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ICE in const fn evaluation - Disagreement between legacy and dataflow-based const validators when a const fn takes &mut self #66167

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@fschutt

Description

@fschutt

Error

error[E0493]: destructors cannot be evaluated at compile-time
   --> cargo/azul-core/../../azul-core/dom.rs:895:69
    |
895 |     pub const fn set_node_type(&mut self, node_type: NodeType<T>) { self.node_type = node_type; }
    |                                                                     ^^^^^^^^^^^^^^ constant functions cannot evaluate destructors

error[E0019]: constant function contains unimplemented expression type
   --> cargo/azul-core/../../azul-core/dom.rs:895:69
    |
895 |     pub const fn set_node_type(&mut self, node_type: NodeType<T>) { self.node_type = node_type; }
    |                                                                     ^^^^^^^^^^^^^^

[ERROR rustc_mir::transform::qualify_consts] old validator: [(cargo/azul-core/../../azul-core/dom.rs:895:69: 895:83, "LiveDrop"), (cargo/azul-core/../../azul-core/dom.rs:895:69: 895:83, "MutDeref")]
[ERROR rustc_mir::transform::qualify_consts] new validator: [(cargo/azul-core/../../azul-core/dom.rs:895:69: 895:83, "MutDeref"), (cargo/azul-core/../../azul-core/dom.rs:895:69: 895:83, "LiveDrop")]
error: internal compiler error: src/librustc_mir/transform/qualify_consts.rs:1986: Disagreement between legacy and dataflow-based const validators.
    After filing an issue, use `-Zsuppress-const-validation-back-compat-ice` to compile your code.
   --> cargo/azul-core/../../azul-core/dom.rs:895:5
    |
895 |     pub const fn set_node_type(&mut self, node_type: NodeType<T>) { self.node_type = node_type; }
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Backtrace

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:890:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:77
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:61
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1028
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1412
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:65
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:50
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:188
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:205
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:468
  12: std::panicking::begin_panic
  13: rustc_errors::HandlerInner::span_bug
  14: rustc_errors::Handler::span_bug
  15: rustc::util::bug::opt_span_bug_fmt::{{closure}}
  16: rustc::ty::context::tls::with_opt::{{closure}}
  17: rustc::ty::context::tls::with_context_opt
  18: rustc::ty::context::tls::with_opt
  19: rustc::util::bug::opt_span_bug_fmt
  20: rustc::util::bug::span_bug_fmt
  21: rustc_mir::transform::qualify_consts::Checker::check_const
  22: <rustc_mir::transform::qualify_consts::QualifyAndPromoteConstants as rustc_mir::transform::MirPass>::run_pass
  23: rustc_mir::transform::run_passes
  24: rustc_mir::transform::mir_validated
  25: rustc::ty::query::__query_compute::mir_validated
  26: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_validated>::compute
  27: rustc::dep_graph::graph::DepGraph::with_task_impl
  28: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  29: rustc_mir::borrow_check::mir_borrowck
  30: rustc::ty::query::__query_compute::mir_borrowck
  31: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_borrowck>::compute
  32: rustc::dep_graph::graph::DepGraph::with_task_impl
  33: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  34: rustc::ty::<impl rustc::ty::context::TyCtxt>::par_body_owners
  35: rustc_interface::passes::analysis
  36: rustc::ty::query::__query_compute::analysis
  37: rustc::dep_graph::graph::DepGraph::with_task_impl
  38: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  39: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  40: rustc_interface::passes::create_global_ctxt::{{closure}}
  41: rustc_interface::passes::BoxedGlobalCtxt::enter
  42: rustc_interface::interface::run_compiler_in_existing_thread_pool
  43: std::thread::local::LocalKey<T>::with
  44: scoped_tls::ScopedKey<T>::set
  45: syntax::with_globals
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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.40.0-nightly (1423bec54 2019-11-05) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type rlib --crate-type cdylib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [mir_validated] processing `dom::NodeData::<T>::set_node_type`
#1 [mir_borrowck] processing `dom::NodeData::<T>::set_node_type`
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 3 previous errors

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

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.40.0-nightly (1423bec54 2019-11-05) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type rlib --crate-type cdylib

note: some of the compiler flags provided by cargo are hidden

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0019, E0493.
For more information about an error, try `rustc --explain E0019`.
error: could not compile `azul-core`.
warning: build failed, waiting for other jobs to finish...
error: build failed

Code

https://github.com/maps4print/azul/tree/rust-const-ice

Commit fschutt/azul@b38fb1b

Changed here: fschutt/azul@b38fb1b#diff-ce9aead5dc9a44d99879f251fc571013R897

Code: https://github.com/maps4print/azul/blob/b38fb1bc3476ee47d313e891642f7673092e080b/azul-core/dom.rs#L895

Reproducing the bug

rustup update
git clone https://github.com/maps4print/azul
git checkout rust-const-ice
cargo check

Description

I was trying to turn a couple of functions that take a &mut self into const fns - which should technically work:

struct Dom { a: usize }

impl Dom {
    const fn do_thing(&mut self, a: usize) { self.a = a; }
}

As far as I can see, only the ordering of liveness-checking is wrong, the old liveness checker reports ["LiveDrop", "MutDeref"] while the new liveness checker reports ["MutDeref", "LiveDrop"]. Should be an easy (?) bug to fix (famous last words).

Rust version:

rustc 1.40.0-nightly (1423bec54 2019-11-05)
binary: rustc
commit-hash: 1423bec54cf2db283b614e527cfd602b481485d1
commit-date: 2019-11-05
host: x86_64-unknown-linux-gnu
release: 1.40.0-nightly
LLVM version: 9.0

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A-const-evalArea: Constant evaluation, covers all const contexts (static, const fn, ...)C-bugCategory: This is a bug.I-ICEIssue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.

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