"do not use `optimized_mir` for constants" panic when generating MIR cfg

[osa1]

The working example in #72956 builds fine, but if I add -Zunpretty=mir-cfg it panics. Repro:

use std::ops::Index;

struct Banana(*mut ());
unsafe impl Send for Banana {}

impl Banana {
    fn new() -> &'static mut Banana {
        static mut BANANA: Banana = Banana(std::ptr::null_mut());
        unsafe { &mut BANANA }
    }
}

struct Peach<'a>(&'a mut Banana);

impl<'a> std::ops::Index<usize> for Peach<'a> {
    type Output = ();
    fn index(&self, idx: usize) -> &() {
        &()
    }
}

async fn baz(v: &()) {}

async fn bar() -> () {
    let peach = Peach(Banana::new());
    let r: &() = &*std::ops::Index::index(&peach, 0);
    baz(r).await;
    peach.index(0);
}

fn assert_send<T: Send>(_: T) {}

pub fn main() {
    assert_send(bar())
}

It builds fine:

$ rustc +stage1 test_works.rs --edition=2018
warning: unused variable: `v`
  --> test_works.rs:22:14
   |
22 | async fn baz(v: &()) {}
   |              ^ help: if this is intentional, prefix it with an underscore: `_v`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: unused variable: `idx`
  --> test_works.rs:17:21
   |
17 |     fn index(&self, idx: usize) -> &() {
   |                     ^^^ help: if this is intentional, prefix it with an underscore: `_idx`

warning: 2 warnings emitted

Panics with -Zunpretty=mir-cfg:

$ rustc +stage1 test_works.rs --edition=2018 -Zunpretty=mir-cfg
warning: unused variable: `v`
  --> test_works.rs:22:14
   |
22 | async fn baz(v: &()) {}
   |              ^ help: if this is intentional, prefix it with an underscore: `_v`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: unused variable: `idx`
  --> test_works.rs:17:21
   |
17 |     fn index(&self, idx: usize) -> &() {
   |                     ^^^ help: if this is intentional, prefix it with an underscore: `_idx`

thread 'rustc' panicked at 'do not use `optimized_mir` for constants: Static(Mut)', compiler/rustc_mir/src/transform/mod.rs:579:24
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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.52.0-dev running on x86_64-unknown-linux-gnu

note: compiler flags: -Z unpretty=mir-cfg

query stack during panic:
#0 [optimized_mir] optimizing MIR for `Banana::new::BANANA`
end of query stack
warning: 2 warnings emitted

