Strongly-typed str-wrapper const results in ICE in pattern match

[neoeinstein]

Code

#![feature(const_fn_union)]

#[derive(PartialEq, Eq)]
#[repr(transparent)]
pub struct Username(str);

pub const ROOT_USER: &Username = Username::from_str("root");

impl Username {
    pub const fn from_str(raw: &str) -> &Self {
        union Transmute<'a> {
            raw: &'a str,
            typed: &'a Username,
        }
        
        unsafe { Transmute { raw }.typed }
    }
    
    pub const fn as_str(&self) -> &str {
        &self.0
    }
    
    pub fn is_root(&self) -> bool {
        match self {
            ROOT_USER => true,
            _ => false,
        }
    }
}

Changing Username::is_root and introducing ROOT_USER_STR compiles successfully:

pub const ROOT_USER_STR: &str = ROOT_USER.as_str();

impl Username {
    …
    pub fn is_root(&self) -> bool {
        match self.as_str() {
            ROOT_USER_STR => true,
            _ => false,
        }
    }
}

Meta

rustc --version --verbose (Was from the Rust Playground):

1.55.0-nightly (2021-07-08 8b87e85394aa583b01e5)

Error output

error: internal compiler error: compiler/rustc_mir/src/const_eval/mod.rs:198:18: type Username should not have metadata, but had Meta(0x0000000000000004)

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 1.55.0-nightly (8b87e8539 2021-07-08) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type bin

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
#0 [deref_const] deref constant
#1 [check_match] match-checking `<impl at src/main.rs:9:1: 31:2>::is_root`
#2 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

error: could not compile `playground`

Backtrace

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1035:9
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_mir::const_eval::deref_const
   8: rustc_query_system::query::plumbing::get_query_impl
   9: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::deref_const
  10: rustc_mir_build::thir::pattern::const_to_pat::ConstToPat::recur
  11: rustc_infer::infer::InferCtxtBuilder::enter
  12: rustc_mir_build::thir::pattern::const_to_pat::<impl rustc_mir_build::thir::pattern::PatCtxt>::const_to_pat
  13: rustc_mir_build::thir::pattern::PatCtxt::lower_path
  14: rustc_mir_build::thir::pattern::PatCtxt::lower_pattern
  15: rustc_mir_build::thir::pattern::check_match::MatchVisitor::lower_pattern
  16: <rustc_mir_build::thir::pattern::check_match::MatchVisitor as rustc_hir::intravisit::Visitor>::visit_expr
  17: <rustc_mir_build::thir::pattern::check_match::MatchVisitor as rustc_hir::intravisit::Visitor>::visit_expr
  18: rustc_mir_build::thir::pattern::check_match::check_match
  19: rustc_query_system::query::plumbing::get_query_impl
  20: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::check_match
  21: rustc_middle::ty::<impl rustc_middle::ty::context::TyCtxt>::par_body_owners
  22: rustc_session::utils::<impl rustc_session::session::Session>::time
  23: rustc_interface::passes::analysis
  24: rustc_query_system::query::plumbing::get_query_impl
  25: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
  26: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  27: rustc_span::with_source_map
  28: rustc_interface::interface::create_compiler_and_run
  29: scoped_tls::ScopedKey<T>::set