Panic when calling `Ty::layout` with escaping bound vars

[dkcumming]

When attempting to collect all the Layouts for Tys in a program I ran into a panic if there are escaping bound vars, which occurs if there is a dummy binder. Here is a (relatively) minimised example:

#![feature(rustc_private)]
extern crate rustc_driver;
extern crate rustc_interface;
extern crate rustc_middle;
extern crate rustc_smir;
extern crate stable_mir;

use rustc_driver::Compilation;
use rustc_interface::interface::Compiler;
use rustc_middle::ty::TyCtxt;
use rustc_smir::rustc_internal;
use stable_mir::mir::{
    mono::{Instance, MonoItem},
    visit::MirVisitor,
};
use std::{collections::HashMap, io::Write, iter::Iterator, vec::Vec};

fn main() {
    let path = "str-to-string.rs";
    generate_input(path).unwrap();
    let args = vec!["panicking-program".to_string(), path.to_string()];

    let mut callbacks = StableMirCallbacks {};
    let _ = rustc_driver::RunCompiler::new(&args, &mut callbacks).run();
}

struct StableMirCallbacks {}

impl rustc_driver::Callbacks for StableMirCallbacks {
    fn after_analysis(&mut self, _compiler: &Compiler, tcx: TyCtxt<'_>) -> Compilation {
        let _ = rustc_internal::run(tcx, || emit_mono_item_layouts(tcx));

        Compilation::Continue
    }
}

// Removed any emitting since it will panic beforehand
fn emit_mono_item_layouts(tcx: TyCtxt<'_>) {
    let items = mono_collect(tcx);

    // Filtering out the MonoItem "copy_nonoverlappings" that causes the layout to panic
    let erroring_instance: Instance = *items
        .iter()
        .find_map(|item| match item {
            MonoItem::Fn(instance) if instance.name().contains("copy_nonoverlapping") => {
                Some(instance)
            }
            _ => None,
        })
        .unwrap();

    // Call the layout collector
    let mut visited_tys = HashMap::new();
    LayoutCollector {
        visited_tys: &mut visited_tys,
    }
    .visit_body(&erroring_instance.body().unwrap())
}

fn mono_collect(tcx: TyCtxt<'_>) -> Vec<MonoItem> {
    let units = tcx.collect_and_partition_mono_items(()).1;
    units
        .iter()
        .flat_map(|unit| {
            unit.items_in_deterministic_order(tcx)
                .iter()
                .map(|(internal_item, _)| rustc_internal::stable(internal_item))
                .collect::<Vec<_>>()
        })
        .collect()
}

//-Layout Collector-//////////////////////////////
type TyAndLayoutMap = HashMap<u64, Result<stable_mir::abi::Layout, stable_mir::Error>>;

struct LayoutCollector<'local> {
    visited_tys: &'local mut TyAndLayoutMap,
}

fn collect_vec_tys(collector: &mut LayoutCollector, tys: Vec<stable_mir::ty::Ty>) {
    tys.into_iter().for_each(|ty| collect_ty(collector, ty));
}

fn collect_ty(val_collector: &mut LayoutCollector, val: stable_mir::ty::Ty) {
    use stable_mir::ty::{RigidTy::*, TyKind::RigidTy};

    let layout_or_err = val.layout(); // panics

    if val_collector
        .visited_tys
        .insert(hash(val), layout_or_err)
        .is_some()
    {
        if let kind @ RigidTy(FnDef(..)) = val.kind() {
            collect_vec_tys(
                val_collector,
                kind.fn_sig().unwrap().value.inputs_and_output,
            );
        }
    }
}

impl MirVisitor for LayoutCollector<'_> {
    fn visit_ty(&mut self, ty: &stable_mir::ty::Ty, _location: stable_mir::mir::visit::Location) {
        collect_ty(self, *ty);
    }
}

//-Utilities-////////////////////////////////////
fn hash<T: std::hash::Hash>(obj: T) -> u64 {
    use std::hash::Hasher;
    let mut hasher = std::hash::DefaultHasher::new();
    obj.hash(&mut hasher);
    hasher.finish()
}

// The failing code
fn generate_input(path: &str) -> std::io::Result<()> {
    let mut file = std::fs::File::create(path)?;
    write!(
        file,
        r#"
        fn main() {{
            "abcd".to_string();
        }}
        "#
    )?;
    Ok(())
}

With output:

thread 'rustc' panicked at compiler/rustc_middle/src/ty/predicate.rs:533:9:
`<[core::fmt::rt::Argument<'_>] as std::marker::Sized>` has escaping bound vars, so it cannot be wrapped in a dummy binder.
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

And back trace:

