Comment ext::tt::transcribe

[mark-i-m]

Also did a bit of minor cleanup (remove unidiomatic use of Add and an unneeded clone). No functionality changes.

r? @petrochenkov

[rust-highfive]

The job x86_64-gnu-llvm-6.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:07d2fecc:start=1557270259665799738,finish=1557270346337693066,duration=86671893328
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout
Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#pull-requests-and-security-restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
$ export GCP_CACHE_BUCKET=rust-lang-ci-cache
$ export AWS_ACCESS_KEY_ID=AKIA46X5W6CZEJZ6XT55
---
[00:26:11] Compiling core v0.0.0 (/checkout/src/libcore)
[00:26:11] Compiling libc v0.2.54
[00:26:11] Compiling build_helper v0.1.0 (/checkout/src/build_helper)
[00:26:12] Compiling unwind v0.0.0 (/checkout/src/libunwind)
[00:26:12] thread 'rustc' panicked at 'assertion failed: src.len() > 0', src/libsyntax/ext/tt/transcribe.rs:74:5
[00:26:12] [00:26:12] error: internal compiler error: unexpected panic
[00:26:12] [00:26:12] note: the compiler unexpectedly panicked. this is a bug.
[00:26:12] note: the compiler unexpectedly panicked. this is a bug.
[00:26:12] [00:26:12] note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
[00:26:12] [00:26:12] note: rustc 1.36.0-dev running on x86_64-unknown-linux-gnu
[00:26:12] [00:26:12] note: compiler flags: -Z external-macro-backtrace -Z unstable-options -Z force-unstable-if-unmarked -C opt-level=2 -C prefer-dynamic -C debug-assertions=y -C codegen-units=1 -C link-args=-Wl,-rpath,$ORIGIN/../lib --crate-type lib
[00:26:12] note: some of the compiler flags provided by cargo are hidden
[00:26:12] [00:26:12] error: Could not compile `core`.
[00:26:12] warning: build failed, waiting for other jobs to finish...

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[mark-i-m]

@jseyfried If you have any feedback here, that would be appreciated too.

[petrochenkov]

@bors r+

[bors]

📌 Commit eb7d47c has been approved by petrochenkov

[bors]

⌛ Testing commit eb7d47c with merge 03bd2f6...