diff --git a/compiler/rustc_ty_utils/src/layout.rs b/compiler/rustc_ty_utils/src/layout.rs
index 62f3667ad7f4f..bebc0707e26d9 100644
--- a/compiler/rustc_ty_utils/src/layout.rs
+++ b/compiler/rustc_ty_utils/src/layout.rs
@@ -764,14 +764,20 @@ fn layout_of_uncached<'tcx>(
         }
 
         ty::Alias(..) => {
-            // NOTE(eddyb) `layout_of` query should've normalized these away,
-            // if that was possible, so there's no reason to try again here.
-            let err = if ty.has_param() {
+            // In case we're still in a generic context, aliases might be rigid. E.g.
+            // if we've got a `T: Trait` where-bound, `T::Assoc` cannot be normalized
+            // in the current context.
+            //
+            // For some builtin traits, generic aliases can be rigid even in an empty environment,
+            // e.g. `<T as Pointee>::Metadata`.
+            //
+            // Due to trivial bounds, this can even be the case if the alias does not reference
+            // any generic parameters, e.g. a `for<'a> u32: Trait<'a>` where-bound means that
+            // `<u32 as Trait<'static>>::Assoc` is rigid.
+            let err = if ty.has_param() || !cx.typing_env.param_env.caller_bounds().is_empty() {
                 LayoutError::TooGeneric(ty)
             } else {
-                // This is only reachable with unsatisfiable predicates. For example, if we have
-                // `u8: Iterator`, then we can't compute the layout of `<u8 as Iterator>::Item`.
-                LayoutError::Unknown(ty)
+                unreachable!("invalid rigid alias in layout_of after normalization: {ty:?}");
             };
             return Err(error(cx, err));
         }
diff --git a/tests/rustdoc-html/type-layout.rs b/tests/rustdoc-html/type-layout.rs
index 482b8b597dd30..84d2aa8060079 100644
--- a/tests/rustdoc-html/type-layout.rs
+++ b/tests/rustdoc-html/type-layout.rs
@@ -64,6 +64,12 @@ pub type GenericTypeAlias = (Generic<(u32, ())>, Generic<u32>);
 //@ hasraw type_layout/type.Edges.html 'Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.'
 pub type Edges<'a, E> = std::borrow::Cow<'a, [E]>;
 
+pub trait Project { type Assoc; }
+// We can't compute layout as the alias stays rigid. A `LayoutError::TooGeneric` is returned.
+//@ hasraw type_layout/struct.RigidAlias.html 'Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.'
+//@ !hasraw - 'Size: '
+pub struct RigidAlias(<() as Project>::Assoc) where for<'a> (): Project;
+
 //@ !hasraw type_layout/trait.MyTrait.html 'Size: '
 pub trait MyTrait {}
 
@@ -92,9 +98,3 @@ pub enum Uninhabited {}
 //@ hasraw type_layout/struct.Uninhabited2.html 'Size: '
 //@ hasraw - '8 bytes (<a href="{{channel}}/reference/glossary.html#uninhabited">uninhabited</a>)'
 pub struct Uninhabited2(std::convert::Infallible, u64);
-
-pub trait Project { type Assoc; }
-// We can't compute layout. A `LayoutError::Unknown` is returned.
-//@ hasraw type_layout/struct.Unknown.html 'Unable to compute type layout.'
-//@ !hasraw - 'Size: '
-pub struct Unknown(<() as Project>::Assoc) where for<'a> (): Project;
diff --git a/tests/ui/layout/rigid-alias-no-params.rs b/tests/ui/layout/rigid-alias-no-params.rs
new file mode 100644
index 0000000000000..bc2622eb7ca64
--- /dev/null
+++ b/tests/ui/layout/rigid-alias-no-params.rs
@@ -0,0 +1,30 @@
+//@ compile-flags: -O -Cdebug-assertions=on
+//@ build-pass
+
+// A regression test for #151791. Computing the layout of
+// `<AsOwned as ArchiveWith<'a>>::Archived` fails as the alias
+// is still rigid as the where-bound in scope shadows the impl.
+//
+// This previously caused an incorrect error during MIR optimizations.
+
+struct ArchivedString;
+
+pub trait ArchiveWith<'a> {
+    type Archived;
+}
+
+struct AsOwned;
+impl ArchiveWith<'_> for AsOwned {
+    type Archived = ArchivedString;
+}
+
+fn foo<'a>()
+where
+    AsOwned: ArchiveWith<'a>,
+{
+    let _ = unsafe { &*std::ptr::dangling::<<AsOwned as ArchiveWith<'a>>::Archived>() };
+}
+
+fn main() {
+    foo();
+}
diff --git a/tests/ui/where-clauses/projection-bound-unsatisfied-item-bounds-mit-opt-ice.rs b/tests/ui/where-clauses/projection-bound-unsatisfied-item-bounds-mit-opt-ice.rs
index 80eec709eecbc..9448c8a2f9112 100644
--- a/tests/ui/where-clauses/projection-bound-unsatisfied-item-bounds-mit-opt-ice.rs
+++ b/tests/ui/where-clauses/projection-bound-unsatisfied-item-bounds-mit-opt-ice.rs
@@ -2,7 +2,7 @@
 //@ build-pass
 
 // A regression test for #149081. The environment of `size` and `align`
-// currently means that the item bound of`T::Assoc` doesn't hold. This can
+// currently means that the item bound of `T::Assoc` doesn't hold. This can
 // result in normalization failures and ICE during MIR optimizations.
 //
 // This will no longer be an issue once #149283 is implemented.