Source is missing from docs for crates that use a procedural macro

[jdm]

Running ./mach doc on Servo's codebase yields a bunch of crates with the following warning:

warning: source code was requested to be rendered, but processing `<proc-macro source code>` had an error: No such file or directory (os error 2)
         skipping rendering of source code

This means that the source is available for some types, but not others.

I'll try to figure out when this regressed.

[jdm]

Minimal reproduction:

extern crate heapsize;
#[macro_use] extern crate heapsize_derive;

#[derive(HeapSizeOf)]
pub struct Range;

[package]
name = "foo"
version = "0.1.0"
authors = ["Josh Matthews <josh@joshmatthews.net>"]

[dependencies]
heapsize = "0.4"
heapsize_derive = "0.1"

If I remove the pub or HeapSizeOf derivation then it works fine.

[jdm]

If I use cargo +beta doc --open then it works fine; cargo +nightly doc --open is broken.

[jdm]

godot:foo jdm$ cargo --version
cargo 0.21.0-nightly (eb6cf012a 2017-07-02)
godot:foo jdm$ rustc --version
rustc 1.20.0-nightly (b2c070787 2017-07-13)

[jdm]

The 7/6 nightly (3610a70) works fine, and the 7/7 nightly (696412d) is broken.

[jdm]

My money is on #40939.

[alexcrichton]

cc @jseyfried, @nrc

[nrc]

I believe the issue is that (since #40939) we no longer accurately track expansion info for names declared in a macro. That means that tools think some names are hand-written, when they are macro generated (this is also causing problems for the RLS). #43179 was meant to fix this, but it did not (at least not for all cases). I haven't looked into the details.

[oli-obk]

Not just names. Clippy gets confused by the true or false inside the expansion of cfg!: #43268