[WIP] undo #49719 for rust 2015 with feature flag

[mark-i-m]

Do not merge yet

#49719 was a breaking change that became insta-stable with an FCP. Pending the decision made in that thread, this PR can be used to roll back that PR for rust 2015. The change would only go into effect in rust 2018 edition.

cc @alexreg @sgrif @pietroalbini @durka @oli-obk, who were involved in that discussion

r? @petrochenkov

[mark-i-m]

@petrochenkov I think we can wait for an answer from the compiler team on the #49719 thread before you review this PR (to avoid wasting your time if this isn't the direction they want to go).

[rust-highfive]

The job x86_64-gnu-llvm-3.9 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.

[00:25:21]    Compiling syntax_pos v0.0.0 (file:///checkout/src/libsyntax_pos)
[00:25:25]    Compiling rustc_errors v0.0.0 (file:///checkout/src/librustc_errors)
[00:26:27]    Compiling proc_macro v0.0.0 (file:///checkout/src/libproc_macro)
[00:26:38]    Compiling syntax_ext v0.0.0 (file:///checkout/src/libsyntax_ext)
[00:26:39] warning: `?` as a separator is deprecated and will be removed in an upcoming edition.
[00:26:39]    --> librustc/lint/mod.rs:111:55
[00:26:39]     |
[00:26:39] 111 |      $lint_edition: expr => $edition_level: ident $(,)?
[00:26:39] 
[00:26:39] 
[00:26:39] error: expected `*` or `+`
[00:26:39]    --> librustc/lint/mod.rs:111:55
[00:26:39]     |
[00:26:39] 111 |      $lint_edition: expr => $edition_level: ident $(,)?
[00:26:39] 
[00:26:39] 
[00:26:39] warning: `?` as a separator is deprecated and will be removed in an upcoming edition.
[00:26:39]    --> librustc/lint/mod.rs:125:28
[00:26:39]     |
[00:26:39] 125 |     ($( $lint:expr ),* $(,)?) => {{
[00:26:39] 
[00:26:39] 
[00:26:39] error: expected `*` or `+`
[00:26:39]    --> librustc/lint/mod.rs:125:28
[00:26:39]     |
[00:26:39] 125 |     ($( $lint:expr ),* $(,)?) => {{
[00:26:39] 
[00:27:13] error: aborting due to 2 previous errors
[00:27:13] 
[00:27:13] error: Could not compile `rustc`.
[00:27:13] error: Could not compile `rustc`.
[00:27:13] 
[00:27:13] Caused by:
[00:27:13]   process didn't exit successfully: `/checkout/obj/build/bootstrap/debug/rustc --crate-name rustc librustc/lib.rs --color always --error-format json --crate-type dylib --emit=dep-info,link -C prefer-dynamic -C opt-level=2 -C metadata=75c66186192fe9ef -C extra-filename=-75c66186192fe9ef --out-dir /checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps --target x86_64-unknown-linux-gnu -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps -L dependency=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/release/deps --extern rustc_data_structures=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/librustc_data_structures-b06458981f2f8b08.so --extern flate2=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/libflate2-e0b46302cb6adfc1.rlib --extern byteorder=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/libbyteorder-da75a1391e0819dc.rlib --extern chalk_engine=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/libchalk_engine-ad6626ca6f9c7adf.rlib --extern serialize=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/libserialize-c8132f366b701b34.so --extern serialize=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/libserialize-c8132f366b701b34.rlib --extern tempdir=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/libtempdir-1f3205fe042ccbd0.rlib --extern rustc_errors=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/librustc_errors-c1fe07f143b7c210.so --extern bitflags=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/libbitflags-5281aac7329915d9.rlib --extern rustc_target=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/librustc_target-688e8d8d7a3790aa.so --extern lazy_static=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/liblazy_static-f80cd5bfeff4f735.rlib --extern arena=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/libarena-fb260ffdf5223bb7.so --extern proc_macro=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/libproc_macro-5f271ace49e1c4e5.so --extern backtrace=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/libbacktrace-a77c8710dcd8c364.rlib --extern syntax=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/libsyntax-a1814c1a386023f7.so --extern jobserver=/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/libjobserver-7e250215e5863400.rlib --extern fmt_macros=/checkout/obj/build/x86_64-unknown-linux-gn86_64-unknown-linux-gnu/stage0-rustc/x86_64-unknown-linux-gnu/release
13780 ./src/llvm/include/llvm
13476 ./.git/modules/src/stdsimd
13428 ./src/test/ui
travis_time:end:00ca13bb:start=1528080844921429757,finish=1528080845142746210,duration=221316453

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[pietroalbini]

Closing this, it was decided to revert the original PR. Following up in #51417.