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ICE: lazy type alias: IOOB compiler/rustc_hir_analysis/src/variance/constraints.rs #114468

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matthiaskrgr opened this issue Aug 4, 2023 · 3 comments · Fixed by #114566
Closed

ICE: lazy type alias: IOOB compiler/rustc_hir_analysis/src/variance/constraints.rs #114468

matthiaskrgr opened this issue Aug 4, 2023 · 3 comments · Fixed by #114566
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@fmease
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C-bug Category: This is a bug. F-lazy_type_alias `#![feature(lazy_type_alias)]` I-ICE Issue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️ requires-nightly This issue requires a nightly compiler in some way. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@matthiaskrgr
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matthiaskrgr commented Aug 4, 2023 •
edited by rustbot
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Code

original code from tests/ui/issues/issue-20797.rs

// build-pass

// Regression test for #20797.

use std::default::Default;
use std::io;
use std::fs;
use std::path::PathBuf;

pub trait PathExtensions {
    fn is_dir(&self) -> bool { false }
}

impl PathExtensions for PathBuf {}

/// A strategy for acquiring more subpaths to walk.
pub trait Strategy {
    type P: PathExtensions;
    /// Gets additional subpaths from a given path.
    fn get_more(&self, item: &Self::P) -> io::Result<Vec<Self::P>>;
    /// Determine whether a path should be walked further.
    /// This is run against each item from `get_more()`.
    fn prune(&self, p: &Self::P) -> bool;
}

/// The basic fully-recursive strategy. Nothing is pruned.
#[derive(Copy, Clone, Default)]
pub struct Recursive;

impl Strategy for Recursive {
    type P = PathBuf;
    fn get_more(&self, p: &PathBuf) -> io::Result<Vec<PathBuf>> {
        Ok(fs::read_dir(p).unwrap().map(|s| s.unwrap().path()).collect())
    }

    fn prune(&self, _: &PathBuf) -> bool { false }
}

/// A directory walker of `P` using strategy `S`.
pub struct Subpaths<S: Strategy> {
    stack: Vec<S::P>,
    strategy: S,
}

impl<S: Strategy> Subpaths<S> {
    /// Creates a directory walker with a root path and strategy.
    pub fn new(p: &S::P, strategy: S) -> io::Result<Subpaths<S>> {
        let stack = strategy.get_more(p)?;
        Ok(Subpaths { stack: stack, strategy: strategy })
    }
}

impl<S: Default + Strategy> Subpaths<S> {
    /// Creates a directory walker with a root path and a default strategy.
    pub fn walk(p: &S::P) -> io::Result<Subpaths<S>> {
        Subpaths::new(p, Default::default())
    }
}

impl<S: Default + Strategy> Default for Subpaths<S> {
    fn default() -> Subpaths<S> {
        Subpaths { stack: Vec::new(), strategy: Default::default() }
    }
}

impl<S: Strategy> Iterator for Subpaths<S> {
    type Item = S::P;
    fn next (&mut self) -> Option<S::P> {
        let mut opt_path = self.stack.pop();
        while opt_path.is_some() && self.strategy.prune(opt_path.as_ref().unwrap()) {
            opt_path = self.stack.pop();
        }
        match opt_path {
            Some(path) => {
                if path.is_dir() {
                    let result = self.strategy.get_more(&path);
                    match result {
                        Ok(dirs) => { self.stack.extend(dirs); },
                        Err(..) => { }
                    }
                }
                Some(path)
            }
            None => None,
        }
    }
}

fn _foo() {
    let _walker: Subpaths<Recursive> = Subpaths::walk(&PathBuf::from("/home")).unwrap();
}

fn main() {}

reduced

#![feature(lazy_type_alias)]

use std::io;

pub trait Strategy {}

pub struct Subpaths<S: Strategy> {}

impl<S: Strategy> Subpaths<S> {
    pub fn new() -> io::Result<Subpaths<S>> {}
}

Meta

rustc --version --verbose:

rustc 1.73.0-nightly (474709a9a 2023-08-03)
binary: rustc
commit-hash: 474709a9a2a74a8bcf0055fadb335d0ca0d2d939
commit-date: 2023-08-03
host: x86_64-unknown-linux-gnu
release: 1.73.0-nightly
LLVM version: 16.0.5

