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.

[oli-obk]

This code is 100% reasonable and should not be ICEing or erroring. I think the reason it is ICEing is the raw pointer causing the allocation to not get interned as immutable, even though it must be.

Basically the inner & does not get its own promoted:

pub const FOO:  &'static *const i32 = &(&0 as _);

gives the following MIR

// WARNING: This output format is intended for human consumers only
// and is subject to change without notice. Knock yourself out.
const  FOO: &*const i32 = {
    let mut _0: &*const i32;             // return place in scope 0 at src/lib.rs:1:17: 1:36
    let _1: &*const i32;                 // in scope 0 at src/lib.rs:1:39: 1:49

    bb0: {
        StorageLive(_1);                 // bb0[0]: scope 0 at src/lib.rs:1:39: 1:49
        _1 = &(promoted[0]: *const i32); // bb0[1]: scope 0 at src/lib.rs:1:39: 1:49
        _0 = _1;                         // bb0[2]: scope 0 at src/lib.rs:1:39: 1:49
        StorageDead(_1);                 // bb0[3]: scope 0 at src/lib.rs:1:48: 1:49
        return;                          // bb0[4]: scope 0 at src/lib.rs:1:1: 1:50
    }
}

promoted[0] in  FOO: *const i32 = {
    let mut _0: *const i32;              // return place in scope 0 at src/lib.rs:1:39: 1:49
    let mut _1: *const i32;              // in scope 0 at src/lib.rs:1:40: 1:49
    let mut _2: &i32;                    // in scope 0 at src/lib.rs:1:41: 1:43
    let mut _3: i32;                     // in scope 0 at src/lib.rs:1:42: 1:43

    bb0: {
        _3 = const 0i32;                 // bb0[0]: scope 0 at src/lib.rs:1:42: 1:43
                                         // ty::Const
                                         // + ty: i32
                                         // + val: Value(Scalar(0x00000000))
                                         // mir::Constant
                                         // + span: src/lib.rs:1:42: 1:43
                                         // + literal: Const { ty: i32, val: Value(Scalar(0x00000000)) }
        _2 = &_3;                        // bb0[1]: scope 0 at src/lib.rs:1:41: 1:43
        _1 = &raw const (*_2);           // bb0[2]: scope 0 at src/lib.rs:1:41: 1:43
        _0 = move _1;                    // bb0[3]: scope 0 at src/lib.rs:1:39: 1:49
        return;                          // bb0[4]: scope 0 at src/lib.rs:1:39: 1:49
    }
}

cc @ecstatic-morse promoteds still contain the inner promoteds as just locals without StorageDead markers. It's probably fine, just wanted to let you know

[Centril]

( cc also @RalfJung & @eddyb )

[oli-obk]

I'll prep a fix PR.

[RalfJung]

This code is 100% reasonable

Well I beg to disagree, this creates non-Sync shared allocations in constants, which is very much not reasonable (Cc #49206). But that's a separate issue.

[oli-obk]

well... 100% reasonable from our current rule set, which does a const body based analysis instead of relying on our type system. I didn't say the rule set is 100% reasonable.

[SimonSapin]

The ICE persists if we eliminate sharing of non-Sync values:

pub struct SyncPtr(*const u32);
unsafe impl Sync for SyncPtr {}
pub const FOO:  &'static SyncPtr = &SyncPtr(&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:3:36
  |
3 | pub const FOO:  &'static SyncPtr = &SyncPtr(&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

[RalfJung]

Yeah, that's what I meant by "separate issue". The ICE has nothing to do with Sync. But seeing such code in the wild just makes me wonder if we'll ever be able to fix the Sync thing. Sorry for the noise.