ICE error was constructed but not emitted

[matthiaskrgr]

cfg_if::cfg_if! {
    if #[cfg(windows)] {
    } else if #(&cpus) {
    } else [libc::CTL_HW, libc::HW_NCPU, 0, 0]
}

error: internal compiler error: the following error was constructed but not emitted

error: expected one of `!` or `[`, found `(`
 --> /tmp/im/D148C7688C5A01A6B9DD473ACF70135F85C41E99C693835C947E942C30ECD884.rs:3:16
  |
3 |     } else if #(&cpus) {
  |                ^ expected one of `!` or `[`

thread 'main' panicked at 'error was constructed but not emitted', compiler/rustc_errors/src/diagnostic_builder.rs:780:21
stack backtrace:
   0:     0x7f4fb0f65e8a - std::backtrace_rs::backtrace::libunwind::trace::hc07ec51854030801
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7f4fb0f65e8a - std::backtrace_rs::backtrace::trace_unsynchronized::hd5249f53d09c2c41
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f4fb0f65e8a - std::sys_common::backtrace::_print_fmt::h7a58bb30aee44196
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/sys_common/backtrace.rs:65:5
   3:     0x7f4fb0f65e8a - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h4f2b7c295e3f8019
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f4fb0fc9e6f - core::fmt::write::h10777037b979b8b8
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/core/src/fmt/mod.rs:1254:17
   5:     0x7f4fb0f58ac5 - std::io::Write::write_fmt::he1dc4aa4bd2563db
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/io/mod.rs:1698:15
   6:     0x7f4fb0f65c55 - std::sys_common::backtrace::_print::h163c7799d3150ce8
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/sys_common/backtrace.rs:47:5
   7:     0x7f4fb0f65c55 - std::sys_common::backtrace::print::h3cf13564ddfd5c0e
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/sys_common/backtrace.rs:34:9
   8:     0x7f4fb0f688fe - std::panicking::default_hook::{{closure}}::h999811fec35f64bf
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/panicking.rs:269:22
   9:     0x7f4fb0f686a5 - std::panicking::default_hook::he182d48ca482e8b2
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/panicking.rs:288:9
  10:     0x7f4fb0f68f6e - std::panicking::rust_panic_with_hook::h2bc066147eaf9845
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/panicking.rs:690:13
  11:     0x7f4fb0f68e22 - std::panicking::begin_panic_handler::{{closure}}::h1e4368c427832fe9
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/panicking.rs:579:13
  12:     0x7f4fb0f662f6 - std::sys_common::backtrace::__rust_end_short_backtrace::h5466480ef33b438e
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/sys_common/backtrace.rs:150:18
  13:     0x7f4fb0f68bc2 - rust_begin_unwind
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/panicking.rs:577:5
  14:     0x7f4fb0fc6273 - core::panicking::panic_fmt::ha28b6d803d12671b
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/core/src/panicking.rs:67:14
  15:     0x7f4fb2377875 - <rustc_errors[10b8f3037483a69]::diagnostic_builder::DiagnosticBuilderInner as core[a19e7c0c634483f1]::ops::drop::Drop>::drop
  16:     0x55b4b8a04b32 - rustfmt_nightly[e3445ded4feb3b46]::parse::macros::cfg_if::parse_cfg_if_inner
  17:     0x55b4b8ab2219 - <rustfmt_nightly[e3445ded4feb3b46]::modules::visitor::CfgIfVisitor as rustc_ast[b583ee7534cdfdf7]::visit::Visitor>::visit_mac_call
  18:     0x55b4b8a442f1 - rustc_ast[b583ee7534cdfdf7]::visit::walk_item::<rustfmt_nightly[e3445ded4feb3b46]::modules::visitor::CfgIfVisitor>
  19:     0x55b4b8ada58d - <rustfmt_nightly[e3445ded4feb3b46]::modules::ModResolver>::visit_cfg_if
  20:     0x55b4b8ada9c6 - <rustfmt_nightly[e3445ded4feb3b46]::modules::ModResolver>::visit_mod_from_ast
  21:     0x55b4b8ada015 - <rustfmt_nightly[e3445ded4feb3b46]::modules::ModResolver>::visit_crate
  22:     0x55b4b8998533 - rustfmt_nightly[e3445ded4feb3b46]::formatting::format_project::<rustfmt_nightly[e3445ded4feb3b46]::Session<std[6ff7f37a454f70cc]::io::stdio::Stdout>>
  23:     0x55b4b89959d0 - <scoped_tls[abd3470abb840dd8]::ScopedKey<rustc_span[e810f079a636b72c]::SessionGlobals>>::with::<<rustfmt_nightly[e3445ded4feb3b46]::Session<std[6ff7f37a454f70cc]::io::stdio::Stdout>>::format_input_inner::{closure#0}, core[a19e7c0c634483f1]::result::Result<rustfmt_nightly[e3445ded4feb3b46]::FormatReport, rustfmt_nightly[e3445ded4feb3b46]::ErrorKind>>
  24:     0x55b4b8992a7f - <scoped_tls[abd3470abb840dd8]::ScopedKey<rustc_span[e810f079a636b72c]::SessionGlobals>>::set::<rustc_span[e810f079a636b72c]::create_session_if_not_set_then<core[a19e7c0c634483f1]::result::Result<rustfmt_nightly[e3445ded4feb3b46]::FormatReport, rustfmt_nightly[e3445ded4feb3b46]::ErrorKind>, <rustfmt_nightly[e3445ded4feb3b46]::Session<std[6ff7f37a454f70cc]::io::stdio::Stdout>>::format_input_inner::{closure#0}>::{closure#0}, core[a19e7c0c634483f1]::result::Result<rustfmt_nightly[e3445ded4feb3b46]::FormatReport, rustfmt_nightly[e3445ded4feb3b46]::ErrorKind>>
  25:     0x55b4b89bb650 - <rustfmt_nightly[e3445ded4feb3b46]::Session<std[6ff7f37a454f70cc]::io::stdio::Stdout>>::format
  26:     0x55b4b89aab05 - rustfmt[fd29d8685e71f599]::format_and_emit_report::<std[6ff7f37a454f70cc]::io::stdio::Stdout>
  27:     0x55b4b89b9b89 - <rustfmt_nightly[e3445ded4feb3b46]::Session<std[6ff7f37a454f70cc]::io::stdio::Stdout>>::override_config::<rustfmt[fd29d8685e71f599]::format::{closure#0}, ()>
  28:     0x55b4b89a9865 - rustfmt[fd29d8685e71f599]::execute
  29:     0x55b4b89a7221 - rustfmt[fd29d8685e71f599]::main
  30:     0x55b4b89ba483 - std[6ff7f37a454f70cc]::sys_common::backtrace::__rust_begin_short_backtrace::<fn(), ()>
  31:     0x55b4b89b8cc9 - std[6ff7f37a454f70cc]::rt::lang_start::<()>::{closure#0}
  32:     0x7f4fb0f4969e - core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &F>::call_once::hd8ec26e1d0256733
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/core/src/ops/function.rs:287:13
  33:     0x7f4fb0f4969e - std::panicking::try::do_call::ha93a022fbffe13f1
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/panicking.rs:485:40
  34:     0x7f4fb0f4969e - std::panicking::try::hb1493baa50505ff7
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/panicking.rs:449:19
  35:     0x7f4fb0f4969e - std::panic::catch_unwind::h68d19a9efbc50ca2
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/panic.rs:140:14
  36:     0x7f4fb0f4969e - std::rt::lang_start_internal::{{closure}}::hfefbc424423bf0ab
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/rt.rs:148:48
  37:     0x7f4fb0f4969e - std::panicking::try::do_call::h33a3e76f78055d3b
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/panicking.rs:485:40
  38:     0x7f4fb0f4969e - std::panicking::try::hf16a19af84a23805
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/panicking.rs:449:19
  39:     0x7f4fb0f4969e - std::panic::catch_unwind::hfd39d2618b0c9350
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/panic.rs:140:14
  40:     0x7f4fb0f4969e - std::rt::lang_start_internal::h8eb39434a671d889
                               at /rustc/17c11672167827b0dd92c88ef69f24346d1286dd/library/std/src/rt.rs:148:20
  41:     0x55b4b89ab8a5 - main
  42:     0x7f4fb0bc3790 - <unknown>
  43:     0x7f4fb0bc384a - __libc_start_main
  44:     0x55b4b898ec59 - <unknown>
  45:                0x0 - <unknown>