Error output

<output>
Backtrace 

warning: the feature `lazy_type_alias` is incomplete and may not be safe to use and/or cause compiler crashes
 --> treereduce.out:1:12
  |
1 | #![feature(lazy_type_alias)]
  |            ^^^^^^^^^^^^^^^
  |
  = note: see issue #112792 <https://github.com/rust-lang/rust/issues/112792> for more information
  = note: `#[warn(incomplete_features)]` on by default

error[E0601]: `main` function not found in crate `treereduce`
  --> treereduce.out:11:2
   |
11 | }
   |  ^ consider adding a `main` function to `treereduce.out`

thread 'rustc' panicked at compiler/rustc_hir_analysis/src/variance/constraints.rs:362:36:
index out of bounds: the len is 0 but the index is 0
stack backtrace:
   0:     0x7f2b33b6313c - std::backtrace_rs::backtrace::libunwind::trace::h607c5cae6680b0fb
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7f2b33b6313c - std::backtrace_rs::backtrace::trace_unsynchronized::hc1cbea2e412d60ef
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f2b33b6313c - std::sys_common::backtrace::_print_fmt::h62a9831c7ff163f5
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x7f2b33b6313c - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hf49ca0eb9dc1b6fd
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f2b33bc983c - core::fmt::rt::Argument::fmt::h9cc0ab3f56c44ea1
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/core/src/fmt/rt.rs:138:9
   5:     0x7f2b33bc983c - core::fmt::write::h87aa5f2149ab03a1
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/core/src/fmt/mod.rs:1094:21
   6:     0x7f2b33b556de - std::io::Write::write_fmt::h04dc89e115afce4f
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/io/mod.rs:1714:15
   7:     0x7f2b33b62f25 - std::sys_common::backtrace::_print::h4e2feb31096e94de
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f2b33b62f25 - std::sys_common::backtrace::print::h2c16c4ace2baa3ab
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f2b33b6607a - std::panicking::panic_hook_with_disk_dump::{{closure}}::h18ad7c40aef108e0
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/panicking.rs:278:22
  10:     0x7f2b33b65d13 - std::panicking::panic_hook_with_disk_dump::hce08e9b5439b012b
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/panicking.rs:312:9
  11:     0x7f2b36e59f99 - rustc_driver_impl[2f92a0ccc46c059]::install_ice_hook::{closure#0}
  12:     0x7f2b33b66920 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h02014ddf1cce8192
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/alloc/src/boxed.rs:2021:9
  13:     0x7f2b33b66920 - std::panicking::rust_panic_with_hook::h07367ca72cc1b034
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/panicking.rs:733:13
  14:     0x7f2b33b666a7 - std::panicking::begin_panic_handler::{{closure}}::h1091911fd2e9526e
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/panicking.rs:621:13
  15:     0x7f2b33b63676 - std::sys_common::backtrace::__rust_end_short_backtrace::h6770f526fbcecfe3
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/sys_common/backtrace.rs:170:18
  16:     0x7f2b33b663f2 - rust_begin_unwind
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/panicking.rs:617:5
  17:     0x7f2b33bc5bf3 - core::panicking::panic_fmt::h2ea3095a9e99667b
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/core/src/panicking.rs:67:14
  18:     0x7f2b33bc5d52 - core::panicking::panic_bounds_check::hf798c1571563a745
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/core/src/panicking.rs:162:5
  19:     0x7f2b36fbe3c3 - <rustc_hir_analysis[1f87ebcc916a8bb0]::variance::constraints::ConstraintContext>::add_constraints_from_args
  20:     0x7f2b35734c40 - <rustc_hir_analysis[1f87ebcc916a8bb0]::variance::constraints::ConstraintContext>::build_constraints_for_item
  21:     0x7f2b357337d9 - rustc_hir_analysis[1f87ebcc916a8bb0]::variance::crate_variances
  22:     0x7f2b36509694 - rustc_query_impl[49f881e28274e957]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[49f881e28274e957]::query_impl::crate_variances::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1d4c141020d5ff1d]::query::erase::Erased<[u8; 8usize]>>
  23:     0x7f2b36509679 - <rustc_query_impl[49f881e28274e957]::query_impl::crate_variances::dynamic_query::{closure#2} as core[71fbaba24b011fa4]::ops::function::FnOnce<(rustc_middle[1d4c141020d5ff1d]::ty::context::TyCtxt, ())>>::call_once
