[Rust 2018] rustdoc fails completely

[CryZe]

My base crate uses Rust 2018 and has a bunch of Rust 2015 dependencies. Running cargo doc fails to document most Rust 2015 crates:

 Documenting num-traits v0.2.4
 Documenting base64 v0.9.1
 Documenting serde v1.0.62
error: unused extern crate
 --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\base64-0.9.1\src\line_wrap.rs:1:1
  |
1 | extern crate safemem;
  | ^^^^^^^^^^^^^^^^^^^^^ help: remove it
warning: `[cfg]` cannot be resolved, ignoring it...
 --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\cfg-if-0.1.3\src\lib.rs:1:28
  |
1 | //! A macro for defining #[cfg] if-else statements.
  |                            ^^^ cannot be resolved, ignoring
  |
  |
note: lint level defined here
 --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\base64-0.9.1\src\lib.rs:59:57
  |
59|     missing_docs, trivial_casts, trivial_numeric_casts, unused_extern_crates, unused_import_braces,
  |                                                         ^^^^^^^^^^^^^^^^^^^^

  = note: #[warn(intra_doc_link_resolution_failure)] on by default
error: Compilation failed, aborting rustdoc

  = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`

warning: `[cfg]` cannot be resolved, ignoring it...
 --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\cfg-if-0.1.3\src\lib.rs:7:59
  |
7 | //! This allows you to conveniently provide a long list #[cfg]'d blocks of code
  |                                                           ^^^ cannot be resolved, ignoring
  |
  = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`

error: Could not document `base64`.

Caused by:
  process didn't exit successfully: `rustdoc --crate-name base64 C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\base64-0.9.1\src\lib.rs -o C:\Projekte\one-offs\LiveSplitOne\livesplit-core\target\doc -L dependency=C:\Projekte\one-offs\LiveSplitOne\livesplit-core\target\debug\deps --extern byteorder=C:\Projekte\one-offs\LiveSplitOne\livesplit-core\target\debug\deps\libbyteorder-198fa0f87c04f399.rmeta --extern safemem=C:\Projekte\one-offs\LiveSplitOne\livesplit-core\target\debug\deps\libsafemem-9af8dc15e19938f1.rmeta` (exit code: 101)
warning: build failed, waiting for other jobs to finish...
error: unused import: `SeqAccess`
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\de\value.rs:41:44
   |
41 | use de::{self, Expected, IntoDeserializer, SeqAccess};
   |                                            ^^^^^^^^^
   |
note: lint level defined here
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\lib.rs:126:23
   |
126| #![deny(missing_docs, unused_imports)]
   |                       ^^^^^^^^^^^^^^

error: unused import: `private::de::size_hint`
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\de\value.rs:42:5
   |
42 | use private::de::size_hint;
   |     ^^^^^^^^^^^^^^^^^^^^^^

error: unused import: `Unexpected`
    --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\de\value.rs:1274:20
     |
1274 |     use de::{self, Unexpected};
     |                    ^^^^^^^^^^

error: unused imports: `Unexpected`, `VariantAccess`
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\de\impls.rs:12:62
   |
12 |     Deserialize, Deserializer, EnumAccess, Error, SeqAccess, Unexpected, VariantAccess, Visitor,
   |                                                              ^^^^^^^^^^  ^^^^^^^^^^^^^

error: unused import: `de::MapAccess`
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\de\impls.rs:16:5
   |
16 | use de::MapAccess;
   |     ^^^^^^^^^^^^^

error: unused import: `de::from_primitive::FromPrimitive`
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\de\impls.rs:18:5
   |
18 | use de::from_primitive::FromPrimitive;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: unused import: `private::de::InPlaceSeed`
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\de\impls.rs:19:5
   |
19 | use private::de::InPlaceSeed;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^

error: unused import: `private::de::size_hint`
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\de\impls.rs:22:5
   |
22 | use private::de::size_hint;
   |     ^^^^^^^^^^^^^^^^^^^^^^

error: unused import: `SerializeTuple`
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\ser\impls.rs:11:22
   |
11 | use ser::{Serialize, SerializeTuple, Serializer};
   |                      ^^^^^^^^^^^^^^

error: unused import: `ser::Error`
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\ser\impls.rs:14:5
   |
14 | use ser::Error;
   |     ^^^^^^^^^^

error: unused import: `Unexpected`
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\private\de.rs:14:21
   |
14 | use de::{MapAccess, Unexpected};
   |                     ^^^^^^^^^^

error: unused import: `super::size_hint`
   --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\private\de.rs:232:9
    |
232 |     use super::size_hint;
    |         ^^^^^^^^^^^^^^^^

error: unused import: `SerializeStruct`
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\private\ser.rs:11:54
   |
11 | use ser::{self, Impossible, Serialize, SerializeMap, SerializeStruct, Serializer};
   |                                                      ^^^^^^^^^^^^^^^

error: unused import: `ContentSerializer`
  --> C:\Users\Christopher Serr\.cargo\registry\src\git.colasdn.top-1ecc6299db9ec823\serde-1.0.62\src\private\ser.rs:15:14
   |
15 |     Content, ContentSerializer, SerializeStructVariantAsMapValue, SerializeTupleVariantAsMapValue,
   |              ^^^^^^^^^^^^^^^^^

error: Compilation failed, aborting rustdoc

error: Could not document `serde`.

[dtolnay]

This was a consequence of a deny(unused_imports) regression that has been fixed. See #51661.