Recursion limit reached while evaluating requirement

[Michael-F-Bryan]

It looks like the latest nightly rustc overflowed while trying to prove proc_macro2::Group satisfies the Unpin oibit.

Strangely, I couldn't reproduce this by adding just proc-macro2 to a dummy project and building. It needed to be used by another crate (may have something to do with generics or feature flags?).

Documenting proc-macro2 v0.4.30
    Checking proc-macro2 v0.4.30
 
...

error[E0275]: overflow evaluating the requirement `proc_macro2::Group: std::marker::Unpin`
  |
  = help: consider adding a `#![recursion_limit="128"]` attribute to your crate
  = note: required because it appears within the type `proc_macro2::TokenTree`
  = note: required because it appears within the type `*const proc_macro2::TokenTree`
  = note: required because it appears within the type `std::ptr::Unique<proc_macro2::TokenTree>`
  = note: required because it appears within the type `alloc::raw_vec::RawVec<proc_macro2::TokenTree>`
  = note: required because it appears within the type `std::vec::Vec<proc_macro2::TokenTree>`
  = note: required because it appears within the type `proc_macro2::fallback::TokenStream`
  = note: required because it appears within the type `proc_macro2::imp::TokenStream`
  = note: required because it appears within the type `proc_macro2::TokenStream`
  = note: required because it appears within the type `syn::Macro`
  = note: required because it appears within the type `syn::PatMacro`
  = note: required because it appears within the type `syn::Pat`
  = note: required because it appears within the type `(syn::Pat, syn::token::Or)`
  = note: required because it appears within the type `*const (syn::Pat, syn::token::Or)`
  = note: required because it appears within the type `std::ptr::Unique<(syn::Pat, syn::token::Or)>`
  = note: required because it appears within the type `alloc::raw_vec::RawVec<(syn::Pat, syn::token::Or)>`
  = note: required because it appears within the type `std::vec::Vec<(syn::Pat, syn::token::Or)>`
  = note: required because it appears within the type `syn::punctuated::Punctuated<syn::Pat, syn::token::Or>`
  = note: required because it appears within the type `syn::ExprLet`
  = note: required because it appears within the type `syn::Expr`
  = note: required because it appears within the type `syn::TypeArray`
  = note: required because it appears within the type `syn::Type`
  = note: required because it appears within the type `syn::GenericArgument`
  = note: required because it appears within the type `(syn::GenericArgument, syn::token::Comma)`
  = note: required because it appears within the type `*const (syn::GenericArgument, syn::token::Comma)`
  = note: required because it appears within the type `std::ptr::Unique<(syn::GenericArgument, syn::token::Comma)>`
  = note: required because it appears within the type `alloc::raw_vec::RawVec<(syn::GenericArgument, syn::token::Comma)>`
  = note: required because it appears within the type `std::vec::Vec<(syn::GenericArgument, syn::token::Comma)>`
  = note: required because it appears within the type `syn::punctuated::Punctuated<syn::GenericArgument, syn::token::Comma>`
  = note: required because it appears within the type `syn::AngleBracketedGenericArguments`
  = note: required because it appears within the type `syn::PathArguments`
  = note: required because it appears within the type `syn::PathSegment`
  = note: required because it appears within the type `(syn::PathSegment, syn::token::Colon2)`
  = note: required because it appears within the type `*const (syn::PathSegment, syn::token::Colon2)`
  = note: required because it appears within the type `std::ptr::Unique<(syn::PathSegment, syn::token::Colon2)>`
  = note: required because it appears within the type `alloc::raw_vec::RawVec<(syn::PathSegment, syn::token::Colon2)>`
  = note: required because it appears within the type `std::vec::Vec<(syn::PathSegment, syn::token::Colon2)>`
  = note: required because it appears within the type `syn::punctuated::Punctuated<syn::PathSegment, syn::token::Colon2>`
  = note: required because it appears within the type `syn::Path`
  = note: required because it appears within the type `syn::Attribute`
  = note: required because it appears within the type `*const syn::Attribute`
  = note: required because it appears within the type `std::ptr::Unique<syn::Attribute>`
  = note: required because it appears within the type `alloc::raw_vec::RawVec<syn::Attribute>`
  = note: required because it appears within the type `std::vec::Vec<syn::Attribute>`
  = note: required because it appears within the type `syn::LifetimeDef`
  = note: required because it appears within the type `(syn::LifetimeDef, syn::token::Comma)`
  = note: required because it appears within the type `*const (syn::LifetimeDef, syn::token::Comma)`
  = note: required because it appears within the type `std::ptr::Unique<(syn::LifetimeDef, syn::token::Comma)>`
  = note: required because it appears within the type `alloc::raw_vec::RawVec<(syn::LifetimeDef, syn::token::Comma)>`
  = note: required because it appears within the type `std::vec::Vec<(syn::LifetimeDef, syn::token::Comma)>`
  = note: required because it appears within the type `syn::punctuated::Punctuated<syn::LifetimeDef, syn::token::Comma>`
  = note: required because it appears within the type `syn::BoundLifetimes`
  = note: required because it appears within the type `std::option::Option<syn::BoundLifetimes>`
  = note: required because it appears within the type `syn::PredicateType`
  = note: required because it appears within the type `syn::WherePredicate`
  = note: required because it appears within the type `*const syn::WherePredicate`
  = note: required because it appears within the type `std::ptr::Unique<syn::WherePredicate>`
  = note: required because it appears within the type `alloc::raw_vec::RawVec<syn::WherePredicate>`
  = note: required because it appears within the type `std::vec::Vec<syn::WherePredicate>`
  = note: required because it appears within the type `std::option::Option<std::vec::Vec<syn::WherePredicate>>`
  = note: required because it appears within the type `attr::InputClone`
  = note: required because it appears within the type `std::option::Option<attr::InputClone>`
  = note: required because it appears within the type `attr::Input`
  = note: required because it appears within the type `ast::Input<'a>`

error: aborting due to previous error

I was able to reproduce this quite reliably by doing the following on my Linux laptop:

$ rustc --version
rustc 1.37.0-nightly (8aa42ed7c 2019-06-24)
$ cargo --version
cargo 1.37.0-nightly (807429e1b 2019-06-11)
$ cd /tmp
$cargo new --lib whatever
     Created library `whatever` package
$ cd whatever
$ cargo add specs
      Adding specs v0.14.3 to dependencies
$ cargo doc
    BOOM!!1!

[dtolnay]

Thanks, this is being tracked in rust-lang/rust#62059.