..215 '{     ...     }': ()
+            194..195 '_': &'? Struct<'?, T>
+            198..202 'self': &'? Struct<'a, T>
+            198..208 'self.foo()': &'? Struct<'?, T>
+        "#]],
+    );
+}