Confusing diagnostic when improperly using constant as type.

[rylev]

The following code...

#[allow(non_upper_case_globals)]
pub const Foo: &str = "";

mod submod {
    use super::Foo;
    fn function() {
        println!("{}", Foo::Bar);
    }
}

...yields the following error and warning:

error[E0433]: failed to resolve: use of undeclared type `Foo`
 --> src/lib.rs:7:24
  |
7 |         println!("{}", Foo::Bar);
  |                        ^^^ use of undeclared type `Foo`

warning: unused import: `super::Foo`
 --> src/lib.rs:5:9
  |
5 |     use super::Foo;
  |         ^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

The warning says that Foo is not used but it clearly is, just not properly. This error is normally easier to spot when using the proper casing for constants, but that might not always be the case. Perhaps in this instance, we should look for other items with the exact same name and point out that the user is trying to use a constant as a type.

[estebank]

Every single one of these errors should point at similarly named things of the wrong namespace.

[henryboisdequin]

Do you know where in the code this is coming from?

[estebank]

@henryboisdequin running the following will tell you where the error is being emitted:

% RUST_BACKTRACE=1 rustc +dev f5.rs -Ztreat-err-as-bug
error[E0433]: failed to resolve: use of undeclared type `Foo`
 --> f5.rs:7:24
  |
7 |         println!("{}", Foo::Bar);
  |                        ^^^ use of undeclared type `Foo`

thread 'rustc' panicked at 'aborting due to `-Z treat-err-as-bug=1`', compiler/rustc_errors/src/lib.rs:990:27
stack backtrace:
   0: std::panicking::begin_panic
   1: rustc_errors::HandlerInner::emit_diagnostic
   2: rustc_errors::Handler::emit_diagnostic
   3: rustc_errors::diagnostic_builder::DiagnosticBuilder::emit
   4: rustc_resolve::diagnostics::<impl rustc_resolve::Resolver>::report_error
   5: rustc_resolve::late::LateResolutionVisitor::report_error
   6: rustc_resolve::late::LateResolutionVisitor::smart_resolve_path_fragment
   7: rustc_resolve::late::LateResolutionVisitor::resolve_expr
   8: rustc_resolve::late::LateResolutionVisitor::resolve_expr
   9: rustc_ast::visit::walk_expr
  10: rustc_resolve::late::LateResolutionVisitor::resolve_expr
  11: rustc_ast::visit::walk_expr
  12: rustc_resolve::late::LateResolutionVisitor::resolve_expr
  13: rustc_resolve::late::LateResolutionVisitor::resolve_expr
  14: rustc_resolve::late::LateResolutionVisitor::resolve_expr
  15: rustc_resolve::late::LateResolutionVisitor::resolve_expr
  16: <rustc_resolve::late::LateResolutionVisitor as rustc_ast::visit::Visitor>::visit_block
  17: rustc_resolve::late::LateResolutionVisitor::resolve_expr
  18: <rustc_resolve::late::LateResolutionVisitor as rustc_ast::visit::Visitor>::visit_block
  19: rustc_resolve::late::LateResolutionVisitor::with_rib
  20: <rustc_resolve::late::LateResolutionVisitor as rustc_ast::visit::Visitor>::visit_fn
  21: rustc_ast::visit::walk_item
  22: rustc_resolve::late::LateResolutionVisitor::with_generic_param_rib
  23: <rustc_resolve::late::LateResolutionVisitor as rustc_ast::visit::Visitor>::visit_item
  24: rustc_ast::visit::walk_item
  25: rustc_resolve::late::LateResolutionVisitor::with_scope
  26: <rustc_resolve::late::LateResolutionVisitor as rustc_ast::visit::Visitor>::visit_item
  27: rustc_ast::visit::walk_crate
  28: rustc_resolve::late::<impl rustc_resolve::Resolver>::late_resolve_crate
  29: rustc_session::utils::<impl rustc_session::session::Session>::time
  30: rustc_resolve::Resolver::resolve_crate
  31: rustc_interface::passes::configure_and_expand_inner
  32: rustc_interface::passes::configure_and_expand::{{closure}}
  33: alloc::boxed::<impl core::ops::generator::Generator<R> for core::pin::Pin<alloc::boxed::Box<G,A>>>::resume
  34: rustc_data_structures::box_region::PinnedGenerator<I,A,R>::new
  35: rustc_interface::passes::BoxedResolver::new
  36: rustc_interface::passes::configure_and_expand
  37: rustc_interface::queries::Query<T>::compute
  38: rustc_interface::queries::Queries::expansion
  39: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  40: rustc_span::with_source_map
  41: rustc_interface::interface::create_compiler_and_run
  42: scoped_tls::ScopedKey<T>::set
  43: rustc_span::with_session_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose 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 1.51.0-dev running on x86_64-apple-darwin

note: compiler flags: -Z treat-err-as-bug

query stack during panic:
end of query stack

From there the place you might want to start looking at is smart_resolve_path_fragment, which is a bit hard to follow at the moment (I'm doing a slight clean up of that method at the moment), but should be reasonably straight forward to add another check for this case to that pile :)

[henryboisdequin]

@rustbot claim

[Krout0n]

Can I take this issue?

[henryboisdequin]

Can I take this issue?

Yes feel free to take it, I removed my assignment.

[Krout0n]

I’m going to be tied up until May, feel free to take this issue:pray:

[kwj2104]

Hey I'm looking to get to know the Rust codebase better and contribute to the project - would love to take a crack at this issue!

[kwj2104]

@rustbot claim