Solve ProjectionPredicate during normalization (#830)

* add test

* fix #829 hopefully

* Update crates/flux-middle/src/rty/projections.rs

Co-authored-by: Nico Lehmann <nico.lehmannm@gmail.com>

* change todo to tracked-span-bug

crates/flux-driver/src/collector/mod.rs

@@ -22,7 +22,7 @@ use rustc_errors::ErrorGuaranteed;
 use rustc_hir::{
     self as hir,
     def::DefKind,
-    def_id::{DefId, LocalDefId, CRATE_DEF_ID},
+    def_id::{LocalDefId, CRATE_DEF_ID},
     EnumDef, ImplItemKind, Item, ItemKind, OwnerId, VariantData, CRATE_OWNER_ID,
 };
 use rustc_middle::ty::TyCtxt;

crates/flux-middle/src/rty/projections.rs

@@ -13,8 +13,9 @@ use rustc_trait_selection::traits::SelectionContext;
 
 use super::{
     fold::{FallibleTypeFolder, TypeFoldable, TypeSuperFoldable},
-    AliasKind, AliasReft, AliasTy, BaseTy, Binder, Clause, ClauseKind, Const, Expr, ExprKind,
-    GenericArg, ProjectionPredicate, RefineArgs, Region, SubsetTy, Ty, TyKind,
+    subst::{GenericsSubstDelegate, GenericsSubstFolder},
+    AliasKind, AliasReft, AliasTy, BaseTy, Binder, Clause, ClauseKind, Const, EarlyBinder, Expr,
+    ExprKind, GenericArg, ProjectionPredicate, RefineArgs, Region, SubsetTy, Ty, TyKind,
 };
 use crate::{
     global_env::GlobalEnv,
@@ -136,7 +137,65 @@ impl<'genv, 'tcx, 'cx> Normalizer<'genv, 'tcx, 'cx> {
         Ok((true, ty))
     }
 
-    fn confirm_candidate(&self, candidate: Candidate, obligation: &AliasTy) -> QueryResult<Ty> {
+    fn find_resolved_predicates(
+        &self,
+        subst: &mut TVarSubst,
+        preds: Vec<EarlyBinder<ProjectionPredicate>>,
+    ) -> (Vec<ProjectionPredicate>, Vec<EarlyBinder<ProjectionPredicate>>) {
+        let mut resolved = vec![];
+        let mut unresolved = vec![];
+        for pred in preds {
+            let term = pred.clone().skip_binder().term;
+            let alias_ty = pred.clone().map(|p| p.projection_ty);
+            match subst.instantiate_partial(alias_ty) {
+                Some(projection_ty) => {
+                    let pred = ProjectionPredicate { projection_ty, term };
+                    resolved.push(pred);
+                }
+                None => unresolved.push(pred.clone()),
+            }
+        }
+        (resolved, unresolved)
+    }
+
+    // See issue-829*.rs for an example of what this function is for.
+    fn resolve_projection_predicates(
+        &mut self,
+        subst: &mut TVarSubst,
+        impl_def_id: DefId,
+    ) -> QueryResult {
+        let mut projection_preds: Vec<_> = self
+            .genv
+            .predicates_of(impl_def_id)?
+            .skip_binder()
+            .predicates
+            .iter()
+            .filter_map(|pred| {
+                if let ClauseKind::Projection(pred) = pred.kind_skipping_binder() {
+                    Some(EarlyBinder(pred.clone()))
+                } else {
+                    None
+                }
+            })
+            .collect();
+
+        while !projection_preds.is_empty() {
+            let (resolved, unresolved) = self.find_resolved_predicates(subst, projection_preds);
+
+            if resolved.is_empty() {
+                break; // failed: there is some unresolved projection pred!
+            }
+            for p in resolved {
+                let obligation = &p.projection_ty;
+                let (_, ty) = self.normalize_projection_ty(obligation)?;
+                subst.tys(&p.term, &ty);
+            }
+            projection_preds = unresolved;
+        }
+        Ok(())
+    }
+
+    fn confirm_candidate(&mut self, candidate: Candidate, obligation: &AliasTy) -> QueryResult<Ty> {
         match candidate {
             Candidate::ParamEnv(pred) | Candidate::TraitDef(pred) => Ok(pred.term),
             Candidate::UserDefinedImpl(impl_def_id) => {
@@ -145,9 +204,9 @@ impl<'genv, 'tcx, 'cx> Normalizer<'genv, 'tcx, 'cx> {
                 // and the id of a rust impl block
                 //     impl<T, A: Allocator> Iterator for IntoIter<T, A>
 
