External specification declared on a trait implementation did not resolve to a concrete type

[nishanthkarthik]

I'm trying to test extern_spec for a TryFrom impl. parse_u32 is supposed to prove a cast from a slice to an array. The extern spec currently throws an error internal error: entered unreachable code: External specification declared on a trait implementation did not resolve to a concrete type

extern crate prusti_contracts;
use prusti_contracts::*;

#[extern_spec]
impl<T, const N: usize> std::convert::TryFrom<&[T]> for [T; N]
where T: Copy,
{
    #[inline]
    // <Specifications removed here>
    fn try_from(slice: &[T]) -> Result<[T; N], std::array::TryFromSliceError>;
}

#[requires(a.len() == 4)]
fn parse_u32(a: &[u8]) -> u32 {
    let bytes: Result<[u8; 4], _> = std::convert::TryFrom::try_from(a);
    match bytes {
        Ok(x) => u32::from_le_bytes(x),
        Err(_) => {
            prusti_assert!(false);
            0
        },
    }
}

My goal is to specify try_from such that the assert-false is not triggered. Currently, prusti fails with an ICE

stacktrace

``` thread 'rustc' panicked at prusti-interface/src/specs/external.rs:155:17: internal error: entered unreachable code: External specification declared on a trait implementation did not resolve to a concrete type stack backtrace: 0: 0x792da1362efc - std::backtrace_rs::backtrace::libunwind::trace::h652247f520429b18 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5 1: 0x792da1362efc - std::backtrace_rs::backtrace::trace_unsynchronized::h20ba733a518048ae at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5 2: 0x792da1362efc - std::sys_common::backtrace::_print_fmt::ha9cb2d71bba5eb16 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/sys_common/backtrace.rs:67:5 3: 0x792da1362efc - ::fmt::h527f3c0db321cf86 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/sys_common/backtrace.rs:44:22 4: 0x792da13c915c - core::fmt::rt::Argument::fmt::hc5a8cd063e05c609 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/core/src/fmt/rt.rs:138:9 5: 0x792da13c915c - core::fmt::write::h818c732e4e373aa5 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/core/src/fmt/mod.rs:1094:21 6: 0x792da1355b1e - std::io::Write::write_fmt::h9fe6c7e095e96a32 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/io/mod.rs:1714:15 7: 0x792da1362ce4 - std::sys_common::backtrace::_print::h4b50c3b478ae2a37 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/sys_common/backtrace.rs:47:5 8: 0x792da1362ce4 - std::sys_common::backtrace::print::hf2c7643f5414af94 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/sys_common/backtrace.rs:34:9 9: 0x792da1365dda - std::panicking::panic_hook_with_disk_dump::{{closure}}::h62ff4ef3ec32306d at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/panicking.rs:280:22 10: 0x792da1365a98 - std::panicking::panic_hook_with_disk_dump::hcd58ca7cb67f8702 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/panicking.rs:307:9 11: 0x792da45690b9 - >::call_once::{shim:vtable#0} 12: 0x792da1366693 - as core::ops::function::Fn>::call::h2b79b1e8b8bd4402 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/alloc/src/boxed.rs:2021:9 13: 0x792da1366693 - std::panicking::rust_panic_with_hook::ha2c93276d1208654 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/panicking.rs:757:13 14: 0x792da13663c6 - std::panicking::begin_panic_handler::{{closure}}::hb78d7a76234f0397 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/panicking.rs:623:13 15: 0x792da1363426 - std::sys_common::backtrace::__rust_end_short_backtrace::h96e02fd19b415b36 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/sys_common/backtrace.rs:170:18 16: 0x792da1366152 - rust_begin_unwind at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/panicking.rs:619:5 17: 0x792da13c5505 - core::panicking::panic_fmt::h62ee289ca1991433 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/core/src/panicking.rs:72:14 18: 0x60b11d35a8f2 - prusti_interface::specs::external::ExternSpecResolver::add_extern_fn::hbd4b0f6255a4e2fc 19: 0x60b11d36369e - ::visit_fn::h1c00a4816b11d7be 20: 0x60b11d32443c - rustc_hir::intravisit::walk_item::h0bee9e35ac9a576c 21: 0x60b11d38f594 - prusti_interface::specs::SpecCollector::collect_specs::h4dc91f6926130477 22: 0x60b11ca2419d - ::after_analysis::h7d8f5df69404394f 23: 0x792da3a789d6 - ::enter::, rustc_span[82af0e0afe7e8690]::ErrorGuaranteed>> 24: 0x792da3a71c04 - std[843b1ee06368cddb]::sys_common::backtrace::__rust_begin_short_backtrace::, rustc_driver_impl[8b2874cda94e7cd4]::run_compiler::{closure#1}>::{closure#0}, core[b0493a3e457862f3]::result::Result<(), rustc_span[82af0e0afe7e8690]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[b0493a3e457862f3]::result::Result<(), rustc_span[82af0e0afe7e8690]::ErrorGuaranteed>> 25: 0x792da3a7135e - <::spawn_unchecked_, rustc_driver_impl[8b2874cda94e7cd4]::run_compiler::{closure#1}>::{closure#0}, core[b0493a3e457862f3]::result::Result<(), rustc_span[82af0e0afe7e8690]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[b0493a3e457862f3]::result::Result<(), rustc_span[82af0e0afe7e8690]::ErrorGuaranteed>>::{closure#1} as core[b0493a3e457862f3]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0} 26: 0x792da1371075 - as core::ops::function::FnOnce>::call_once::h7bff668e3fcc7cec at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/alloc/src/boxed.rs:2007:9 27: 0x792da1371075 - as core::ops::function::FnOnce>::call_once::h6cf1c11e2e0c58d1 at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/alloc/src/boxed.rs:2007:9 28: 0x792da1371075 - std::sys::unix::thread::Thread::new::thread_start::hfa7d5fcc9039f5da at /rustc/ca2b74f1ae5075d62e223c0a91574a1fc3f51c7c/library/std/src/sys/unix/thread.rs:108:17 29: 0x792d9fdbbded - 30: 0x792d9fe3f0dc - 31: 0x0 - rustc version: 1.74.0-nightly (ca2b74f1a 2023-09-14) platform: x86_64-unknown-linux-gnu query stack during panic: end of query stack ```