Rustc dumping AST in json fails on nightly

[alobb]

Attempting to use the '-Z ast-json' option to nightly fails to print any output.

Steps to reproduce:

cargo new --bin temp
cd temp
rustc -Z ast-json src/main.rs

I expected to see this happen: JSON output to the terminal

Instead, this happened: Nothing is printed to stdout

Meta

rustc --version --verbose:
rustc 1.10.0-nightly (476fe6e 2016-05-21)
binary: rustc
commit-hash: 476fe6e
commit-date: 2016-05-21
host: x86_64-apple-darwin
release: 1.10.0-nightly

I am using the new version of rustup to manage toolchains, but I don't think that is important here. Just in case:
rustup --version:
rustup 0.1.12 (c6e430a 2016-05-12)

rustup show:

installed toolchains

stable-x86_64-apple-darwin
beta-x86_64-apple-darwin
nightly-x86_64-apple-darwin (default)

active toolchain

nightly-x86_64-apple-darwin (default)
rustc 1.10.0-nightly (476fe6e 2016-05-21)

[jseyfried]

This was caused by #33450 and fixed in #33997 (cc @nrc).