  24:     0x7f2b362ae62c - rustc_query_system[cfe4b6087db7ea89]::query::plumbing::try_execute_query::<rustc_query_impl[49f881e28274e957]::DynamicConfig<rustc_query_system[cfe4b6087db7ea89]::query::caches::SingleCache<rustc_middle[1d4c141020d5ff1d]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[49f881e28274e957]::plumbing::QueryCtxt, false>
  25:     0x7f2b367bbc45 - rustc_query_impl[49f881e28274e957]::query_impl::crate_variances::get_query_non_incr::__rust_end_short_backtrace
  26:     0x7f2b35a7ef4c - rustc_hir_analysis[1f87ebcc916a8bb0]::variance::variances_of
  27:     0x7f2b34e23893 - rustc_query_impl[49f881e28274e957]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[49f881e28274e957]::query_impl::variances_of::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1d4c141020d5ff1d]::query::erase::Erased<[u8; 16usize]>>
  28:     0x7f2b34e23855 - <rustc_query_impl[49f881e28274e957]::query_impl::variances_of::dynamic_query::{closure#2} as core[71fbaba24b011fa4]::ops::function::FnOnce<(rustc_middle[1d4c141020d5ff1d]::ty::context::TyCtxt, rustc_span[87ac8800f9b2bd45]::def_id::DefId)>>::call_once
  29:     0x7f2b34ea5964 - rustc_query_system[cfe4b6087db7ea89]::query::plumbing::try_execute_query::<rustc_query_impl[49f881e28274e957]::DynamicConfig<rustc_query_system[cfe4b6087db7ea89]::query::caches::DefaultCache<rustc_span[87ac8800f9b2bd45]::def_id::DefId, rustc_middle[1d4c141020d5ff1d]::query::erase::Erased<[u8; 16usize]>>, false, false, false>, rustc_query_impl[49f881e28274e957]::plumbing::QueryCtxt, false>
  30:     0x7f2b367bbfd3 - rustc_query_impl[49f881e28274e957]::query_impl::variances_of::get_query_non_incr::__rust_end_short_backtrace
  31:     0x7f2b35771bdf - rustc_middle[1d4c141020d5ff1d]::query::plumbing::query_get_at::<rustc_query_system[cfe4b6087db7ea89]::query::caches::DefaultCache<rustc_span[87ac8800f9b2bd45]::def_id::DefId, rustc_middle[1d4c141020d5ff1d]::query::erase::Erased<[u8; 16usize]>>>
  32:     0x7f2b357925ec - rustc_hir_analysis[1f87ebcc916a8bb0]::check::wfcheck::check_variances_for_type_defn
  33:     0x7f2b3576fd0f - rustc_hir_analysis[1f87ebcc916a8bb0]::check::wfcheck::check_well_formed
  34:     0x7f2b34e11a7c - rustc_query_impl[49f881e28274e957]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[49f881e28274e957]::query_impl::check_well_formed::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1d4c141020d5ff1d]::query::erase::Erased<[u8; 0usize]>>
  35:     0x7f2b35799986 - rustc_query_system[cfe4b6087db7ea89]::query::plumbing::try_execute_query::<rustc_query_impl[49f881e28274e957]::DynamicConfig<rustc_query_system[cfe4b6087db7ea89]::query::caches::VecCache<rustc_hir[4b4119f5a2aa97da]::hir_id::OwnerId, rustc_middle[1d4c141020d5ff1d]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[49f881e28274e957]::plumbing::QueryCtxt, false>
  36:     0x7f2b357996a2 - rustc_query_impl[49f881e28274e957]::query_impl::check_well_formed::get_query_non_incr::__rust_end_short_backtrace
  37:     0x7f2b3608a5bc - rustc_data_structures[d4f5b56d4da9ff99]::sync::par_for_each_in::<&[rustc_hir[4b4119f5a2aa97da]::hir::ImplItemId], <rustc_middle[1d4c141020d5ff1d]::hir::ModuleItems>::par_impl_items<rustc_hir_analysis[1f87ebcc916a8bb0]::check::wfcheck::check_mod_type_wf::{closure#1}>::{closure#0}>
  38:     0x7f2b3608a39a - rustc_hir_analysis[1f87ebcc916a8bb0]::check::wfcheck::check_mod_type_wf
  39:     0x7f2b35ab967c - rustc_query_impl[49f881e28274e957]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[49f881e28274e957]::query_impl::check_mod_type_wf::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1d4c141020d5ff1d]::query::erase::Erased<[u8; 0usize]>>
  40:     0x7f2b35ab965e - <rustc_query_impl[49f881e28274e957]::query_impl::check_mod_type_wf::dynamic_query::{closure#2} as core[71fbaba24b011fa4]::ops::function::FnOnce<(rustc_middle[1d4c141020d5ff1d]::ty::context::TyCtxt, rustc_span[87ac8800f9b2bd45]::def_id::LocalDefId)>>::call_once
  41:     0x7f2b356d75df - rustc_query_system[cfe4b6087db7ea89]::query::plumbing::try_execute_query::<rustc_query_impl[49f881e28274e957]::DynamicConfig<rustc_query_system[cfe4b6087db7ea89]::query::caches::VecCache<rustc_span[87ac8800f9b2bd45]::def_id::LocalDefId, rustc_middle[1d4c141020d5ff1d]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[49f881e28274e957]::plumbing::QueryCtxt, false>
