Unexpected panic with derive debug on non-item

[alexbowers]

Compiling mount v0.3.0 (file:///var/www/sites/current/mount)
error: `derive` can be only be applied to items
  --> /var/www/sites/current/mount/src/mount.rs:63:14
   |
63 |     #[derive(Debug)]
   |              ^^^^^

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/blob/master/CONTRIBUTING.md#bug-reports

thread 'rustc' panicked at 'adding a def'n for node-id NodeId(4294967295) and data TypeParam("H") but a previous def'n exists: DefKey { parent: Some(DefIndex(29)), disambiguated_data: DisambiguatedDefPathData { data: ValueNs("mount"), disambiguator: 0 } }', /checkout/src/librustc/hir/map/definitions.rs:479
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: Could not compile `mount`.

---

Code:

impl Mount {
    /// Creates a new instance of `Mount`.
    pub fn new() -> Mount {
        Mount {
            inner: SequenceTrie::new()
        }
    }

    /// Mounts a given `Handler` onto a route.
    ///
    /// This method may be called multiple times with different routes.
    /// For a given request, the *most specific* handler will be selected.
    ///
    /// Existing handlers on the same route will be overwritten.
    #[derive(Debug)]
    pub fn mount<H: Handler>(&mut self, route: &str, handler: H) -> &mut Mount {
        // Parse the route into a list of strings. The unwrap is safe because strs are UTF-8.
        let key: Vec<&str> = Path::new(route).components().flat_map(|c|
            match c {
                Component::RootDir => None,
                c => Some(c.as_os_str().to_str().unwrap())
            }
        ).collect();

        // Insert a match struct into the trie.
        let match_length = key.len();
        self.inner.insert(key, Match {
            handler: Box::new(handler) as Box<Handler>,
            length: match_length,
        });
        self
    }
}

I assume this is because of where I put the derive debug line.

[strangeglyph]

Minimized:

struct Mount;
impl Mount {
    #[derive(Debug)]
    fn mount(a: ()) {}
}

Though I'm only able to reproduce on stable and beta, not nightly

[alexbowers]

This is on stable

rustc 1.18.0 (03fc9d622 2017-06-06)
cargo 0.19.0 (28d1d60d4 2017-05-16)

[iWritei]

I also seen this bug with the following code:

fn main() {
let test = test_it::GoodMessage(String::from("Hi Good Looking") );
test.printme();
}
enum test_it{
GoodMessage(String),
BadMessage(String),
}
impl test_it{
#[derive(Debug)]
fn printme(&self)
{
println!("{:?}",self);
}
}

rustc 1.19.0 (0ade339 2017-07-17)
binary: rustc
commit-hash: 0ade339
commit-date: 2017-07-17
host: x86_64-unknown-linux-gnu
release: 1.19.0
LLVM version: 4.0

[kennytm]

@iWritei's ICE seems to be a different issue from @Boreeas's. Several reduced test cases:

// ICE everywhere.
struct S;
impl S {
    #[derive(Debug)]
    fn f() {
        file!(); // must invoke a macro.
    }
}

// ICE on beta, nightly; no ICE on stable.
trait TT {
    #[derive(Debug)]
    fn f();
}

// ICE on beta, nightly; no ICE on stable.
trait TT {
    #[derive(Debug)]
    type F;
}

The first new ICE happens with the unwrap on this line: https://github.com/rust-lang/rust/blob/3f977baf/src/libsyntax/ext/placeholders.rs#L105

impl<'a, 'b> PlaceholderExpander<'a, 'b> {
    fn remove(&mut self, id: ast::NodeId) -> Expansion {
        self.expansions.remove(&id).unwrap()
    }
}

Not sure about the rest, but they all involve an unwrap() somewhere.

[alexbowers]

I ran into this again today, can this be improved somehow?

[arielb1]

Current compiler output is:

   Compiling playground v0.0.1 (file:///playground)
error: `derive` can be only be applied to items
  --> src/main.rs:10:10
   |
10 | #[derive(Debug)]
   |          ^^^^^

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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.22.0-nightly (744dd6c1d 2017-09-02) running on x86_64-unknown-linux-gnu

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', /checkout/src/libcore/option.rs:335:20
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: Could not compile `playground`.

To learn more, run the command again with --verbose.

[nikomatsakis]

triage: P-high

[jseyfried]

Fixed in #44757.

[alexbowers]

@jseyfried Thanks for the fix, hope it gets merged soon.

For my understanding and learning, is it possible for you to detail how you went about debugging this?

I wouldn't know where to start, and since this PR is fairly simple in its changes, it would be probably a good one to learn from.

Thanks :)