Lint against nested opaque types that don't satisfy associated type bounds

compiler/rustc_error_messages/locales/en-US/lint.ftl

@@ -433,3 +433,7 @@ lint_check_name_unknown_tool = unknown lint tool: `{$tool_name}`
 lint_check_name_warning = {$msg}
 
 lint_check_name_deprecated = lint name `{$lint_name}` is deprecated and does not have an effect anymore. Use: {$new_name}
+
+lint_opaque_hidden_inferred_bound = opaque type `{$ty}` does not satisfy its associated type bounds
+    .specifically = this associated type bound is unsatisfied for `{$proj_ty}`
+    .suggestion = add this bound

compiler/rustc_lint/src/lib.rs

@@ -62,6 +62,7 @@ mod non_ascii_idents;
 mod non_fmt_panic;
 mod nonstandard_style;
 mod noop_method_call;
+mod opaque_hidden_inferred_bound;
 mod pass_by_value;
 mod passes;
 mod redundant_semicolon;
@@ -93,6 +94,7 @@ use non_ascii_idents::*;
 use non_fmt_panic::NonPanicFmt;
 use nonstandard_style::*;
 use noop_method_call::*;
+use opaque_hidden_inferred_bound::*;
 use pass_by_value::*;
 use redundant_semicolon::*;
 use traits::*;
@@ -223,6 +225,7 @@ macro_rules! late_lint_mod_passes {
                 EnumIntrinsicsNonEnums: EnumIntrinsicsNonEnums,
                 InvalidAtomicOrdering: InvalidAtomicOrdering,
                 NamedAsmLabels: NamedAsmLabels,
+                OpaqueHiddenInferredBound: OpaqueHiddenInferredBound,
             ]
         );
     };

