Rollup merge of rust-lang#131420 - compiler-errors:post-mono-layout-c…

…ycle, r=wesleywiser

Dont ICE when encountering post-mono layout cycle error

It's possible to encounter post-mono layout cycle errors in `fn_abi_of_instance`. Don't ICE in those cases.

This was originally discovered in an async fn, but that's not the only way to encounter such an error (which the other test I added should demonstrate).

Error messsages suck, but this fix is purely about suppressing the ICE.

Fixes rust-lang#131409

compiler/rustc_codegen_llvm/src/context.rs

@@ -1187,10 +1187,11 @@ impl<'tcx> FnAbiOfHelpers<'tcx> for CodegenCx<'_, 'tcx> {
         span: Span,
         fn_abi_request: FnAbiRequest<'tcx>,
     ) -> ! {
-        if let FnAbiError::Layout(LayoutError::SizeOverflow(_)) = err {
-            self.tcx.dcx().emit_fatal(Spanned { span, node: err })
-        } else {
-            match fn_abi_request {
+        match err {
+            FnAbiError::Layout(LayoutError::SizeOverflow(_) | LayoutError::Cycle(_)) => {
+                self.tcx.dcx().emit_fatal(Spanned { span, node: err });
+            }
+            _ => match fn_abi_request {
                 FnAbiRequest::OfFnPtr { sig, extra_args } => {
                     span_bug!(span, "`fn_abi_of_fn_ptr({sig}, {extra_args:?})` failed: {err:?}",);
                 }
@@ -1200,7 +1201,7 @@ impl<'tcx> FnAbiOfHelpers<'tcx> for CodegenCx<'_, 'tcx> {
                         "`fn_abi_of_instance({instance}, {extra_args:?})` failed: {err:?}",
                     );
                 }
-            }
+            },
         }
     }
 }

tests/ui/layout/post-mono-layout-cycle-2.rs

@@ -0,0 +1,59 @@
+//@ build-fail
+//@ edition: 2021
+
+#![feature(async_closure, noop_waker)]
+
+use std::future::Future;
+use std::pin::pin;
+use std::task::*;
+
+pub fn block_on<T>(fut: impl Future<Output = T>) -> T {
+    let mut fut = pin!(fut);
+    // Poll loop, just to test the future...
+    let ctx = &mut Context::from_waker(Waker::noop());
+
+    loop {
+        match fut.as_mut().poll(ctx) {
+            Poll::Pending => {}
+            Poll::Ready(t) => break t,
+        }
+    }
+}
+
+trait Blah {
+    async fn iter<T>(&mut self, iterator: T)
+    where
+        T: IntoIterator<Item = ()>;
+}
+
+impl Blah for () {
+    async fn iter<T>(&mut self, iterator: T)
+    //~^ ERROR recursion in an async fn requires boxing
+    where
+        T: IntoIterator<Item = ()>,
+    {
+        Blah::iter(self, iterator).await
+    }
+}
+
+struct Wrap<T: Blah> {
+    t: T,
+}
+
+impl<T: Blah> Wrap<T>
+where
+    T: Blah,
+{
+    async fn ice(&mut self) {
+        //~^ ERROR a cycle occurred during layout computation
+        let arr: [(); 0] = [];
+        self.t.iter(arr.into_iter()).await;
+    }
+}
+
+fn main() {
+    block_on(async {
+        let mut t = Wrap { t: () };
+        t.ice();
+    })
+}

tests/ui/layout/post-mono-layout-cycle-2.stderr

@@ -0,0 +1,23 @@
+error[E0733]: recursion in an async fn requires boxing
+  --> $DIR/post-mono-layout-cycle-2.rs:30:5
+   |
+LL | /     async fn iter<T>(&mut self, iterator: T)
+LL | |
+LL | |     where
+LL | |         T: IntoIterator<Item = ()>,
+   | |___________________________________^
+LL |       {
+LL |           Blah::iter(self, iterator).await
+   |           -------------------------------- recursive call here
+   |
+   = note: a recursive `async fn` call must introduce indirection such as `Box::pin` to avoid an infinitely sized future
+
+error: a cycle occurred during layout computation
+  --> $DIR/post-mono-layout-cycle-2.rs:47:5
+   |
+LL |     async fn ice(&mut self) {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0733`.

tests/ui/layout/post-mono-layout-cycle.rs

@@ -0,0 +1,25 @@
+//@ build-fail
+//~^ cycle detected when computing layout of `Wrapper<()>`
+
+trait Trait {
+    type Assoc;
+}
+
+impl Trait for () {
+    type Assoc = Wrapper<()>;
+}
+
+struct Wrapper<T: Trait> {
+    _x: <T as Trait>::Assoc,
+}
+
+fn abi<T: Trait>(_: Option<Wrapper<T>>) {}
+//~^ ERROR a cycle occurred during layout computation
+
+fn indirect<T: Trait>() {
+    abi::<T>(None);
+}
+
+fn main() {
+    indirect::<()>();
+}

tests/ui/layout/post-mono-layout-cycle.stderr

@@ -0,0 +1,16 @@
+error[E0391]: cycle detected when computing layout of `Wrapper<()>`
+   |
+   = note: ...which requires computing layout of `<() as Trait>::Assoc`...
+   = note: ...which again requires computing layout of `Wrapper<()>`, completing the cycle
+   = note: cycle used when computing layout of `core::option::Option<Wrapper<()>>`
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: a cycle occurred during layout computation
+  --> $DIR/post-mono-layout-cycle.rs:16:1
+   |
+LL | fn abi<T: Trait>(_: Option<Wrapper<T>>) {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0391`.