[1.38] Regression: failed to resolve: could not find rustc_serialize in {{root}}

[petrochenkov]

#63628 (comment)

Caused by #63449.
Tests in rustc-serialize and its fork rustc-serialize2 are broken, no other crates are affected.
(Can be fixed with extern crate self as rustc_serivalize;.)
Probably no action required.

[nikomatsakis]

Check-in from compiler team:

This is some sort of issue triggered by upgrading RustcSerialize to Rust 2018. It is specific to the rustc-serialize crate itself, as partially caused by having derive(RustcSerialize) in a "semi-blessed" state, i.e., built-in to compiler but also in a crate. Not a general purpose problem. Closing as "no action required".

[petrochenkov]

The reason of breakage is that #63449 migrated derive(RustcEncodable, RustcDecodable) to 2018 edition as a part of compiler simplification (these macros now take their edition from libcore in which they are defined).

As a result, code like this was broken:

extern crate serialize;
use serialize as rustc_serialize;

// uses `::rustc_serialize` internally,
// on 2018 edition it means *crate* rustc_serialize,
// not a name in root module.
#[derive(RustcEncodable)]
struct S;

Code like this appears either in code using unstable crate serialize from sysroot, or inside rustc-serialize itself in testing mode, which uses a similar construction:

mod rustc_serialize {
    pub use serialize::*;
}

// `::rustc_serialize` no longer resolves to the module in the root on 2018 edition
#[derive(RustcEncodable)]
struct S;