compiler/rustc_lint/src/opaque_hidden_inferred_bound.rs

@@ -0,0 +1,156 @@
+use rustc_hir as hir;
+use rustc_infer::infer::TyCtxtInferExt;
+use rustc_macros::LintDiagnostic;
+use rustc_middle::ty::{self, fold::BottomUpFolder, Ty, TypeFoldable};
+use rustc_span::Span;
+use rustc_trait_selection::traits;
+use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
+
+use crate::{LateContext, LateLintPass, LintContext};
+
+declare_lint! {
+    /// The `opaque_hidden_inferred_bound` lint detects cases in which nested
+    /// `impl Trait` in associated type bounds are not written generally enough
+    /// to satisfy the bounds of the associated type.
+    ///
+    /// ### Explanation
+    ///
+    /// This functionality was removed in #97346, but then rolled back in #99860
+    /// because it caused regressions.
+    ///
+    /// We plan on reintroducing this as a hard error, but in the mean time,
+    /// this lint serves to warn and suggest fixes for any use-cases which rely
+    /// on this behavior.
+    ///
+    /// ### Example
+    ///
+    /// ```
+    /// trait Trait {
+    ///     type Assoc: Send;
+    /// }
+    ///
+    /// struct Struct;
+    ///
+    /// impl Trait for Struct {
+    ///     type Assoc = i32;
+    /// }
+    ///
+    /// fn test() -> impl Trait<Assoc = impl Sized> {
+    ///     Struct
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// In this example, `test` declares that the associated type `Assoc` for
+    /// `impl Trait` is `impl Sized`, which does not satisfy the `Send` bound
+    /// on the associated type.
+    ///
+    /// Although the hidden type, `i32` does satisfy this bound, we do not
+    /// consider the return type to be well-formed with this lint. It can be
+    /// fixed by changing `impl Sized` into `impl Sized + Send`.
+    pub OPAQUE_HIDDEN_INFERRED_BOUND,
+    Warn,
+    "detects the use of nested `impl Trait` types in associated type bounds that are not general enough"
+}
+
+declare_lint_pass!(OpaqueHiddenInferredBound => [OPAQUE_HIDDEN_INFERRED_BOUND]);
+
+impl<'tcx> LateLintPass<'tcx> for OpaqueHiddenInferredBound {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
+        let hir::ItemKind::OpaqueTy(_) = &item.kind else { return; };
+        let def_id = item.def_id.def_id.to_def_id();
+        cx.tcx.infer_ctxt().enter(|ref infcx| {
+            // For every projection predicate in the opaque type's explicit bounds,
+            // check that the type that we're assigning actually satisfies the bounds
+            // of the associated type.
+            for &(pred, pred_span) in cx.tcx.explicit_item_bounds(def_id) {
+                // Liberate bound regions in the predicate since we
+                // don't actually care about lifetimes in this check.
+                let predicate = cx.tcx.liberate_late_bound_regions(
+                    def_id,
+                    pred.kind(),
+                );
+                let ty::PredicateKind::Projection(proj) = predicate else {
+                    continue;
+                };
+                // Only check types, since those are the only things that may
+                // have opaques in them anyways.
+                let Some(proj_term) = proj.term.ty() else { continue };
+
+                let proj_ty =
+                    cx
+                    .tcx
+                    .mk_projection(proj.projection_ty.item_def_id, proj.projection_ty.substs);
+                // For every instance of the projection type in the bounds,
+                // replace them with the term we're assigning to the associated
+                // type in our opaque type.
+                let proj_replacer = &mut BottomUpFolder {
+                    tcx: cx.tcx,
+                    ty_op: |ty| if ty == proj_ty { proj_term } else { ty },
+                    lt_op: |lt| lt,
+                    ct_op: |ct| ct,
+                };
+                // For example, in `impl Trait<Assoc = impl Send>`, for all of the bounds on `Assoc`,
+                // e.g. `type Assoc: OtherTrait`, replace `<impl Trait as Trait>::Assoc: OtherTrait`
+                // with `impl Send: OtherTrait`.
+                for assoc_pred_and_span in cx
+                    .tcx
+                    .bound_explicit_item_bounds(proj.projection_ty.item_def_id)
+                    .transpose_iter()
+                {
+                    let assoc_pred_span = assoc_pred_and_span.0.1;
+                    let assoc_pred = assoc_pred_and_span
+                        .map_bound(|(pred, _)| *pred)
+                        .subst(cx.tcx, &proj.projection_ty.substs)
+                        .fold_with(proj_replacer);
+                    let Ok(assoc_pred) = traits::fully_normalize(infcx, traits::ObligationCause::dummy(), cx.param_env, assoc_pred) else {
+                        continue;
+                    };
+                    // If that predicate doesn't hold modulo regions (but passed during type-check),
+                    // then we must've taken advantage of the hack in `project_and_unify_types` where
+                    // we replace opaques with inference vars. Emit a warning!
+                    if !infcx.predicate_must_hold_modulo_regions(&traits::Obligation::new(
+                        traits::ObligationCause::dummy(),
+                        cx.param_env,
+                        assoc_pred,
+                    )) {
+                        // If it's a trait bound and an opaque that doesn't satisfy it,
+                        // then we can emit a suggestion to add the bound.
+                        let (suggestion, suggest_span) =
+                            match (proj_term.kind(), assoc_pred.kind().skip_binder()) {
+                                (ty::Opaque(def_id, _), ty::PredicateKind::Trait(trait_pred)) => (
+                                    format!(" + {}", trait_pred.print_modifiers_and_trait_path()),
+                                    Some(cx.tcx.def_span(def_id).shrink_to_hi()),
+                                ),
+                                _ => (String::new(), None),
+                            };
+                        cx.emit_spanned_lint(
+                            OPAQUE_HIDDEN_INFERRED_BOUND,
+                            pred_span,
+                            OpaqueHiddenInferredBoundLint {
+                                ty: cx.tcx.mk_opaque(def_id, ty::InternalSubsts::identity_for_item(cx.tcx, def_id)),
+                                proj_ty: proj_term,
+                                assoc_pred_span,
+                                suggestion,
+                                suggest_span,
+                            },
+                        );
+                    }
+                }
+            }
+        });
+    }
+}
+
+#[derive(LintDiagnostic)]
+#[diag(lint::opaque_hidden_inferred_bound)]
+struct OpaqueHiddenInferredBoundLint<'tcx> {
+    ty: Ty<'tcx>,
+    proj_ty: Ty<'tcx>,
+    #[label(lint::specifically)]
+    assoc_pred_span: Span,
+    #[suggestion_verbose(applicability = "machine-applicable", code = "{suggestion}")]
+    suggest_span: Option<Span>,
+    suggestion: String,
+}

