docs: Trying to access the source code (`[src]` link) of `str::rc::Rc` yields a "Not Found" page

[japaric]

STR

• Head to the docs of std::rc::Rc [1]
• Click the [src] link [2]
• Yields a "Not Found" page [3]
• Head to the docs of alloc::rc::Rc [4]
• Click the [src] link [5] - this one works

Here are the full text links:

[1] http://doc.rust-lang.org/std/rc/struct.Rc.html
[2] http://doc.rust-lang.org/alloc/rc/struct.Rc.html?gotosrc=1835
[3] http://doc.rust-lang.org/alloc/rc/undefined
[4] http://doc.rust-lang.org/alloc/rc/struct.Rc.html
[5] http://doc.rust-lang.org/src/alloc/home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/liballoc/rc.rs.html#174-180

[japaric]

Duplicate of #15309. Closing.