Merge #9975

9975: minor: Fix panic caused by #9966 r=flodiebold a=flodiebold

Chalk can introduce new type variables when doing lazy normalization, so we have to do the proper 'fudging' after all.

Co-authored-by: Florian Diebold <flodiebold@gmail.com>

crates/hir_ty/src/infer/expr.rs

@@ -986,20 +986,21 @@ impl<'a> InferenceContext<'a> {
         inputs: Vec<Ty>,
     ) -> Vec<Ty> {
         if let Some(expected_ty) = expected_output.to_option(&mut self.table) {
-            let snapshot = self.table.snapshot();
-            let result = if self.table.try_unify(&expected_ty, &output).is_ok() {
-                // FIXME: the unification could introduce lifetime variables, which we'd need to handle here
-                self.table.resolve_with_fallback(inputs, |var, kind, _, _| match kind {
-                    chalk_ir::VariableKind::Ty(tk) => var.to_ty(&Interner, tk).cast(&Interner),
-                    chalk_ir::VariableKind::Lifetime => var.to_lifetime(&Interner).cast(&Interner),
-                    chalk_ir::VariableKind::Const(ty) => {
-                        var.to_const(&Interner, ty).cast(&Interner)
-                    }
-                })
-            } else {
-                Vec::new()
-            };
-            self.table.rollback_to(snapshot);
+            let result = self.table.fudge_inference(|table| {
+                if table.try_unify(&expected_ty, &output).is_ok() {
+                    table.resolve_with_fallback(inputs, |var, kind, _, _| match kind {
+                        chalk_ir::VariableKind::Ty(tk) => var.to_ty(&Interner, tk).cast(&Interner),
+                        chalk_ir::VariableKind::Lifetime => {
+                            var.to_lifetime(&Interner).cast(&Interner)
+                        }
+                        chalk_ir::VariableKind::Const(ty) => {
+                            var.to_const(&Interner, ty).cast(&Interner)
+                        }
+                    })
+                } else {
+                    Vec::new()
+                }
+            });
             result
         } else {
             Vec::new()

crates/hir_ty/src/infer/unify.rs

@@ -11,9 +11,9 @@ use ena::unify::UnifyKey;
 
 use super::{InferOk, InferResult, InferenceContext, TypeError};
 use crate::{
-    db::HirDatabase, fold_tys, static_lifetime, AliasEq, AliasTy, BoundVar, Canonical,
-    DebruijnIndex, GenericArg, Goal, Guidance, InEnvironment, InferenceVar, Interner, ProjectionTy,
-    Scalar, Solution, Substitution, TraitEnvironment, Ty, TyKind, VariableKind,
+    db::HirDatabase, fold_tys, static_lifetime, AliasEq, AliasTy, BoundVar, Canonical, Const,
+    DebruijnIndex, GenericArg, Goal, Guidance, InEnvironment, InferenceVar, Interner, Lifetime,
+    ProjectionTy, Scalar, Solution, Substitution, TraitEnvironment, Ty, TyKind, VariableKind,
 };
 
 impl<'a> InferenceContext<'a> {
@@ -273,6 +273,16 @@ impl<'a> InferenceTable<'a> {
         self.new_var(TyVariableKind::General, true)
     }
 
+    pub(crate) fn new_const_var(&mut self, ty: Ty) -> Const {
+        let var = self.var_unification_table.new_variable(UniverseIndex::ROOT);
+        var.to_const(&Interner, ty)
+    }
+
+    pub(crate) fn new_lifetime_var(&mut self) -> Lifetime {
+        let var = self.var_unification_table.new_variable(UniverseIndex::ROOT);
+        var.to_lifetime(&Interner)
+    }
+
     pub(crate) fn resolve_with_fallback<T>(
         &mut self,
         t: T,
@@ -388,6 +398,76 @@ impl<'a> InferenceTable<'a> {
         }
     }
 
+    pub(crate) fn fudge_inference<T: Fold<Interner>>(
+        &mut self,
+        f: impl FnOnce(&mut Self) -> T,
+    ) -> T::Result {
+        use chalk_ir::fold::Folder;
+        struct VarFudger<'a, 'b> {
+            table: &'a mut InferenceTable<'b>,
+            highest_known_var: InferenceVar,
+        }
+        impl<'a, 'b> Folder<'static, Interner> for VarFudger<'a, 'b> {
+            fn as_dyn(&mut self) -> &mut dyn Folder<'static, Interner> {
+                self
+            }
+
+            fn interner(&self) -> &'static Interner {
+                &Interner
+            }
+
+            fn fold_inference_ty(
+                &mut self,
+                var: chalk_ir::InferenceVar,
+                kind: TyVariableKind,
+                _outer_binder: chalk_ir::DebruijnIndex,
+            ) -> chalk_ir::Fallible<chalk_ir::Ty<Interner>> {
+                Ok(if var < self.highest_known_var {
+                    var.to_ty(&Interner, kind)
+                } else {
+                    self.table.new_type_var()
+                })
+            }
+
+            fn fold_inference_lifetime(
+                &mut self,
+                var: chalk_ir::InferenceVar,
+                _outer_binder: chalk_ir::DebruijnIndex,
+            ) -> chalk_ir::Fallible<chalk_ir::Lifetime<Interner>> {
+                Ok(if var < self.highest_known_var {
+                    var.to_lifetime(&Interner)
+                } else {
+                    self.table.new_lifetime_var()
+                })
+            }
+
+            fn fold_inference_const(
+                &mut self,
+                ty: chalk_ir::Ty<Interner>,
+                var: chalk_ir::InferenceVar,
+                _outer_binder: chalk_ir::DebruijnIndex,
+            ) -> chalk_ir::Fallible<chalk_ir::Const<Interner>> {
+                Ok(if var < self.highest_known_var {
+                    var.to_const(&Interner, ty)
+                } else {
+                    self.table.new_const_var(ty)
+                })
+            }
+        }
+
+        let snapshot = self.snapshot();
+        let highest_known_var =
+            self.new_type_var().inference_var(&Interner).expect("inference_var");
+        let result = f(self);
+        self.rollback_to(snapshot);
+
+        let result = result
+            .fold_with(&mut VarFudger { table: self, highest_known_var }, DebruijnIndex::INNERMOST)
+            .expect("fold_with with VarFudger");
+
+        result
+    }
+
     /// This checks whether any of the free variables in the `canonicalized`
     /// have changed (either been unified with another variable, or with a
     /// value). If this is not the case, we don't need to try to solve the goal

crates/hir_ty/src/tests/regression.rs

@@ -1114,3 +1114,34 @@ fn test() {
         "#,
     );
 }
+
+#[test]
+fn coerce_diesel_panic() {
+    check_no_mismatches(
+        r#"
+//- minicore: option
+
+trait TypeMetadata {
+    type MetadataLookup;
+}
+
+pub struct Output<'a, T, DB>
+where
+    DB: TypeMetadata,
+    DB::MetadataLookup: 'a,
+{
+    out: T,
+    metadata_lookup: Option<&'a DB::MetadataLookup>,
+}
+
+impl<'a, T, DB: TypeMetadata> Output<'a, T, DB> {
+    pub fn new(out: T, metadata_lookup: &'a DB::MetadataLookup) -> Self {
+        Output {
+            out,
+            metadata_lookup: Some(metadata_lookup),
+        }
+    }
+}
+        "#,
+    );
+}