compiler/rustc_middle/src/lint.rs

@@ -444,7 +444,9 @@ pub fn in_external_macro(sess: &Session, span: Span) -> bool {
     match expn_data.kind {
         ExpnKind::Inlined
         | ExpnKind::Root
-        | ExpnKind::Desugaring(DesugaringKind::ForLoop | DesugaringKind::WhileLoop) => false,
+        | ExpnKind::Desugaring(
+            DesugaringKind::ForLoop | DesugaringKind::WhileLoop | DesugaringKind::OpaqueTy,
+        ) => false,
         ExpnKind::AstPass(_) | ExpnKind::Desugaring(_) => true, // well, it's "external"
         ExpnKind::Macro(MacroKind::Bang, _) => {
             // Dummy span for the `def_site` means it's an external macro.

src/test/ui/impl-trait/nested-return-type2-tait.rs

@@ -26,6 +26,7 @@ type Sendable = impl Send;
 // var to make it uphold the `: Duh` bound on `Trait::Assoc`. The opaque
 // type does not implement `Duh`, but if its hidden type does.
 fn foo() -> impl Trait<Assoc = Sendable> {
+    //~^ WARN opaque type `impl Trait<Assoc = Sendable>` does not satisfy its associated type bounds
     || 42
 }
 

src/test/ui/impl-trait/nested-return-type2-tait.stderr

@@ -0,0 +1,17 @@
+warning: opaque type `impl Trait<Assoc = Sendable>` does not satisfy its associated type bounds
+  --> $DIR/nested-return-type2-tait.rs:28:24
+   |
+LL |     type Assoc: Duh;
+   |                 --- this associated type bound is unsatisfied for `Sendable`
+...
+LL | fn foo() -> impl Trait<Assoc = Sendable> {
+   |                        ^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default
+help: add this bound
+   |
+LL | type Sendable = impl Send + Duh;
+   |                           +++++
+
+warning: 1 warning emitted
+

src/test/ui/impl-trait/nested-return-type2.rs

@@ -23,6 +23,7 @@ impl<R: Duh, F: FnMut() -> R> Trait for F {
 // Lazy TAIT would error out, but we inserted a hack to make it work again,
 // keeping backwards compatibility.
 fn foo() -> impl Trait<Assoc = impl Send> {
+    //~^ WARN opaque type `impl Trait<Assoc = impl Send>` does not satisfy its associated type bounds
     || 42
 }
 

src/test/ui/impl-trait/nested-return-type2.stderr

@@ -0,0 +1,17 @@
+warning: opaque type `impl Trait<Assoc = impl Send>` does not satisfy its associated type bounds
+  --> $DIR/nested-return-type2.rs:25:24
+   |
+LL |     type Assoc: Duh;
+   |                 --- this associated type bound is unsatisfied for `impl Send`
+...
+LL | fn foo() -> impl Trait<Assoc = impl Send> {
+   |                        ^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default
+help: add this bound
+   |
+LL | fn foo() -> impl Trait<Assoc = impl Send + Duh> {
+   |                                          +++++
+
+warning: 1 warning emitted
+

src/test/ui/impl-trait/nested-return-type3-tait.rs

@@ -17,6 +17,7 @@ impl<F: Duh> Trait for F {
 type Sendable = impl Send;
 
