Auto merge of #123240 - compiler-errors:assert-args-compat, r=fmease

Assert that args are actually compatible with their generics, rather than just their count

Right now we just check that the number of args is right, rather than actually checking the kinds. Uplift a helper fn that I wrote from trait selection to do just that. Found a couple bugs along the way.

r? `@lcnr` or `@fmease` (or anyone really lol)

10 files changed, 151 insertions, 114 deletions

diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/object_safety.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/object_safety.rs
index d3ca35ba481..97ba946b7e0 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/object_safety.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/object_safety.rs
@@ -6,9 +6,10 @@ use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
 use rustc_lint_defs::builtin::UNUSED_ASSOCIATED_TYPE_BOUNDS;
-use rustc_middle::ty::{self, Ty};
+use rustc_middle::ty::fold::BottomUpFolder;
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable};
 use rustc_middle::ty::{DynKind, ToPredicate};
-use rustc_span::Span;
+use rustc_span::{ErrorGuaranteed, Span};
 use rustc_trait_selection::traits::error_reporting::report_object_safety_error;
 use rustc_trait_selection::traits::{self, hir_ty_lowering_object_safety_violations};
 
@@ -228,12 +229,17 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                         if arg == dummy_self.into() {
                             let param = &generics.params[index];
                             missing_type_params.push(param.name);
-                            return Ty::new_misc_error(tcx).into();
+                            Ty::new_misc_error(tcx).into()
                         } else if arg.walk().any(|arg| arg == dummy_self.into()) {
                             references_self = true;
-                            return Ty::new_misc_error(tcx).into();
+                            let guar = tcx.dcx().span_delayed_bug(
+                                span,
+                                "trait object trait bounds reference `Self`",
+                            );
+                            replace_dummy_self_with_error(tcx, arg, guar)
+                        } else {
+                            arg
                         }
-                        arg
                     })
                     .collect();
                 let args = tcx.mk_args(&args);
@@ -288,18 +294,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     let guar = tcx
                         .dcx()
                         .span_delayed_bug(span, "trait object projection bounds reference `Self`");
-                    let args: Vec<_> = b
-                        .projection_ty
-                        .args
-                        .iter()
-                        .map(|arg| {
-                            if arg.walk().any(|arg| arg == dummy_self.into()) {
-                                return Ty::new_error(tcx, guar).into();
-                            }
-                            arg
-                        })
-                        .collect();
-                    b.projection_ty.args = tcx.mk_args(&args);
+                    b.projection_ty = replace_dummy_self_with_error(tcx, b.projection_ty, guar);
                 }
 
                 ty::ExistentialProjection::erase_self_ty(tcx, b)
@@ -357,3 +352,18 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         Ty::new_dynamic(tcx, existential_predicates, region_bound, representation)
     }
 }
+
+fn replace_dummy_self_with_error<'tcx, T: TypeFoldable<TyCtxt<'tcx>>>(
+    tcx: TyCtxt<'tcx>,
+    t: T,
+    guar: ErrorGuaranteed,
+) -> T {
+    t.fold_with(&mut BottomUpFolder {
+        tcx,
+        ty_op: |ty| {
+            if ty == tcx.types.trait_object_dummy_self { Ty::new_error(tcx, guar) } else { ty }
+        },
+        lt_op: |lt| lt,
+        ct_op: |ct| ct,
+    })
+}
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 23881c4b124..b8d68c1b8be 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -1961,33 +1961,104 @@ impl<'tcx> TyCtxt<'tcx> {
         if pred.kind() != binder { self.mk_predicate(binder) } else { pred }
     }
 
-    #[inline(always)]
-    pub(crate) fn check_and_mk_args(
+    pub fn check_args_compatible(self, def_id: DefId, args: &'tcx [ty::GenericArg<'tcx>]) -> bool {
+        self.check_args_compatible_inner(def_id, args, false)
+    }
+
+    fn check_args_compatible_inner(
         self,
-        _def_id: DefId,
-        args: impl IntoIterator<Item: Into<GenericArg<'tcx>>>,
-    ) -> GenericArgsRef<'tcx> {
-        let args = args.into_iter().map(Into::into);
-        #[cfg(debug_assertions)]
+        def_id: DefId,
+        args: &'tcx [ty::GenericArg<'tcx>],
+        nested: bool,
+    ) -> bool {
+        let generics = self.generics_of(def_id);
+
+        // IATs themselves have a weird arg setup (self + own args), but nested items *in* IATs
+        // (namely: opaques, i.e. ATPITs) do not.
+        let own_args = if !nested
+            && let DefKind::AssocTy = self.def_kind(def_id)
+            && let DefKind::Impl { of_trait: false } = self.def_kind(self.parent(def_id))
         {
-            let generics = self.generics_of(_def_id);
+            if generics.params.len() + 1 != args.len() {
+                return false;
+            }
 
