`rustdoc --output-format json` assert while documenting the `syn` crate

[Urgau]

Code

From the syn source code:

pub trait Spanned {
    /// Returns a `Span` covering the complete contents of this syntax tree
    /// node, or [`Span::call_site()`] if this node is empty.
    ///
    /// [`Span::call_site()`]: proc_macro2::Span::call_site
    fn span(&self) -> Span;
}

impl<T: ?Sized + ToTokens> Spanned for T {
    fn span(&self) -> Span {
        self.__span()
    }
}

Meta

rustc +nightly --version

rustc 1.56.0-nightly (ccffcafd5 2021-08-11)

rustdoc +nightly --version

rustdoc 1.56.0-nightly (ccffcafd5 2021-08-11)

Error output

thread 'rustc' panicked at 'assertion failed: `(left == right)`
  left: `Item { id: Id("0:3511"), crate_id: 0, name: Some("span"), span: Some(Span { filename: "REDACTED/.cargo/registry/src/github.com-1285ae84e5963aae/syn-1.0.74/src/spanned.rs", begin: (111, 4), end: (113, 5) }), visibility: Public, docs: None, links: {}, attrs: [], deprecation: None, inner: Method(Method { decl: FnDecl { inputs: [("", BorrowedRef { lifetime: None, mutable: false, type_: Generic("Self") })], output: Some(ResolvedPath { name: "Span", id: Id("20:909"), args: Some(AngleBracketed { args: [], bindings: [] }), param_names: [] }), c_variadic: false }, generics: Generics { params: [], where_predicates: [] }, header: {}, abi: "\"Rust\"", has_body: true }) }`,
 right: `Item { id: Id("0:3511"), crate_id: 0, name: Some("span"), span: Some(Span { filename: "REDACTED/.cargo/registry/src/github.com-1285ae84e5963aae/syn-1.0.74/src/spanned.rs", begin: (111, 4), end: (113, 5) }), visibility: Default, docs: None, links: {}, attrs: [], deprecation: None, inner: Method(Method { decl: FnDecl { inputs: [("self", BorrowedRef { lifetime: None, mutable: false, type_: Generic("Self") })], output: Some(ResolvedPath { name: "Span", id: Id("20:909"), args: Some(AngleBracketed { args: [], bindings: [] }), param_names: [] }), c_variadic: false }, generics: Generics { params: [], where_predicates: [] }, header: {}, abi: "\"Rust\"", has_body: true }) }`', src/librustdoc/json/mod.rs:179:17

Backtrace

stack backtrace:
   0: rust_begin_unwind
             at /rustc/ccffcafd55e58f769d4b0efc0064bf65e76998e4/library/std/src/panicking.rs:517:5
   1: core::panicking::panic_fmt
             at /rustc/ccffcafd55e58f769d4b0efc0064bf65e76998e4/library/core/src/panicking.rs:93:14
   2: core::panicking::assert_failed_inner
   3: core::panicking::assert_failed
   4: <rustdoc::json::JsonRenderer as rustdoc::formats::renderer::FormatRenderer>::item
   5: <rustdoc::json::JsonRenderer as rustdoc::formats::renderer::FormatRenderer>::item
   6: <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
   7: <rustdoc::json::JsonRenderer as rustdoc::formats::renderer::FormatRenderer>::item
   8: <rustdoc::json::JsonRenderer as rustdoc::formats::renderer::FormatRenderer>::item
   9: <rustdoc::json::JsonRenderer as rustdoc::formats::renderer::FormatRenderer>::item
  10: rustdoc::formats::renderer::run_format
  11: rustdoc::run_renderer
  12: rustc_interface::interface::create_compiler_and_run
  13: rustdoc::main_options
  14: scoped_tls::ScopedKey<T>::set

[Urgau]

cc @jyn514 @P1n3appl3 @GuillaumeGomez

@rustbot label +T-rustdoc +A-rustdoc-json -T-compiler

[jyn514]

This is likely a duplicate of #83718.