thread 'rustc' panicked at compiler/rustc_middle/src/ty/predicate.rs:533:9:
`<[core::fmt::rt::Argument<'_>] as std::marker::Sized>` has escaping bound vars, so it cannot be wrapped in a dummy binder.
stack backtrace:
   0: rust_begin_unwind
   1: core::panicking::panic_fmt
   2: rustc_ty_utils::common_traits::is_item_raw
      [... omitted 1 frame ...]
   3: <rustc_middle::ty::Ty>::is_sized
   4: rustc_ty_utils::layout::layout_of_uncached
   5: rustc_ty_utils::layout::layout_of
      [... omitted 1 frame ...]
   6: <core::iter::adapters::GenericShunt<core::iter::adapters::by_ref_sized::ByRefSized<core::iter::adapters::map::Map<core::slice::iter::Iter<rustc_middle::ty::VariantDef>, rustc_ty_utils::layout::layout_of_uncached::{closure#13}>>, core::result::Result<core::convert::Infallible, &rustc_middle::ty::layout::LayoutError>> as core::iter::traits::iterator::Iterator>::next
   7: rustc_ty_utils::layout::layout_of_uncached
   8: rustc_ty_utils::layout::layout_of
      [... omitted 1 frame ...]
   9: rustc_middle::query::plumbing::query_get_at::<rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::PseudoCanonicalInput<rustc_middle::ty::Ty>, rustc_middle::query::erase::Erased<[u8; 16]>>>
  10: <rustc_smir::rustc_smir::context::TablesWrapper as stable_mir::compiler_interface::Context>::ty_layout
  11: <stable_mir::ty::Ty>::layout
  12: ice::collect_ty
             at ./src/main.rs:87:25
  13: ice::collect_vec_tys::{{closure}}
             at ./src/main.rs:81:35
  14: core::iter::traits::iterator::Iterator::for_each::call::{{closure}}
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/iterator.rs:797:29
  15: <alloc::vec::into_iter::IntoIter<T,A> as core::iter::traits::iterator::Iterator>::fold
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/vec/into_iter.rs:318:25
  16: core::iter::traits::iterator::Iterator::for_each
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/iterator.rs:800:9
  17: ice::collect_vec_tys
             at ./src/main.rs:81:5
  18: ice::collect_ty
             at ./src/main.rs:95:13
  19: <ice::LayoutCollector as stable_mir::mir::visit::MirVisitor>::visit_ty
             at ./src/main.rs:105:9
  20: stable_mir::mir::visit::MirVisitor::super_mir_const
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/mir/visit.rs:381:9
  21: stable_mir::mir::visit::MirVisitor::visit_mir_const
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/mir/visit.rs:116:9
  22: stable_mir::mir::visit::MirVisitor::super_const_operand
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/mir/visit.rs:376:9
  23: stable_mir::mir::visit::MirVisitor::visit_const_operand
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/mir/visit.rs:112:9
  24: stable_mir::mir::visit::MirVisitor::super_operand
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/mir/visit.rs:359:17
  25: stable_mir::mir::visit::MirVisitor::visit_operand
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/mir/visit.rs:99:9
  26: stable_mir::mir::visit::MirVisitor::super_terminator
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/mir/visit.rs:247:17
  27: stable_mir::mir::visit::MirVisitor::visit_terminator
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/mir/visit.rs:68:9
  28: stable_mir::mir::visit::MirVisitor::super_basic_block
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/mir/visit.rs:172:9
  29: stable_mir::mir::visit::MirVisitor::visit_basic_block
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/mir/visit.rs:48:9
  30: stable_mir::mir::visit::MirVisitor::super_body
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/mir/visit.rs:146:13
  31: stable_mir::mir::visit::MirVisitor::visit_body
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/mir/visit.rs:44:9
  32: ice::emit_mono_item_layouts
             at ./src/main.rs:54:5
  33: <ice::StableMirCallbacks as rustc_driver_impl::Callbacks>::after_analysis::{{closure}}
             at ./src/main.rs:31:45
  34: rustc_smir::rustc_internal::init::{{closure}}
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/rustc_smir/src/rustc_internal/mod.rs:192:33
  35: scoped_tls::ScopedKey<T>::set
             at /rust/deps/scoped-tls-1.0.1/src/lib.rs:137:9
  36: rustc_smir::rustc_internal::init
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/rustc_smir/src/rustc_internal/mod.rs:192:5
  37: rustc_smir::rustc_internal::run::{{closure}}
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/rustc_smir/src/rustc_internal/mod.rs:223:53
  38: stable_mir::compiler_interface::run::{{closure}}
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/compiler_interface.rs:265:40
  39: scoped_tls::ScopedKey<T>::set
             at /rust/deps/scoped-tls-1.0.1/src/lib.rs:137:9
  40: stable_mir::compiler_interface::run
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/stable_mir/src/compiler_interface.rs:265:9
  41: rustc_smir::rustc_internal::run
             at /home/daniel/.rustup/toolchains/nightly-2024-11-29-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/compiler/rustc_smir/src/rustc_internal/mod.rs:223:5
  42: <ice::StableMirCallbacks as rustc_driver_impl::Callbacks>::after_analysis
             at ./src/main.rs:31:17
  43: rustc_interface::interface::run_compiler::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[celinval]

Thank you for all the details. I'll try to take a look at it next week.

[celinval]

Hi @dkcumming, I finally got the chance to debug this. The issue here is that you are trying to get the layout of a type that hasn't been monomorphized.

In your call to collect_vec_tys, here:

            collect_vec_tys(
                val_collector,
                kind.fn_sig().unwrap().value.inputs_and_output,
            );

You are getting the kind.fn_sig() which is a binder, and skipping it by accessing the value variable directly. That will ignore any instantiation arguments. Instead, you should resolve the instance and get the instance function ABI from where you can extract the type and layout of the inputs and outputs. Something like:

        if let kind @ RigidTy(FnDef(def, args)) = val.kind() {
            let instance = Instance::resolve(def, &args).unwrap();
            let fn_abi = instance.fn_abi().unwrap();
            // TODO: Implement get_input_and_output
            collect_vec_tys(val_collector, get_input_and_output(fn_abi));
        }

That said, StableMIR shouldn't be crashing on you, so the issue is still relevant.

@oli-obk Is there a more direct way that we can expose a monomorphic function signature?

[dkcumming]

Thank you for the feedback. That is very helpful to us