-                // 1. Match the self type of the rust impl block and the flux self type of the obligation
+                // 1. MATCH the self type of the rust impl block and the flux self type of the obligation
                 //    to infer a substitution
-                //        IntoIter<{v. i32[v] | v > 0}, Global> against IntoIter<T, A>
+                //        IntoIter<{v. i32[v] | v > 0}, Global> MATCH IntoIter<T, A>
                 //            => {T -> {v. i32[v] | v > 0}, A -> Global}
 
                 let impl_trait_ref = self
@@ -162,9 +221,13 @@ impl<'genv, 'tcx, 'cx> Normalizer<'genv, 'tcx, 'cx> {
                 for (a, b) in iter::zip(&impl_trait_ref.args, &obligation.args) {
                     subst.generic_args(a, b);
                 }
+
+                // 2. Gather the ProjectionPredicates and solve them see issue-808.rs
+                self.resolve_projection_predicates(&mut subst, impl_def_id)?;
+
                 let args = subst.finish(self.tcx(), generics);
 
-                // 2. Get the associated type in the impl block and apply the substitution to it
+                // 3. Get the associated type in the impl block and apply the substitution to it
                 let assoc_type_id = self
                     .tcx()
                     .associated_items(impl_def_id)
@@ -316,11 +379,51 @@ struct TVarSubst {
     args: Vec<Option<GenericArg>>,
 }
 
+impl GenericsSubstDelegate for &TVarSubst {
+    type Error = ();
+
+    fn ty_for_param(&mut self, param_ty: rustc_middle::ty::ParamTy) -> Result<Ty, Self::Error> {
+        match self.args.get(param_ty.index as usize) {
+            Some(Some(GenericArg::Ty(ty))) => Ok(ty.clone()),
+            Some(None) => Err(()),
+            arg => tracked_span_bug!("expected type for generic parameter, found `{arg:?}`"),
+        }
+    }
+
+    fn sort_for_param(
+        &mut self,
+        _param_ty: rustc_middle::ty::ParamTy,
+    ) -> Result<super::Sort, Self::Error> {
+        tracked_span_bug!()
+    }
+
+    fn ctor_for_param(&mut self, _param_ty: rustc_middle::ty::ParamTy) -> super::SubsetTyCtor {
+        tracked_span_bug!()
+    }
+
+    fn region_for_param(&mut self, _ebr: rustc_middle::ty::EarlyParamRegion) -> Region {
+        tracked_span_bug!()
+    }
+
+    fn expr_for_param_const(&self, _param_const: rustc_middle::ty::ParamConst) -> Expr {
+        tracked_span_bug!()
+    }
+
+    fn const_for_param(&mut self, _param: &Const) -> Const {
+        tracked_span_bug!()
+    }
+}
+
 impl TVarSubst {
     fn new(generics: &rustc_middle::ty::Generics) -> Self {
         Self { args: vec![None; generics.count()] }
     }
 
