3 files changed, 28 insertions, 2 deletions
diff --git a/compiler/rustc_hir_analysis/messages.ftl b/compiler/rustc_hir_analysis/messages.ftl
index 67959d9dfed..f697d331aa6 100644
--- a/compiler/rustc_hir_analysis/messages.ftl
+++ b/compiler/rustc_hir_analysis/messages.ftl
@@ -460,6 +460,10 @@ hir_analysis_static_specialize = cannot specialize on `'static` lifetime
 hir_analysis_tait_forward_compat = item constrains opaque type that is not in its signature
     .note = this item must mention the opaque type in its signature in order to be able to register hidden types
 
+hir_analysis_tait_forward_compat2 = item does not constrain `{$opaque_type}`, but has it in its signature
+    .note = consider moving the opaque type's declaration and defining uses into a separate module
+    .opaque = this opaque type is in the signature
+
 hir_analysis_target_feature_on_main = `main` function is not allowed to have `#[target_feature]`
 
 hir_analysis_too_large_static = extern static is too large for the current architecture
diff --git a/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs b/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
index 1bec8c496ad..2b2f07001d2 100644
--- a/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
+++ b/compiler/rustc_hir_analysis/src/collect/type_of/opaque.rs
@@ -8,7 +8,7 @@ use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitableExt};
 use rustc_span::{sym, ErrorGuaranteed, DUMMY_SP};
 
-use crate::errors::{TaitForwardCompat, TypeOf, UnconstrainedOpaqueType};
+use crate::errors::{TaitForwardCompat, TaitForwardCompat2, TypeOf, UnconstrainedOpaqueType};
 
 pub fn test_opaque_hidden_types(tcx: TyCtxt<'_>) -> Result<(), ErrorGuaranteed> {
     let mut res = Ok(());
@@ -229,13 +229,14 @@ impl TaitConstraintLocator<'_> {
             return;
         }
 
+        let opaque_types_defined_by = self.tcx.opaque_types_defined_by(item_def_id);
+
         let mut constrained = false;
         for (&opaque_type_key, &hidden_type) in &tables.concrete_opaque_types {
             if opaque_type_key.def_id != self.def_id {
                 continue;
             }
             constrained = true;
-            let opaque_types_defined_by = self.tcx.opaque_types_defined_by(item_def_id);
 
             if !opaque_types_defined_by.contains(&self.def_id) {
                 self.tcx.dcx().emit_err(TaitForwardCompat {
@@ -259,6 +260,16 @@ impl TaitConstraintLocator<'_> {
 
         if !constrained {
             debug!("no constraints in typeck results");
+            if opaque_types_defined_by.contains(&self.def_id) {
+                self.tcx.dcx().emit_err(TaitForwardCompat2 {
+                    span: self
+                        .tcx
+                        .def_ident_span(item_def_id)
+                        .unwrap_or_else(|| self.tcx.def_span(item_def_id)),
+                    opaque_type_span: self.tcx.def_span(self.def_id),
+                    opaque_type: self.tcx.def_path_str(self.def_id),
+                });
+            }
             return;
         };
 
diff --git a/compiler/rustc_hir_analysis/src/errors.rs b/compiler/rustc_hir_analysis/src/errors.rs
index 8a9d4cd4ac7..50e0e372295 100644
--- a/compiler/rustc_hir_analysis/src/errors.rs
+++ b/compiler/rustc_hir_analysis/src/errors.rs
@@ -390,6 +390,17 @@ pub struct TaitForwardCompat {
     pub item_span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(hir_analysis_tait_forward_compat2)]
+#[note]
+pub struct TaitForwardCompat2 {
+    #[primary_span]
+    pub span: Span,
+    #[note(hir_analysis_opaque)]
+    pub opaque_type_span: Span,
+    pub opaque_type: String,
+}
+
 pub struct MissingTypeParams {
     pub span: Span,
     pub def_span: Span,