-            let n = if let DefKind::AssocTy = self.def_kind(_def_id)
-                && let DefKind::Impl { of_trait: false } = self.def_kind(self.parent(_def_id))
+            if !matches!(args[0].unpack(), ty::GenericArgKind::Type(_)) {
+                return false;
+            }
+
+            &args[1..]
+        } else {
+            if generics.count() != args.len() {
+                return false;
+            }
+
+            let (parent_args, own_args) = args.split_at(generics.parent_count);
+
+            if let Some(parent) = generics.parent
+                && !self.check_args_compatible_inner(parent, parent_args, true)
             {
-                // If this is an inherent projection.
-                generics.params.len() + 1
-            } else {
-                generics.count()
-            };
-            assert_eq!(
-                (n, Some(n)),
-                args.size_hint(),
-                "wrong number of generic parameters for {_def_id:?}: {:?}",
-                args.collect::<Vec<_>>(),
-            );
+                return false;
+            }
+
+            own_args
+        };
+
+        for (param, arg) in std::iter::zip(&generics.params, own_args) {
+            match (&param.kind, arg.unpack()) {
+                (ty::GenericParamDefKind::Type { .. }, ty::GenericArgKind::Type(_))
+                | (ty::GenericParamDefKind::Lifetime, ty::GenericArgKind::Lifetime(_))
+                | (ty::GenericParamDefKind::Const { .. }, ty::GenericArgKind::Const(_)) => {}
+                _ => return false,
+            }
         }
-        self.mk_args_from_iter(args)
+
+        true
+    }
+
+    /// With `cfg(debug_assertions)`, assert that args are compatible with their generics,
+    /// and print out the args if not.
+    pub fn debug_assert_args_compatible(self, def_id: DefId, args: &'tcx [ty::GenericArg<'tcx>]) {
+        if cfg!(debug_assertions) {
+            if !self.check_args_compatible(def_id, args) {
+                if let DefKind::AssocTy = self.def_kind(def_id)
+                    && let DefKind::Impl { of_trait: false } = self.def_kind(self.parent(def_id))
+                {
+                    bug!(
+                        "args not compatible with generics for {}: args={:#?}, generics={:#?}",
+                        self.def_path_str(def_id),
+                        args,
+                        // Make `[Self, GAT_ARGS...]` (this could be simplified)
+                        self.mk_args_from_iter(
+                            [self.types.self_param.into()].into_iter().chain(
+                                self.generics_of(def_id)
+                                    .own_args(ty::GenericArgs::identity_for_item(self, def_id))
+                                    .iter()
+                                    .copied()
+                            )
+                        )
+                    );
+                } else {
+                    bug!(
+                        "args not compatible with generics for {}: args={:#?}, generics={:#?}",
+                        self.def_path_str(def_id),
+                        args,
+                        ty::GenericArgs::identity_for_item(self, def_id)
+                    );
+                }
+            }
+        }
+    }
+
+    #[inline(always)]
+    pub(crate) fn check_and_mk_args(
+        self,
+        def_id: DefId,
+        args: impl IntoIterator<Item: Into<GenericArg<'tcx>>>,
+    ) -> GenericArgsRef<'tcx> {
+        let args = self.mk_args_from_iter(args.into_iter().map(Into::into));
+        self.debug_assert_args_compatible(def_id, args);
+        args
     }
 
     #[inline]
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 828e48c5be4..0df010f423c 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -1624,13 +1624,7 @@ impl<'tcx> Ty<'tcx> {
 
     #[inline]
     pub fn new_adt(tcx: TyCtxt<'tcx>, def: AdtDef<'tcx>, args: GenericArgsRef<'tcx>) -> Ty<'tcx> {
-        debug_assert_eq!(
-            tcx.generics_of(def.did()).count(),
-            args.len(),
-            "wrong number of args for ADT: {:#?} vs {:#?}",
-            tcx.generics_of(def.did()).params,
-            args
-        );
+        tcx.debug_assert_args_compatible(def.did(), args);
         Ty::new(tcx, Adt(def, args))
     }
 
@@ -1715,11 +1709,7 @@ impl<'tcx> Ty<'tcx> {
         def_id: DefId,
         closure_args: GenericArgsRef<'tcx>,
     ) -> Ty<'tcx> {
-        debug_assert_eq!(
-            closure_args.len(),
-            tcx.generics_of(tcx.typeck_root_def_id(def_id)).count() + 3,
-            "closure constructed with incorrect generic parameters"
-        );
+        tcx.debug_assert_args_compatible(def_id, closure_args);
         Ty::new(tcx, Closure(def_id, closure_args))
     }
 
