Compiler crash during doc test with generic_const_exprs

[ssande7]

Trying to use generic_const_exprs (#76560) to enforce bounds on a const generic is causing a compiler crash for doc tests if #![feature(generic_const_exprs)] is not included in the doc test snippet. This does not occur in all cases - eg. the TryFrom example below causes a crash, but the From example does not. Seems to be related to the Result type.

Code

cargo new --lib const_generics_bug

lib.rs:

#![allow(incomplete_features)]
#![feature(generic_const_exprs)]

pub struct If<const B: bool>;
pub trait True {}
impl True for If<true> {}

#[derive(Debug)]
pub struct FooError;

pub struct Foo<const N: usize> where If<{N>0}>: True {
    pub bar: [i32; N],
}

impl<const N: usize> TryFrom<[i32;N]> for Foo<N> 
where If<{N>0}>: True,
{
    type Error = FooError;

    /// # Examples
    /// ```
    // /// #![feature(generic_const_exprs)]   // <-- Uncommenting this line fixes the crash
    /// use const_generics_bug::{FooError, Foo};
    /// fn main() -> Result<(), FooError> {
    ///     let _baz = Foo::<4>::try_from([1,2,3,4])?;
    ///     Ok(())
    /// }
    /// ```
    fn try_from(value: [i32;N]) -> Result<Self, Self::Error> {
        Ok(Self {
            bar: value,
        })
    }
}

// This test passes
#[test]
fn test() -> Result<(), FooError> {
    let _baz = Foo::<4>::try_from([1,2,3,4])?;
    Ok(())
}


// Uncommenting the below (and commenting out test() and impl TryFrom) works fine

// impl<const N: usize> From<[i32;N]> for Foo<N> 
// where If<{N>0}>: True,
// {
//     /// # Examples
//     /// ```
//     /// use const_generics_bug::Foo;
//     /// fn main() {
//     ///     let _baz = Foo::<4>::from([1,2,3,4]);
//     /// }
//     /// ```
//     fn from(value: [i32;N]) -> Self {
//         Self {
//             bar: value,
//         }
//     }
// }

Meta

rustc --version --verbose:

rustc 1.61.0-nightly (c274e4969 2022-03-05)
binary: rustc
commit-hash: c274e4969f058b1c644243181ece9f829efa7594
commit-date: 2022-03-05
host: x86_64-unknown-linux-gnu
release: 1.61.0-nightly
LLVM version: 14.0.0

Error output

---- src/lib.rs - Foo<N>::try_from (line 22) stdout ----
error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:179:90: Failed to normalize std::result::Result<const_generics_bug::Foo<N>, <const_generics_bug::Foo<N> as std::convert::TryFrom<[i32; N]>>::Error>, maybe try to call `try_normalize_erasing_regions` instead

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1236:9

Backtrace

   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: std::panic::panic_any::<rustc_errors::ExplicitBug>
   2: <rustc_errors::HandlerInner>::bug
   3: <rustc_errors::Handler>::bug
   4: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, ()>
   5: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>
   6: rustc_middle::util::bug::bug_fmt
   7: <rustc_middle::ty::Ty as rustc_middle::ty::fold::TypeFoldable>::try_fold_with::<rustc_middle::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder>
   8: <&rustc_middle::ty::list::List<rustc_middle::ty::Ty> as rustc_middle::ty::fold::TypeFoldable>::try_fold_with::<rustc_middle::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder>
   9: <rustc_middle::ty::instance::Instance>::fn_sig_for_fn_abi
  10: rustc_middle::ty::layout::fn_abi_of_instance
  11: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<rustc_middle::ty::ParamEnvAnd<(rustc_middle::ty::instance::Instance, &rustc_middle::ty::list::List<rustc_middle::ty::Ty>)>, core::result::Result<&rustc_target::abi::call::FnAbi<rustc_middle::ty::Ty>, rustc_middle::ty::layout::FnAbiError>>>
  12: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::fn_abi_of_instance
  13: <rustc_codegen_llvm::context::CodegenCx as rustc_codegen_ssa::traits::declare::PreDefineMethods>::predefine_fn
  14: rustc_codegen_llvm::base::compile_codegen_unit::module_codegen
  15: <rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::with_task::<rustc_middle::ty::context::TyCtxt, rustc_span::symbol::Symbol, rustc_codegen_ssa::ModuleCodegen<rustc_codegen_llvm::ModuleLlvm>>
  16: rustc_codegen_llvm::base::compile_codegen_unit
  17: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  18: <rustc_session::session::Session>::time::<alloc::boxed::Box<dyn core::any::Any>, rustc_interface::passes::start_codegen::{closure#0}>
  19: <rustc_interface::passes::QueryContext>::enter::<<rustc_interface::queries::Queries>::ongoing_codegen::{closure#0}::{closure#0}, core::result::Result<alloc::boxed::Box<dyn core::any::Any>, rustc_errors::ErrorGuaranteed>>
  20: <rustc_interface::queries::Queries>::ongoing_codegen
  21: <rustc_interface::interface::Compiler>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_errors::ErrorGuaranteed>>
  22: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_interface::interface::create_compiler_and_run<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>::{closure#1}>
  23: rustc_interface::interface::create_compiler_and_run::<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>
  24: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.61.0-nightly (c274e4969 2022-03-05) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -C codegen-units=1 -C embed-bitcode=no

query stack during panic:
#0 [fn_abi_of_instance] computing call ABI of `<const_generics_bug::Foo<4_usize> as core::convert::TryFrom<[i32; 4]>>::try_from`
end of query stack

[BoxyUwU]

same as #79018 probably

[ssande7]

Sounds like that's probably the case. Feel free to close this if appropriate