Regression in Servo: "internal compiler error: mutable allocation in constant"

[SimonSapin]

https://community-tc.services.mozilla.com/tasks/bdytAYuMTdWK1kbybZ6FWA/runs/0/logs/https%3A%2F%2Fcommunity-tc.services.mozilla.com%2Fapi%2Fqueue%2Fv1%2Ftask%2FbdytAYuMTdWK1kbybZ6FWA%2Fruns%2F0%2Fartifacts%2Fpublic%2Flogs%2Flive.log#L1943

   --> /repo/target/debug/build/script-a3d7e9e184a37789/out/Bindings/ANGLEInstancedArraysBinding.rs:852:61
    |
852 |   const sMethods_specs: &'static [&'static[JSFunctionSpec]] = &[
    |  _____________________________________________________________^
853 | | &[
854 | |     JSFunctionSpec {
855 | |         name: b"drawArraysInstancedANGLE\0" as *const u8 as *const libc::c_char,
...   |
882 | |
883 | | ];
    | |_^

(Many other occurrences)

error: internal compiler error: mutable allocation in constant
    --> /repo/target/debug/build/script-a3d7e9e184a37789/out/Bindings/XRWebGLLayerBinding.rs:1088:64
     |
1088 |   const sAttributes_specs: &'static [&'static[JSPropertySpec]] = &[
     |  ________________________________________________________________^
1089 | | &[
1090 | |     JSPropertySpec {
1091 | |         name: b"context\0" as *const u8 as *const libc::c_char,
...    |
1203 | |
1204 | | ];
     | |_^
thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:349:17
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:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1057
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1426
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:195
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:215
  10: rustc_driver::report_ice
  11: <alloc::boxed::Box<F> as core::ops::function::Fn<A>>::call
             at /rustc/a9c1c04e986dbf610be8cbe6a8107f90b4db61ce/src/liballoc/boxed.rs:1029
  12: proc_macro::bridge::client::<impl proc_macro::bridge::Bridge>::enter::{{closure}}::{{closure}}
             at /rustc/a9c1c04e986dbf610be8cbe6a8107f90b4db61ce/src/libproc_macro/bridge/client.rs:305
  13: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:476
  14: std::panicking::begin_panic
  15: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
  16: core::ptr::real_drop_in_place
  17: core::ptr::real_drop_in_place
  18: core::ptr::real_drop_in_place
  19: rustc_interface::interface::run_compiler_in_existing_thread_pool
  20: std::thread::local::LocalKey<T>::with
  21: scoped_tls::ScopedKey<T>::set
  22: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
error: internal compiler error: unexpected panic
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 1.42.0-nightly (a9c1c04e9 2019-12-24) running on x86_64-unknown-linux-gnu
note: compiler flags: -C debuginfo=2 -C link-args=-fuse-ld=gold --crate-type rlib
note: some of the compiler flags provided by cargo are hidden
query stack during panic:
end of query stack
error: could not compile `script`.

The crate uses #![feature(const_fn)].

Example of one of the affected const items:

const sMethods_specs: &'static [&'static[JSFunctionSpec]] = &[
&[
    JSFunctionSpec {
        name: b"getViewport\0" as *const u8 as *const libc::c_char,
        call: JSNativeWrapper { 
            // `generic_method` is a free function
            op: Some(generic_method),
            // getViewport_methodinfo is a const item
            info: &getViewport_methodinfo as *const _ as *const JSJitInfo
        },
        nargs: 1,
        flags: (JSPROP_ENUMERATE) as u16,
        selfHostedName: 0 as *const libc::c_char
    },
    JSFunctionSpec {
        name: 0 as *const libc::c_char,
        call: JSNativeWrapper { 
            op: None, 
            info: 0 as *const JSJitInfo
        },
        nargs: 0,
        flags: 0,
        selfHostedName: 0 as *const libc::c_char
    }
]
];

Definitions of types involved:

pub struct JSFunctionSpec {
    pub name: *const i8,
    pub call: JSNativeWrapper,
    pub nargs: u16,
    pub flags: u16,
    pub selfHostedName: *const i8,
}
pub struct JSNativeWrapper {
    pub op: Option<unsafe extern "C" fn(*mut JSContext, u32, *mut Value) -> bool>,
    pub info: *const JSJitInfo,
}
pub struct JSJitInfo {
    pub call: *const c_void,
    pub protoID: u16,
    pub depth: u16,
    pub _bitfield_1: u32,
}

Regression range:

git log --merges --oneline 9ae6ced...a9c1c04
a9c1c04 Auto merge of #67241 - mark-i-m:simplify-borrow_check-3, r=matthewjasper
f6dca76 Auto merge of #66221 - ohadravid:doc-constants, r=Dylan-DPC
84d8f9d Auto merge of #67579 - RalfJung:miri, r=RalfJung
6253754 Auto merge of #67575 - Centril:rollup-feikoir, r=Centril
a76d67f Rollup merge of #67572 - aidanhs:aphs-choco-direct-cdn, r=Mark-Simulacrum
20d5df9 Rollup merge of #67569 - Mark-Simulacrum:opt-char-encode, r=oli-obk
75b27ef Rollup merge of #67561 - euclio:remove-description, r=jonas-schievink
a75968a Rollup merge of #67551 - ldm0:E0627, r=Dylan-DPC
d130e8d Rollup merge of #67547 - GuillaumeGomez:cleanup-err-codes, r=Dylan-DPC
07effe1 Rollup merge of #67543 - JohnTitor:regression-tests, r=Centril
1ac8fc7 Rollup merge of #67337 - oli-obk:no_mut_static_ref_from_const, r=RalfJung
a4cd03d Auto merge of #66296 - Centril:bindings_after_at-init, r=pnkfelix

The error message appears to be from #67337, cc @oli-obk

If this (modulo ICE vs proper diagnostic) is not a rustc bug, I don’t understand what change we’re supposed to make to this code. I don’t see any heap allocation or mutability happening.

[Centril]

If possible, providing a self-contained example that fits into the playground would make minimization and finding the cause of the ICE easier.

[oli-obk]

Repro extracted from example: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=d437e043d28ec84f02dd5467c36fc645

minifying now

[SimonSapin]

Reduced to:

pub const FOO:  &'static *const u32 = &(&BAR as _);
pub const BAR: u32 = 0;

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error: internal compiler error: mutable allocation in constant
 --> src/lib.rs:1:39
  |
1 | pub const FOO:  &'static *const u32 = &(&BAR as _);
  |                                       ^^^^^^^^^^^^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:349:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

error: internal compiler error: unexpected panic

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 1.42.0-nightly (a9c1c04e9 2019-12-24) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type lib

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

error: could not compile `playground`.

To learn more, run the command again with --verbose.