  42:     0x7f2b367c0eb2 - rustc_query_impl[49f881e28274e957]::query_impl::check_mod_type_wf::get_query_non_incr::__rust_end_short_backtrace
  43:     0x7f2b362c2ed7 - rustc_data_structures[d4f5b56d4da9ff99]::sync::par_for_each_in::<&[rustc_hir[4b4119f5a2aa97da]::hir_id::OwnerId], <rustc_middle[1d4c141020d5ff1d]::hir::map::Map>::par_for_each_module<rustc_hir_analysis[1f87ebcc916a8bb0]::check_crate::{closure#5}::{closure#0}::{closure#0}>::{closure#0}>
  44:     0x7f2b362c228e - rustc_hir_analysis[1f87ebcc916a8bb0]::check_crate
  45:     0x7f2b362bcb7a - rustc_interface[6d12d522b9b133dc]::passes::analysis
  46:     0x7f2b36331c9a - rustc_query_impl[49f881e28274e957]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[49f881e28274e957]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1d4c141020d5ff1d]::query::erase::Erased<[u8; 1usize]>>
  47:     0x7f2b36331c89 - <rustc_query_impl[49f881e28274e957]::query_impl::analysis::dynamic_query::{closure#2} as core[71fbaba24b011fa4]::ops::function::FnOnce<(rustc_middle[1d4c141020d5ff1d]::ty::context::TyCtxt, ())>>::call_once
  48:     0x7f2b364cf5d8 - rustc_query_system[cfe4b6087db7ea89]::query::plumbing::try_execute_query::<rustc_query_impl[49f881e28274e957]::DynamicConfig<rustc_query_system[cfe4b6087db7ea89]::query::caches::SingleCache<rustc_middle[1d4c141020d5ff1d]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[49f881e28274e957]::plumbing::QueryCtxt, false>
  49:     0x7f2b364cf367 - rustc_query_impl[49f881e28274e957]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  50:     0x7f2b360ca1c5 - <rustc_middle[1d4c141020d5ff1d]::ty::context::GlobalCtxt>::enter::<rustc_driver_impl[2f92a0ccc46c059]::run_compiler::{closure#1}::{closure#2}::{closure#4}, core[71fbaba24b011fa4]::result::Result<(), rustc_span[87ac8800f9b2bd45]::ErrorGuaranteed>>
  51:     0x7f2b360c97f2 - <rustc_interface[6d12d522b9b133dc]::interface::Compiler>::enter::<rustc_driver_impl[2f92a0ccc46c059]::run_compiler::{closure#1}::{closure#2}, core[71fbaba24b011fa4]::result::Result<core[71fbaba24b011fa4]::option::Option<rustc_interface[6d12d522b9b133dc]::queries::Linker>, rustc_span[87ac8800f9b2bd45]::ErrorGuaranteed>>
  52:     0x7f2b360c28c8 - std[b67388f384878aed]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[6d12d522b9b133dc]::util::run_in_thread_pool_with_globals<rustc_interface[6d12d522b9b133dc]::interface::run_compiler<core[71fbaba24b011fa4]::result::Result<(), rustc_span[87ac8800f9b2bd45]::ErrorGuaranteed>, rustc_driver_impl[2f92a0ccc46c059]::run_compiler::{closure#1}>::{closure#0}, core[71fbaba24b011fa4]::result::Result<(), rustc_span[87ac8800f9b2bd45]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[71fbaba24b011fa4]::result::Result<(), rustc_span[87ac8800f9b2bd45]::ErrorGuaranteed>>
  53:     0x7f2b360c204e - <<std[b67388f384878aed]::thread::Builder>::spawn_unchecked_<rustc_interface[6d12d522b9b133dc]::util::run_in_thread_pool_with_globals<rustc_interface[6d12d522b9b133dc]::interface::run_compiler<core[71fbaba24b011fa4]::result::Result<(), rustc_span[87ac8800f9b2bd45]::ErrorGuaranteed>, rustc_driver_impl[2f92a0ccc46c059]::run_compiler::{closure#1}>::{closure#0}, core[71fbaba24b011fa4]::result::Result<(), rustc_span[87ac8800f9b2bd45]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[71fbaba24b011fa4]::result::Result<(), rustc_span[87ac8800f9b2bd45]::ErrorGuaranteed>>::{closure#1} as core[71fbaba24b011fa4]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  54:     0x7f2b33b710c5 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h50b018fa15c55b61
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/alloc/src/boxed.rs:2007:9
  55:     0x7f2b33b710c5 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h30bf8c9edaaa8962
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/alloc/src/boxed.rs:2007:9
  56:     0x7f2b33b710c5 - std::sys::unix::thread::Thread::new::thread_start::hd4703b35fe8f9aee
                               at /rustc/4f7bb9890c0402cd145556ac1929d13d7524959e/library/std/src/sys/unix/thread.rs:108:17
  57:     0x7f2b3390244b - <unknown>
  58:     0x7f2b33985e40 - <unknown>
  59:                0x0 - <unknown>