 fn foo() -> impl Trait<Assoc = Sendable> {
+    //~^ WARN opaque type `impl Trait<Assoc = Sendable>` does not satisfy its associated type bounds
     42
 }
 

src/test/ui/impl-trait/nested-return-type3-tait.stderr

@@ -0,0 +1,17 @@
+warning: opaque type `impl Trait<Assoc = Sendable>` does not satisfy its associated type bounds
+  --> $DIR/nested-return-type3-tait.rs:19:24
+   |
+LL |     type Assoc: Duh;
+   |                 --- this associated type bound is unsatisfied for `Sendable`
+...
+LL | fn foo() -> impl Trait<Assoc = Sendable> {
+   |                        ^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default
+help: add this bound
+   |
+LL | type Sendable = impl Send + Duh;
+   |                           +++++
+
+warning: 1 warning emitted
+

src/test/ui/impl-trait/nested-return-type3-tait2.rs

@@ -16,6 +16,7 @@ impl<F: Duh> Trait for F {
 
 type Sendable = impl Send;
 type Traitable = impl Trait<Assoc = Sendable>;
+//~^ WARN opaque type `Traitable` does not satisfy its associated type bounds
 
 fn foo() -> Traitable {
     42

src/test/ui/impl-trait/nested-return-type3-tait2.stderr

@@ -0,0 +1,17 @@
+warning: opaque type `Traitable` does not satisfy its associated type bounds
+  --> $DIR/nested-return-type3-tait2.rs:18:29
+   |
+LL |     type Assoc: Duh;
+   |                 --- this associated type bound is unsatisfied for `Sendable`
+...
+LL | type Traitable = impl Trait<Assoc = Sendable>;
+   |                             ^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default
+help: add this bound
+   |
+LL | type Sendable = impl Send + Duh;
+   |                           +++++
+
+warning: 1 warning emitted
+

src/test/ui/impl-trait/nested-return-type3-tait3.rs

@@ -15,6 +15,7 @@ impl<F: Duh> Trait for F {
 }
 
 type Traitable = impl Trait<Assoc = impl Send>;
+//~^ WARN opaque type `Traitable` does not satisfy its associated type bounds
 
 fn foo() -> Traitable {
     42

src/test/ui/impl-trait/nested-return-type3-tait3.stderr

@@ -0,0 +1,17 @@
+warning: opaque type `Traitable` does not satisfy its associated type bounds
+  --> $DIR/nested-return-type3-tait3.rs:17:29
+   |
+LL |     type Assoc: Duh;
+   |                 --- this associated type bound is unsatisfied for `impl Send`
+...
+LL | type Traitable = impl Trait<Assoc = impl Send>;
+   |                             ^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default
+help: add this bound
+   |
+LL | type Traitable = impl Trait<Assoc = impl Send + Duh>;
+   |                                               +++++
+
+warning: 1 warning emitted
+

src/test/ui/impl-trait/nested-return-type3.rs

@@ -13,6 +13,7 @@ impl<F: Duh> Trait for F {
 }
 
 fn foo() -> impl Trait<Assoc = impl Send> {
+    //~^ WARN opaque type `impl Trait<Assoc = impl Send>` does not satisfy its associated type bounds
     42
 }
 

src/test/ui/impl-trait/nested-return-type3.stderr

@@ -0,0 +1,17 @@
+warning: opaque type `impl Trait<Assoc = impl Send>` does not satisfy its associated type bounds
+  --> $DIR/nested-return-type3.rs:15:24
+   |
+LL |     type Assoc: Duh;
+   |                 --- this associated type bound is unsatisfied for `impl Send`
+...
+LL | fn foo() -> impl Trait<Assoc = impl Send> {
+   |                        ^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default
+help: add this bound
+   |
+LL | fn foo() -> impl Trait<Assoc = impl Send + Duh> {
+   |                                          +++++
+
+warning: 1 warning emitted
+