Avoid ICEing miri on layout query cycles

compiler/rustc_middle/messages.ftl

@@ -32,6 +32,9 @@ middle_values_too_big =
 middle_cannot_be_normalized =
     unable to determine layout for `{$ty}` because `{$failure_ty}` cannot be normalized
 
+middle_cycle =
+    a cycle occurred during layout computation
+
 middle_strict_coherence_needs_negative_coherence =
     to use `strict_coherence` on this trait, the `with_negative_coherence` feature must be enabled
     .label = due to this attribute

compiler/rustc_middle/src/ty/layout.rs

@@ -210,6 +210,7 @@ pub enum LayoutError<'tcx> {
     Unknown(Ty<'tcx>),
     SizeOverflow(Ty<'tcx>),
     NormalizationFailure(Ty<'tcx>, NormalizationError<'tcx>),
+    Cycle,
 }
 
 impl IntoDiagnostic<'_, !> for LayoutError<'_> {
@@ -230,6 +231,9 @@ impl IntoDiagnostic<'_, !> for LayoutError<'_> {
                 diag.set_arg("failure_ty", e.get_type_for_failure());
                 diag.set_primary_message(fluent::middle_cannot_be_normalized);
             }
+            LayoutError::Cycle => {
+                diag.set_primary_message(fluent::middle_cycle);
+            }
         }
         diag
     }
@@ -250,6 +254,7 @@ impl<'tcx> fmt::Display for LayoutError<'tcx> {
                 t,
                 e.get_type_for_failure()
             ),
+            LayoutError::Cycle => write!(f, "a cycle occurred during layout computation"),
         }
     }
 }

compiler/rustc_middle/src/values.rs

@@ -106,6 +106,12 @@ impl<'tcx> Value<TyCtxt<'tcx>, DepKind> for ty::EarlyBinder<ty::Binder<'_, ty::F
     }
 }
 
+impl<'tcx, T> Value<TyCtxt<'tcx>, DepKind> for Result<T, ty::layout::LayoutError<'_>> {
+    fn from_cycle_error(_tcx: TyCtxt<'tcx>, _cycle: &[QueryInfo<DepKind>]) -> Self {
+        Err(ty::layout::LayoutError::Cycle)
+    }
+}
+
 // item_and_field_ids should form a cycle where each field contains the
 // type in the next element in the list
 pub fn recursive_type_error(

src/librustdoc/html/templates/type_layout.html

@@ -49,5 +49,10 @@
             <strong>Note:</strong> Encountered an error during type layout; {#+ #}
             the type failed to be normalized. {# #}
         </p> {# #}
+        {% when Err(LayoutError::Cycle) %}
+        <p> {# #}
+            <strong>Note:</strong> Encountered an error during type layout; {#+ #}
+            the type's layout depended on the type's layout itself. {# #}
+        </p> {# #}
         {% endmatch %}
 </div> {# #}

src/tools/miri/tests/fail/layout_cycle.rs

@@ -0,0 +1,28 @@
+//@error-pattern: a cycle occurred during layout computation
+//~^ ERROR: cycle detected when computing layout of
+
+use std::mem;
+
+pub struct S<T: Tr> {
+    pub f: <T as Tr>::I,
+}
+
+pub trait Tr {
+    type I: Tr;
+}
+
+impl<T: Tr> Tr for S<T> {
+    type I = S<S<T>>;
+}
+
+impl Tr for () {
+    type I = ();
+}
+
+fn foo<T: Tr>() -> usize {
+    mem::size_of::<S<T>>()
+}
+
+fn main() {
+    println!("{}", foo::<S<()>>());
+}

src/tools/miri/tests/fail/layout_cycle.stderr

@@ -0,0 +1,28 @@
+error[E0391]: cycle detected when computing layout of `S<S<()>>`
+   |
+   = note: ...which requires computing layout of `<S<()> as Tr>::I`...
+   = note: ...which again requires computing layout of `S<S<()>>`, completing the cycle
+
+error: post-monomorphization error: a cycle occurred during layout computation
+  --> RUSTLIB/core/src/mem/mod.rs:LL:CC
+   |
+LL |     intrinsics::size_of::<T>()
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ a cycle occurred during layout computation
+   |
+   = note: inside `std::mem::size_of::<S<S<()>>>` at RUSTLIB/core/src/mem/mod.rs:LL:CC
+note: inside `foo::<S<()>>`
+  --> $DIR/layout_cycle.rs:LL:CC
+   |
+LL |     mem::size_of::<S<T>>()
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+note: inside `main`
+  --> $DIR/layout_cycle.rs:LL:CC
+   |
+LL |     println!("{}", foo::<S<()>>());
+   |                    ^^^^^^^^^^^^^^
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0391`.