@@ -1729,11 +1719,7 @@ impl<'tcx> Ty<'tcx> {
         def_id: DefId,
         closure_args: GenericArgsRef<'tcx>,
     ) -> Ty<'tcx> {
-        debug_assert_eq!(
-            closure_args.len(),
-            tcx.generics_of(tcx.typeck_root_def_id(def_id)).count() + 5,
-            "closure constructed with incorrect generic parameters"
-        );
+        tcx.debug_assert_args_compatible(def_id, closure_args);
         Ty::new(tcx, CoroutineClosure(def_id, closure_args))
     }
 
@@ -1743,11 +1729,7 @@ impl<'tcx> Ty<'tcx> {
         def_id: DefId,
         coroutine_args: GenericArgsRef<'tcx>,
     ) -> Ty<'tcx> {
-        debug_assert_eq!(
-            coroutine_args.len(),
-            tcx.generics_of(tcx.typeck_root_def_id(def_id)).count() + 6,
-            "coroutine constructed with incorrect number of generic parameters"
-        );
+        tcx.debug_assert_args_compatible(def_id, coroutine_args);
         Ty::new(tcx, Coroutine(def_id, coroutine_args))
     }
 
diff --git a/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs b/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs
index 66688893235..d8aeadd07b3 100644
--- a/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs
+++ b/compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs
@@ -1,4 +1,4 @@
-use crate::traits::{check_args_compatible, specialization_graph};
+use crate::traits::specialization_graph;
 
 use super::assembly::structural_traits::AsyncCallableRelevantTypes;
 use super::assembly::{self, structural_traits, Candidate};
@@ -247,7 +247,7 @@ impl<'tcx> assembly::GoalKind<'tcx> for NormalizesTo<'tcx> {
                 assoc_def.defining_node,
             );
 
-            if !check_args_compatible(tcx, assoc_def.item, args) {
+            if !tcx.check_args_compatible(assoc_def.item.def_id, args) {
                 return error_response(
                     ecx,
                     "associated item has mismatched generic item arguments",
diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index 3ab4872fffe..2c8116b779b 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -61,12 +61,12 @@ pub use self::specialize::{
 pub use self::structural_match::search_for_structural_match_violation;
 pub use self::structural_normalize::StructurallyNormalizeExt;
 pub use self::util::elaborate;
-pub use self::util::{
-    check_args_compatible, supertrait_def_ids, supertraits, transitive_bounds,
-    transitive_bounds_that_define_assoc_item, SupertraitDefIds,
-};
 pub use self::util::{expand_trait_aliases, TraitAliasExpander, TraitAliasExpansionInfo};
 pub use self::util::{get_vtable_index_of_object_method, impl_item_is_final, upcast_choices};
+pub use self::util::{
+    supertrait_def_ids, supertraits, transitive_bounds, transitive_bounds_that_define_assoc_item,
+    SupertraitDefIds,
+};
 pub use self::util::{with_replaced_escaping_bound_vars, BoundVarReplacer, PlaceholderReplacer};
 
 pub use rustc_infer::traits::*;
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index c33e24b1094..9d744d9a032 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -2,7 +2,6 @@
 
 use std::ops::ControlFlow;
 
-use super::check_args_compatible;
 use super::specialization_graph;
 use super::translate_args;
 use super::util;
@@ -2030,7 +2029,7 @@ fn confirm_impl_candidate<'cx, 'tcx>(
     } else {
         ty.map_bound(|ty| ty.into())
     };
-    if !check_args_compatible(tcx, assoc_ty.item, args) {
+    if !tcx.check_args_compatible(assoc_ty.item.def_id, args) {
         let err = Ty::new_error_with_message(
             tcx,
             obligation.cause.span,
diff --git a/compiler/rustc_trait_selection/src/traits/util.rs b/compiler/rustc_trait_selection/src/traits/util.rs
index 466a53d8ce0..d29fc7921bc 100644
--- a/compiler/rustc_trait_selection/src/traits/util.rs
+++ b/compiler/rustc_trait_selection/src/traits/util.rs
@@ -344,48 +344,6 @@ pub enum TupleArgumentsFlag {
     No,
 }
 