+    fn instantiate_partial<T: TypeFoldable>(&mut self, pred: EarlyBinder<T>) -> Option<T> {
+        let mut folder = GenericsSubstFolder::new(&*self, &[]);
+        pred.skip_binder().try_fold_with(&mut folder).ok()
+    }
+
     fn finish<'tcx>(
         self,
         tcx: TyCtxt<'tcx>,

crates/flux-middle/src/rty/subst.rs

@@ -334,7 +334,7 @@ pub trait GenericsSubstDelegate {
     type Error = !;
 
     fn sort_for_param(&mut self, param_ty: ParamTy) -> Result<Sort, Self::Error>;
-    fn ty_for_param(&mut self, param_ty: ParamTy) -> Ty;
+    fn ty_for_param(&mut self, param_ty: ParamTy) -> Result<Ty, Self::Error>;
     fn ctor_for_param(&mut self, param_ty: ParamTy) -> SubsetTyCtor;
     fn region_for_param(&mut self, ebr: EarlyParamRegion) -> Region;
     fn expr_for_param_const(&self, param_const: ParamConst) -> Expr;
@@ -358,9 +358,9 @@ impl<'a, 'tcx> GenericsSubstDelegate for GenericArgsDelegate<'a, 'tcx> {
         }
     }
 
-    fn ty_for_param(&mut self, param_ty: ParamTy) -> Ty {
+    fn ty_for_param(&mut self, param_ty: ParamTy) -> Result<Ty, !> {
         match self.0.get(param_ty.index as usize) {
-            Some(GenericArg::Ty(ty)) => ty.clone(),
+            Some(GenericArg::Ty(ty)) => Ok(ty.clone()),
             Some(arg) => tracked_span_bug!("expected type for generic parameter, found `{arg:?}`"),
             None => tracked_span_bug!("type parameter out of range {param_ty:?}"),
         }
@@ -433,7 +433,7 @@ where
         (self.sort_for_param)(param_ty)
     }
 
-    fn ty_for_param(&mut self, param_ty: ParamTy) -> Ty {
+    fn ty_for_param(&mut self, param_ty: ParamTy) -> Result<Ty, E> {
         bug!("unexpected type param {param_ty:?}");
     }
 
@@ -497,7 +497,7 @@ impl<D: GenericsSubstDelegate> FallibleTypeFolder for GenericsSubstFolder<'_, D>
 
     fn try_fold_ty(&mut self, ty: &Ty) -> Result<Ty, D::Error> {
         match ty.kind() {
-            TyKind::Param(param_ty) => Ok(self.delegate.ty_for_param(*param_ty)),
+            TyKind::Param(param_ty) => self.delegate.ty_for_param(*param_ty),
             TyKind::Indexed(BaseTy::Param(param_ty), idx) => {
                 let idx = idx.try_fold_with(self)?;
                 Ok(self

tests/tests/pos/surface/issue-829.rs

@@ -0,0 +1,24 @@
+trait Trait1 {
+    type Assoc1;
+}
+
+impl Trait1 for i32 {
+    type Assoc1 = bool;
+}
+
+trait Trait2 {
+    type Assoc2;
+}
+
+struct S<T> {
+    fld: T,
+}
+
+impl<T1, T2> Trait2 for S<T2>
+where
+    T2: Trait1<Assoc1 = T1>,
+{
+    type Assoc2 = T1;
+}
+
+fn test(x: <S<i32> as Trait2>::Assoc2) {}

tests/tests/pos/surface/issue-829b.rs

@@ -0,0 +1,25 @@
+trait Trait1 {
+    type Assoc1;
+}
+
+impl Trait1 for i32 {
+    type Assoc1 = i32;
+}
+
+trait Trait2 {
+    type Assoc2;
+}
+
+struct S<T> {
+    fld: T,
+}
+
+impl<T1, T2, T3> Trait2 for S<T1>
+where
+    T2: Trait1<Assoc1 = T3>,
+    T1: Trait1<Assoc1 = T2>,
+{
+    type Assoc2 = T1;
+}
+
+fn test(x: <S<i32> as Trait2>::Assoc2) {}