rustdoc --test mis-reports origins of #[deny(warnings)]

[kentfredric]

Here's an example I stumbled onto with a copy of rand 0.5.6:

 RUSTFLAGS="--cap-lints warn" cargo test --doc  --verbose
... 
 test src/prng/chacha.rs - prng::chacha::ChaChaRng::new_unseeded (line 115) ... ok
test src/prelude.rs - prelude (line 17) ... ok
test src/rngs/jitter.rs - rngs::jitter::JitterRng (line 72) ... FAILED
test src/lib.rs - sample (line 1020) ... ok
test src/rngs/adapter/read.rs - rngs::adapter::read::ReadRng (line 35) ... ok
test src/rngs/jitter.rs - rngs::jitter::JitterRng::new_with_timer (line 318) ... ok
...
failures:

---- src/rngs/jitter.rs - rngs::jitter::JitterRng (line 72) stdout ----
error: trait objects without an explicit `dyn` are deprecated
  --> src/rngs/jitter.rs:79:33
   |
10 | fn try_main() -> Result<(), Box<Error>> {
   |                                 ^^^^^ help: use `dyn`: `dyn Error`
   |
note: lint level defined here
  --> src/rngs/jitter.rs:71:9
   |
2  | #![deny(warnings)]
   |         ^^^^^^^^
   = note: `#[deny(bare_trait_objects)]` implied by `#[deny(warnings)]`

error: aborting due to previous error

Couldn't compile the test.

failures:
    src/rngs/jitter.rs - rngs::jitter::JitterRng (line 72)

Note there is a lot wrong here:

1. The line numbers on the "source listing" don't correspond at all to the context line given ( 2 vs 71 )
2. The lines cited don't at all correspond with any of the files mentioned.

cat -n src/rngs/jitter.rs | grep -C3 "^\s*71\s"
    68	/// https://github.com/usnistgov/SP800-90B_EntropyAssessment).
    69	///
    70	/// Use the following code using [`timer_stats`] to collect the data:
    71	///
    72	/// ```no_run
    73	/// use rand::jitter::JitterRng;
    74	/// #

 cat -n src/rngs/jitter.rs | grep -C3 "^\s*2\s"
     1	// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
     2	// file at the top-level directory of this distribution and at
     3	// https://rust-lang.org/COPYRIGHT.
     4	//
     5	// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or

No, instead, this rule is inserted somewhere else entirely:

 grep -C3 "deny" -nr src/
src/lib.rs-226-       html_favicon_url = "https://www.rust-lang.org/favicon.ico",
src/lib.rs-227-       html_root_url = "https://docs.rs/rand/0.5.6")]
src/lib.rs-228-
src/lib.rs:229:#![deny(missing_docs)]
src/lib.rs:230:#![deny(missing_debug_implementations)]
src/lib.rs:231:#![doc(test(attr(allow(unused_variables), deny(warnings))))]
src/lib.rs-232-
src/lib.rs-233-#![cfg_attr(not(feature="std"), no_std)]
src/lib.rs-234-#![cfg_attr(all(feature="alloc", not(feature="std")), feature(alloc))]

Obviously macros are to blame here, but as it stands, this makes this reported error source entirely useless, not only is it getting the line and the file wrong, it cites a literal string that doesn't even exist as-stated in the source code, presenting it under the guise of being literal source code.

I'm probably gonna also have to file another bug about that whole "--cap-lints warn does absolutely nothing" deal it has going on there.