-// Verify that the trait item and its implementation have compatible args lists
-pub fn check_args_compatible<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    assoc_item: ty::AssocItem,
-    args: ty::GenericArgsRef<'tcx>,
-) -> bool {
-    fn check_args_compatible_inner<'tcx>(
-        tcx: TyCtxt<'tcx>,
-        generics: &'tcx ty::Generics,
-        args: &'tcx [ty::GenericArg<'tcx>],
-    ) -> bool {
-        if generics.count() != args.len() {
-            return false;
-        }
-
-        let (parent_args, own_args) = args.split_at(generics.parent_count);
-
-        if let Some(parent) = generics.parent
-            && let parent_generics = tcx.generics_of(parent)
-            && !check_args_compatible_inner(tcx, parent_generics, parent_args)
-        {
-            return false;
-        }
-
-        for (param, arg) in std::iter::zip(&generics.params, own_args) {
-            match (&param.kind, arg.unpack()) {
-                (ty::GenericParamDefKind::Type { .. }, ty::GenericArgKind::Type(_))
-                | (ty::GenericParamDefKind::Lifetime, ty::GenericArgKind::Lifetime(_))
-                | (ty::GenericParamDefKind::Const { .. }, ty::GenericArgKind::Const(_)) => {}
-                _ => return false,
-            }
-        }
-
-        true
-    }
-
-    let generics = tcx.generics_of(assoc_item.def_id);
-    // Chop off any additional args (RPITIT) args
-    let args = &args[0..generics.count().min(args.len())];
-    check_args_compatible_inner(tcx, generics, args)
-}
-
 /// Executes `f` on `value` after replacing all escaping bound variables with placeholders
 /// and then replaces these placeholders with the original bound variables in the result.
 ///
diff --git a/compiler/rustc_ty_utils/src/opaque_types.rs b/compiler/rustc_ty_utils/src/opaque_types.rs
index fc16edc6d13..a652bb78116 100644
--- a/compiler/rustc_ty_utils/src/opaque_types.rs
+++ b/compiler/rustc_ty_utils/src/opaque_types.rs
@@ -7,7 +7,6 @@ use rustc_middle::ty::util::{CheckRegions, NotUniqueParam};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::ty::{TypeSuperVisitable, TypeVisitable, TypeVisitor};
 use rustc_span::Span;
-use rustc_trait_selection::traits::check_args_compatible;
 
 use crate::errors::{DuplicateArg, NotParam};
 
@@ -250,7 +249,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OpaqueTypeCollector<'tcx> {
                                 ty::GenericArgs::identity_for_item(self.tcx, parent),
                             );
 
-                            if check_args_compatible(self.tcx, assoc, impl_args) {
+                            if self.tcx.check_args_compatible(assoc.def_id, impl_args) {
                                 self.tcx
                                     .type_of(assoc.def_id)
                                     .instantiate(self.tcx, impl_args)
diff --git a/tests/ui/impl-trait/in-trait/span-bug-issue-121457.rs b/tests/ui/impl-trait/in-trait/span-bug-issue-121457.rs
index ab21dae7dc5..7a51037324f 100644
--- a/tests/ui/impl-trait/in-trait/span-bug-issue-121457.rs
+++ b/tests/ui/impl-trait/in-trait/span-bug-issue-121457.rs
@@ -14,6 +14,7 @@ impl<'a, I: 'a + Iterable> Iterable for &'a I {
     //~^ ERROR binding for associated type `Item` references lifetime `'missing`
     //~| ERROR binding for associated type `Item` references lifetime `'missing`
     //~| ERROR `()` is not an iterator
+    //~| WARNING impl trait in impl method signature does not match trait method signature
 }
 
 fn main() {}
diff --git a/tests/ui/impl-trait/in-trait/span-bug-issue-121457.stderr b/tests/ui/impl-trait/in-trait/span-bug-issue-121457.stderr
index d8a2eef94a1..67c4df0f3a9 100644
--- a/tests/ui/impl-trait/in-trait/span-bug-issue-121457.stderr
+++ b/tests/ui/impl-trait/in-trait/span-bug-issue-121457.stderr
@@ -32,7 +32,24 @@ LL |     fn iter(&self) -> impl for<'missing> Iterator<Item = Self::Item<'missin
    |
    = help: the trait `Iterator` is not implemented for `()`
 
-error: aborting due to 4 previous errors
+warning: impl trait in impl method signature does not match trait method signature
+  --> $DIR/span-bug-issue-121457.rs:13:51
+   |
+LL |     fn iter(&self) -> impl Iterator;
+   |                       ------------- return type from trait method defined here
+...
+LL |     fn iter(&self) -> impl for<'missing> Iterator<Item = Self::Item<'missing>> {}
+   |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^ this bound is stronger than that defined on the trait
+   |
+   = note: add `#[allow(refining_impl_trait)]` if it is intended for this to be part of the public API of this crate
+   = note: we are soliciting feedback, see issue #121718 <https://github.com/rust-lang/rust/issues/121718> for more information
+   = note: `#[warn(refining_impl_trait_reachable)]` on by default
+help: replace the return type so that it matches the trait
+   |
+LL |     fn iter(&self) -> impl Iterator {}
+   |                       ~~~~~~~~~~~~~
+
+error: aborting due to 4 previous errors; 1 warning emitted
 
 Some errors have detailed explanations: E0195, E0277, E0582.
 For more information about an error, try `rustc --explain E0195`.