Backtrace

   0: rust_begin_unwind
             at /home/omer/rust/rust/library/std/src/panicking.rs:493:5
   1: std::panicking::begin_panic_fmt
             at /home/omer/rust/rust/library/std/src/panicking.rs:435:5
   2: rustc_mir::transform::inner_optimized_mir
             at /home/omer/rust/rust/compiler/rustc_mir/src/transform/mod.rs:579:24
   3: rustc_mir::transform::optimized_mir
             at /home/omer/rust/rust/compiler/rustc_mir/src/transform/mod.rs:561:21
   4: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::optimized_mir>::compute
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/query/plumbing.rs:377:17
   5: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
             at /home/omer/rust/rust/compiler/rustc_query_system/src/dep_graph/graph.rs:363:14
   6: rustc_query_system::query::plumbing::force_query_with_job::{{closure}}::{{closure}}
   7: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/query/plumbing.rs:73:74
   8: stacker::maybe_grow
             at /home/omer/.cargo/registry/src/github.com-1ecc6299db9ec823/stacker-0.1.12/src/lib.rs:55:9
   9: rustc_data_structures::stack::ensure_sufficient_stack
             at /home/omer/rust/rust/compiler/rustc_data_structures/src/stack.rs:16:5
  10: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/query/plumbing.rs:73:17
  11: rustc_middle::ty::context::tls::enter_context::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/context.rs:1712:50
  12: rustc_middle::ty::context::tls::set_tlv
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/context.rs:1696:9
  13: rustc_middle::ty::context::tls::enter_context
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/context.rs:1712:9
  14: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/query/plumbing.rs:72:13
  15: rustc_middle::ty::context::tls::with_related_context::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/context.rs:1756:13
  16: rustc_middle::ty::context::tls::with_context::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/context.rs:1740:40
  17: rustc_middle::ty::context::tls::with_context_opt
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/context.rs:1729:22
  18: rustc_middle::ty::context::tls::with_context
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/context.rs:1740:9
  19: rustc_middle::ty::context::tls::with_related_context
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/context.rs:1753:9
  20: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/query/plumbing.rs:61:9
  21: rustc_query_system::query::plumbing::force_query_with_job::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_query_system/src/query/plumbing.rs:598:9
  22: rustc_query_system::query::plumbing::with_diagnostics
             at /home/omer/rust/rust/compiler/rustc_query_system/src/query/plumbing.rs:303:18
  23: rustc_query_system::query::plumbing::force_query_with_job
             at /home/omer/rust/rust/compiler/rustc_query_system/src/query/plumbing.rs:597:51
  24: rustc_query_system::query::plumbing::try_execute_query
             at /home/omer/rust/rust/compiler/rustc_query_system/src/query/plumbing.rs:427:16
  25: rustc_query_system::query::plumbing::get_query_impl::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_query_system/src/query/plumbing.rs:645:23
  26: <rustc_query_system::query::caches::DefaultCache<K,V> as rustc_query_system::query::caches::QueryCache>::lookup
             at /home/omer/rust/rust/compiler/rustc_query_system/src/query/caches.rs:114:79
  27: rustc_query_system::query::plumbing::try_get_cached
             at /home/omer/rust/rust/compiler/rustc_query_system/src/query/plumbing.rs:380:5
  28: rustc_query_system::query::plumbing::get_query_impl
             at /home/omer/rust/rust/compiler/rustc_query_system/src/query/plumbing.rs:637:5
  29: rustc_query_system::query::plumbing::get_query
             at /home/omer/rust/rust/compiler/rustc_query_system/src/query/plumbing.rs:739:5
  30: rustc_middle::ty::query::TyCtxtAt::optimized_mir
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/query/plumbing.rs:487:17
  31: rustc_middle::ty::query::<impl rustc_middle::ty::context::TyCtxt>::optimized_mir
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/query/plumbing.rs:448:17
  32: rustc_mir::util::graphviz::write_mir_graphviz
             at /home/omer/rust/rust/compiler/rustc_mir/src/util/graphviz.rs:25:21
  33: rustc_driver::pretty::print_with_analysis
             at /home/omer/rust/rust/compiler/rustc_driver/src/pretty.rs:499:26
  34: rustc_driver::pretty::print_after_hir_lowering
             at /home/omer/rust/rust/compiler/rustc_driver/src/pretty.rs:431:22
  35: rustc_driver::run_compiler::{{closure}}::{{closure}}::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_driver/src/lib.rs:349:25
  36: rustc_interface::passes::QueryContext::enter::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_interface/src/passes.rs:749:42
  37: rustc_middle::ty::context::tls::enter_context::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/context.rs:1712:50
  38: rustc_middle::ty::context::tls::set_tlv
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/context.rs:1696:9
  39: rustc_middle::ty::context::tls::enter_context
             at /home/omer/rust/rust/compiler/rustc_middle/src/ty/context.rs:1712:9
  40: rustc_interface::passes::QueryContext::enter
             at /home/omer/rust/rust/compiler/rustc_interface/src/passes.rs:749:9
  41: rustc_driver::run_compiler::{{closure}}::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_driver/src/lib.rs:347:21
  42: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
             at /home/omer/rust/rust/compiler/rustc_interface/src/queries.rs:418:19
  43: rustc_driver::run_compiler::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_driver/src/lib.rs:341:22
  44: rustc_interface::interface::create_compiler_and_run::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_interface/src/interface.rs:197:13
  45: rustc_span::with_source_map
             at /home/omer/rust/rust/compiler/rustc_span/src/lib.rs:789:5
  46: rustc_interface::interface::create_compiler_and_run
             at /home/omer/rust/rust/compiler/rustc_interface/src/interface.rs:191:5
  47: rustc_interface::interface::run_compiler::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_interface/src/interface.rs:213:12
  48: rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals::{{closure}}::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_interface/src/util.rs:152:13
  49: scoped_tls::ScopedKey<T>::set
             at /home/omer/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137:9
  50: rustc_span::with_session_globals
             at /home/omer/rust/rust/compiler/rustc_span/src/lib.rs:105:5
  51: rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_interface/src/util.rs:150:9
  52: rustc_interface::util::scoped_thread::{{closure}}
             at /home/omer/rust/rust/compiler/rustc_interface/src/util.rs:125:24

[osa1]

@oli-obk I think the panic was added by you. Could you say a few words on what the issue here? I'd be happy to investigate further and fix, but I have no idea where to start.

[oli-obk]

That's an oversight by me. In

rust/compiler/rustc_mir/src/util/graphviz.rs

Line 25 in ea09825

let body = &tcx.optimized_mir(def_id);

you instead need to do something similar to mir printing. That is, use instance_mir instead of optimized_mir, but also create a second dump for const fn by additionally using mir_for_ctfe to show the graph of the ctfe version of the const fn

[oli-obk]

#81200 is somewhat related, as we should probably first fix that one and then deduplicate the mir selection logic between cfg printing and regular mir dumps.

[osa1]

#82326 has a smaller repro.

[osa1]

@rustbot claim