Rustdoc renders Vec's new allocator type parameter even when set to the default

[sfackler]

For example, the docs for this type use Vec<String, Global> rather than just Vec<String>: https://docs.rs/tokio-postgres/0.7.0/tokio_postgres/types/enum.Kind.html. The definition of the type does not specify the allocator parameter if that matters.

The same is not true for e.g. HashMap's hasher type parameter, so I'm not sure what's different in this case.

[jyn514]

The difference is that this is a cross-crate re-export. MCVE:

// crate1
use std::collections::*;

pub enum Types {
  Map(HashMap<usize, usize>),
  List(Vec<usize>),
  Pointer(Box<usize>),
}

// crate2
pub use crate1::Types;

[jyn514]

This is a bug at least back to 1.22.0.

[jyn514]

In the inner crate, the params never get passes to render in the first place:

DEBUG rustdoc::html::format fmt_type(t=ResolvedPath { path: Path { global: false, res: Def(Struct, DefId(4:4837 ~ alloc[4799]::vec::Vec)), segments: [PathSegment { name: "Vec", args: AngleBracketed { args: [Type(Primitive(Usize))], bindings: [] } }] }, param_names: None, did: DefId(4:4837 ~ alloc[4799]::vec::Vec), is_generic: false }

but in the outer one they do show up:

DEBUG rustdoc::html::format fmt_type(t=ResolvedPath { path: Path { global: false, res: Err, segments: [PathSegment { name: "Vec", args: AngleBracketed { args: [Type(Primitive(Usize)), Type(ResolvedPath { path: Path { global: false, res: Err, segments: [PathSegment { name: "Global", args: AngleBracketed { args: [], bindings: [] } }] }, param_names: None, did: DefId(5:5465 ~ alloc[4799]::alloc::Global), is_generic: false })], bindings: [] } }] }, param_names: None, did: DefId(5:4837 ~ alloc[4799]::vec::Vec), is_generic: false }

I expect this has to do with some difference between HIR and metadata loading.

[jyn514]

I thought this might be related to inlining, but using doc(inline) within the same crate works fine:

[jyn514]

The issue is that metadata types, unlike HIR types, go through external_path:

rust/src/librustdoc/clean/mod.rs

Line 1584 in 89524d0

let path = external_path(cx, cx.tcx.item_name(did), None, false, vec![], substs);

and external_path doesn't pay any attention to the type defaults:

rust/src/librustdoc/clean/utils.rs

Line 103 in 89524d0

let args: Vec<_> = substs

So rustdoc needs to know which of the generic arguments were added by the user and which are just defaults.

[jyn514]

cc https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/What.20is.20the.20difference.20between.20AssocTypeItem.20and.20TypedefItem/near/221461855