"use of local which has no storage here" at -Zmir-opt-level=0

[saethlin]

I ran into this in my PR #131650 (comment) when trying to use GVN more, because MIR "lints" are enabled in the mir-opt suite.

This lint fires a fair bit in the ecosystem, for example in zvariant-5.6.0, compiling with RUSTFLAGS="-Zlint-mir -Zmir-opt-level=0 -Zmir-enable-passes=+GVN" cargo +nightly build

error: internal compiler error: broken MIR in Item(DefId(0:1107 ~ zvariant[4905]::dbus::ser::{impl#4}::serialize_struct_element)) (after pass SanityCheck) at bb46[0]:
                                use of local _10, which has no storage here
   --> src/dbus/ser.rs:540:5
    |
540 |     }
    |     ^
    |
note: delayed at compiler/rustc_mir_transform/src/lint.rs:74:22
         0: <rustc_errors::DiagCtxtInner>::emit_diagnostic
         1: <rustc_errors::DiagCtxtHandle>::emit_diagnostic
         2: <rustc_span::ErrorGuaranteed as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
         3: <rustc_errors::DiagCtxtHandle>::span_delayed_bug::<rustc_span::span_encoding::Span, alloc::string::String>
         4: <rustc_mir_transform::lint::Lint>::fail::<alloc::string::String>
         5: <rustc_mir_transform::lint::Lint as rustc_middle::mir::visit::Visitor>::visit_local
         6: <rustc_mir_transform::lint::Lint as rustc_middle::mir::visit::Visitor>::visit_place
         7: <rustc_mir_transform::lint::Lint as rustc_middle::mir::visit::Visitor>::visit_terminator
         8: rustc_mir_transform::lint::lint_body
         9: rustc_mir_transform::mir_built
<snip>

[cjgillot]

I'm puzzled by this issue. SanityCheck runs just after MIR building. The only thing that happens earlier is a SimplifyCfg.

[saethlin]

Oh, hm. It's quite possible that in my hunt for this same error outside my PR I ran into a different problem.

[saethlin]

Yeah this is definitely not the GVN issue I was hunting. It's a problem before opts.