Prepare proc_macro for decoupling it from the rest of the compiler.

[eddyb]

This is #49219 up to the point where the bridge is introduced. Aside from moving some code around, the largest change is the rewrite of proc_macro::quote to be simpler and do less introspection.

I'd like to also extend quote! with ${stmt;...;expr} instead of just $variable (and maybe even $(... $iter ...)*), which seems pretty straight-forward now, but I don't know if/when I should.

r? @alexcrichton or @dtolnay cc @jseyfried @petrochenkov

[rust-highfive]

The job x86_64-gnu-llvm-5.0 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:start:test_run-pass-fulldeps
Check compiletest suite=run-pass-fulldeps mode=run-pass (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
[00:52:25] 
[00:52:25] running 95 tests
[00:54:14] ..................................................................test [run-pass] run-pass-fulldeps/myriad-closures.rs has been running for over 60 seconds
elp: message: `"_a"` is not a valid identifier
[00:55:37] error: aborting due to previous error
[00:55:37] 
[00:55:37] 
[00:55:37] ------------------------------------------
---
[00:55:37] test result: FAILED. 94 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
[00:55:37] 
[00:55:37] 
[00:55:37] 
[00:55:37] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/compiletest" "--compile-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" "--run-lib-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "--rustc-path" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "--src-base" "/checkout/src/test/run-pass-fulldeps" "--build-base" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass-fulldeps" "--stage-id" "stage2-x86_64-unknown-linux-gnu" "--mode" "run-pass" "--target" "x86_64-unknown-linux-gnu" "--host" "x86_64-unknown-linux-gnu" "--llvm-filecheck" "/usr/lib/llvm-5.0/bin/FileCheck" "--host-rustcflags" "-Crpath -O -Zunstable-options " "--target-rustcflags" "-Crpath -O -Zunstable-options  -Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "--docck-python" "/usr/bin/python2.7" "--lldb-python" "/usr/bin/python2.7" "--gdb" "/usr/bin/gdb" "--quiet" "--llvm-version" "5.0.0\n" "--system-llvm" "--cc" "" "--cxx" "" "--cflags" "" "--llvm-components" "" "--llvm-cxxflags" "" "--adb-path" "adb" "--adb-test-dir" "/data/tmp/work" "--android-cross-path" "" "--color" "always"
[00:55:37] 
[00:55:37] 
[00:55:37] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
[00:55:37] Build completed unsuccessfully in 0:13:43
[00:55:37] Build completed unsuccessfully in 0:13:43
[00:55:37] make: *** [check] Error 1
[00:55:37] Makefile:58: recipe for target 'check' failed

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:0666f117
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)

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)

[alexcrichton]

@bors: r+

[bors]

📌 Commit 99eac01 has been approved by alexcrichton

[bors]

⌛ Testing commit 99eac01 with merge bd455ef...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing bd455ef to master...