error: 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: please attach the file at `/tmp/im/rustc-ice-2023-08-04T15:16:50.896216586Z-99271.txt` to your bug report

query stack during panic:
#0 [crate_variances] computing the variances for items in this crate
#1 [variances_of] computing the variances of `Subpaths`
#2 [check_well_formed] checking that `Subpaths` is well-formed
#3 [check_mod_type_wf] checking that types are well-formed in top-level module
#4 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0601`.

@matthiaskrgr matthiaskrgr added I-ICE Issue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️ T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. C-bug Category: This is a bug. requires-nightly This issue requires a nightly compiler in some way. F-lazy_type_alias `#![feature(lazy_type_alias)]` labels Aug 4, 2023
@rustbot rustbot added the needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. label Aug 4, 2023
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#114253 cc @fmease

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fmease commented Aug 4, 2023 •
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Argh, I kinda knew that external crates were gonna haunt me (external crate: eager type aliases, local crate: lazy type aliases). This problem won't go away either once edition checks are involved.

@rustbot rustbot removed the needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. label Aug 4, 2023
@fmease fmease mentioned this issue Aug 6, 2023
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@bors bors closed this as completed in 3cd0a10 Aug 8, 2023
flip1995 pushed a commit to flip1995/rust that referenced this issue Aug 11, 2023
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Rollup merge of rust-lang#114566 - fmease:type-alias-laziness-is-crat…
5f86009 
…e-specific, r=oli-obk

Store the laziness of type aliases in their `DefKind`

Previously, we would treat paths referring to type aliases as *lazy* type aliases if the current crate had lazy type aliases enabled independently of whether the crate which the alias was defined in had the feature enabled or not.

With this PR, the laziness of a type alias depends on the crate it is defined in. This generally makes more sense to me especially if / once lazy type aliases become the default in a new edition and we need to think about *edition interoperability*:

Consider the hypothetical case where the dependency crate has an older edition (and thus eager type aliases), it exports a type alias with bounds & a where-clause (which are void but technically valid), the dependent crate has the latest edition (and thus lazy type aliases) and it uses that type alias. Arguably, the bounds should *not* be checked since at any time, the dependency crate should be allowed to change the bounds at will with a *non*-major version bump & without negatively affecting downstream crates.

As for the reverse case (dependency: lazy type aliases, dependent: eager type aliases), I guess it rules out anything from slight confusion to mild annoyance from upstream crate authors that would be caused by the compiler ignoring the bounds of their type aliases in downstream crates with older editions.

---

This fixes rust-lang#114468 since before, my assumption that the type alias associated with a given weak projection was lazy (and therefore had its variances computed) did not necessarily hold in cross-crate scenarios (which [I kinda had a hunch about](rust-lang#114253 (comment))) as outlined above. Now it does hold.

`@rustbot` label F-lazy_type_alias
r? `@oli-obk`
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Make the laziness of type aliases dependent on the defining crate fmease/rust
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