[ytmimi]

@matthiaskrgr what version of rustfmt are you using?

I think I know why this is happening, but I'm unable to reproduce it with rustfmt 1.5.2-nightly (34f9ca28 2023-02-16).

[matthiaskrgr]

rustfmt 1.5.2-nightly (17c1167 2023-03-29)

[ytmimi]

@matthiaskrgr we haven't done a subtree sync in a little while so we're still using nightly-2023-01-24 to build rustfmt from source here. I assume you're building rustfmt from rust-lang/rust/src/tools/rustfmt?

[matthiaskrgr]

It's the regular vanilla nightly rustfmt that everyone can install via rustfmt :D

[calebcartwright]

@ytmimi I suspect that grabbing the version of rustfmt that shipped with the most recent nightly toolchain (perhaps going back a day or two) can be used to reproduce this.

Not intended to be a rationalization for ICE behavior, but, rustfmt has a fairly rudimentary handrolled parsing process for cfg_if calls (pre-expansion) to try to peak inside for imported modules so that it can go look for the associated files. That parsing process makes a few assumptions about the structure and validity of the args at the cfg_if call sites and there's a few known issues when those args are invalid (at least based on rustfmt's codified understanding of what the args should be).

The ICE behavior is relatively new, and most likely related to recent updates to rustc's parsing process. We can and should prevent the ICE, but unless else if #(&cpus) is valid and utilized, then doing so is going to be rather low priority for us

[ytmimi]

@matthiaskrgr @calebcartwright thank you both for clarifying! I've installed rustfmt 1.5.2-nightly (17c1167 2023-03-29), and I've actually been able to reproduce the issue with both but rustfmt 1.5.2-nightly (17c1167 2023-03-29) and rustfmt 1.5.2-nightly (34f9ca28 2023-02-16).

@calebcartwright it seems that we don't emit the ICE when the input comes from stdin

$: rustfmt
cfg_if::cfg_if! {
    if #[cfg(windows)] {
    } else if #(&cpus) {
    } else [libc::CTL_HW, libc::HW_NCPU, 0, 0]
}
cfg_if::cfg_if! {
    if #[cfg(windows)] {
    } else if #(&cpus) {
    } else [libc::CTL_HW, libc::HW_NCPU, 0, 0]
}

But we do emit it when the input comes from a file:

$: rustfmt issue_5728.rs 

error: internal compiler error: the following error was constructed but not emitted

error: expected one of `!` or `[`, found `(`
 --> /Users/ytmimi/Documents/Projects/rust_projects/rustfmt/issue_5728.rs:3:16
  |
3 |     } else if #(&cpus) {
  |                ^ expected one of `!` or `[`

thread 'main' panicked at 'error was constructed but not emitted', compiler/rustc_errors/src/diagnostic_builder.rs:728:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[calebcartwright]

And the plot thickens..

[ytmimi]

@calebcartwright The fix for this ICE is pretty simple. I'll have a PR ready a little later.