Improve errors when invoked with wrong sysroot (missing MIR)

[ojeda]

miri behaves like a strange rustc driver:

$ miri main.rs # stable
error: the option `Z` is only accepted on the nightly compiler

$ miri main.rs # nightly
error: internal compiler error: compiler/rustc_metadata/src/rmeta/decoder.rs:1157:17: get_optimized_mir: missing MIR for `DefId(1:8654 ~ std[e170]::rt::lang_start_internal)`

[bjorn3]

$ miri main.rs # stable
error: the option `Z` is only accepted on the nightly compiler

It shouldn't even be possible to install miri on stable AFAIK.

$ miri main.rs # nightly
error: internal compiler error: compiler/rustc_metadata/src/rmeta/decoder.rs:1157:17: get_optimized_mir: missing MIR for `DefId(1:8654 ~ std[e170]::rt::lang_start_internal)`

I guess this error could be better. The sysroot needs to be compiled with -Zalways-encode-mir. Otherwise MIR is missing for non-generic non-#[inline] functions.

[RalfJung]

It shouldn't even be possible to install miri on stable AFAIK.

Indeed, how did you install this?

I guess this error could be better. The sysroot needs to be compiled with -Zalways-encode-mir. Otherwise MIR is missing for non-generic non-#[inline] functions.

Yeah... we used to have a nicer error but that probably bitrotted now that cargo miri can auto-compile the sysroot with xargo.

[ojeda]

Indeed, how did you install this?

It appears to come by default via the standalone installers:

$ curl https://static.rust-lang.org/dist/rust-1.52.1-x86_64-unknown-linux-gnu.tar.gz | tar xz
$ rust-1.52.1-x86_64-unknown-linux-gnu/install.sh
$ miri
error: the option `Z` is only accepted on the nightly compiler
$ RUSTC_BOOTSTRAP=1 miri --version
rustc 1.52.1 (9bc8c42bb 2021-05-09)

[RalfJung]

oops that's a bug. I reported an issue for this: rust-lang/rust#86286. Good thing that Miri dosn't work there, it's not supposed to even be part of this release. ;)

[RalfJung]

For the ICE when a MIR body is missing, we need to check is_mir_available before calling instance_mir. However, this patch does not work:

--- a/compiler/rustc_mir/src/interpret/machine.rs
+++ b/compiler/rustc_mir/src/interpret/machine.rs
@@ -144,7 +144,13 @@ fn load_mir(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         instance: ty::InstanceDef<'tcx>,
     ) -> InterpResult<'tcx, &'tcx mir::Body<'tcx>> {
-        Ok(ecx.tcx.instance_mir(instance))
+        let def_id = instance.def_id();
+        // Avoid ICE when MIR is missing: first check if it is available.
+        if ecx.tcx.is_mir_available(def_id) {
+            Ok(ecx.tcx.instance_mir(instance))
+        } else {
+            throw_unsup!(NoMirFor(def_id))
+        }
     }
 
     /// Entry point to all function calls.

This will now ICE for shims because they claim to have no MIR available.

@oli-obk any suggestions for what to do here? I'd rather not duplicate the entire logic of instance_mir, but I also see no way to avoid that. Can we make instance_mir return an Option?

[oli-obk]

Can we make instance_mir return an Option?

that would invoke is_mir_available on every instance_mir call, probably not a problem, but should bench it at least.

[RalfJung]

I'm open to other options. :D

[oli-obk]

I'm open to other options. :D

I see what you did there, but libcore only has Option

Make instance_mir_inner that does the main logic and takes a closure argument for loading the MIR. instance_mir calls it and passes Some(optimized_mir()) and unwraps the result of instance_mir_inner.

[RalfJung]

Honestly I didn't even see what I did there. Now I do. ;)

Can't the queries called by instance_mir be made to return an Option, instead of ICEing on missing MIR?

[oli-obk]

if you add _opt to their name and add inherent functions of the original name that unwrap, i think that would be fine

[RalfJung]

Yeah that sounds like the best approach to me then.

[bjorn3]

Maybe you could show a warning at startup if tcx.is_mir_available for a known non-generic function in libcore returns false?

[RalfJung]

FWIW, this is the backtrace of the ICE:

error: internal compiler error: compiler/rustc_metadata/src/rmeta/decoder.rs:1167:17: get_optimized_mir: missing MIR for `DefId(1:9633 ~ std[8eb6]::rt::lang_start_internal)`

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1007:9
stack backtrace:
   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::bug
   3: rustc_errors::Handler::bug
   4: rustc_middle::ty::context::tls::with_opt
   5: rustc_middle::util::bug::opt_span_bug_fmt
   6: rustc_middle::util::bug::bug_fmt
   7: rustc_metadata::rmeta::decoder::<impl rustc_metadata::creader::CrateMetadataRef>::get_optimized_mir
   8: rustc_metadata::rmeta::decoder::cstore_impl::provide_extern::optimized_mir
   9: rustc_query_system::query::plumbing::get_query_impl
  10: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::optimized_mir
  11: rustc_middle::ty::<impl rustc_middle::ty::context::TyCtxt>::instance_mir
  12: rustc_mir::interpret::machine::Machine::load_mir
             at /rustc/a50d72158e08e02cfc051b863017bdbd2c45b637/compiler/rustc_mir/src/interpret/machine.rs:147:12
  13: rustc_mir::interpret::eval_context::InterpCx<M>::load_mir
             at /rustc/a50d72158e08e02cfc051b863017bdbd2c45b637/compiler/rustc_mir/src/interpret/eval_context.rs:504:9
  14: miri::shims::EvalContextExt::find_mir_or_eval_fn
             at ./src/shims/mod.rs:57:19
  15: <miri::machine::Evaluator as rustc_mir::interpret::machine::Machine>::find_mir_or_eval_fn
             at ./src/machine.rs:407:9
  16: rustc_mir::interpret::terminator::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_fn_call
             at /rustc/a50d72158e08e02cfc051b863017bdbd2c45b637/compiler/rustc_mir/src/interpret/terminator.rs:303:27
  17: rustc_mir::interpret::terminator::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_terminator
             at /rustc/a50d72158e08e02cfc051b863017bdbd2c45b637/compiler/rustc_mir/src/interpret/terminator.rs:108:17
  18: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::terminator
             at /rustc/a50d72158e08e02cfc051b863017bdbd2c45b637/compiler/rustc_mir/src/interpret/step.rs:313:9
  19: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::step
             at /rustc/a50d72158e08e02cfc051b863017bdbd2c45b637/compiler/rustc_mir/src/interpret/step.rs:73:9
  20: miri::eval::eval_main::{{closure}}
             at ./src/eval.rs:264:29
  21: miri::eval::eval_main
             at ./src/eval.rs:258:38
  22: <miri::MiriCompilerCalls as rustc_driver::Callbacks>::after_analysis::{{closure}}
             at ./src/bin/miri.rs:80:40