rustc: Disallow modules and macros in expansions

[alexcrichton]

This commit feature gates generating modules and macro definitions in procedural
macro expansions. Custom derive is exempt from this check as it would be a large
retroactive breaking change (#50587). It's hoped that we can hopefully stem the
bleeding to figure out a better solution here before opening up the floodgates.

The restriction here is specifically targeted at surprising hygiene results 1
that result in non-"copy/paste" behavior. Hygiene and procedural macros is
intended to be avoided as much as possible for Macros 1.2 by saying everything
is "as if you copy/pasted the code", but modules and macros are sort of weird
exceptions to this rule that aren't fully fleshed out.

cc #50504

[rust-highfive]

r? @eddyb

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[alexcrichton]

r? @petrochenkov

[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.

travis_time:start:tidy
tidy check
[00:04:39] * 542 error codes
[00:04:39] * highest error code: E0911
[00:04:40] tidy error: Found 1 features without a gate test.
[00:04:40] Expected a gate test for the feature 'proc_macro_gen'.
[00:04:40] Hint: create a failing test file named 'feature-gate-proc_macro_gen.rs'
[00:04:40]       in the 'ui' test suite, with its failures due to
[00:04:40]       missing usage of #![feature(proc_macro_gen)].
[00:04:40] Hint: If you already have such a test and don't want to rename it,
[00:04:40]       you can also add a // gate-test-proc_macro_gen line to the test file.
[00:04:40] some tidy checks failed
[00:04:40] 
[00:04:40] 
[00:04:40] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:04:40] 
[00:04:40] 
[00:04:40] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:40] Build completed unsuccessfully in 0:01:47
[00:04:40] Build completed unsuccessfully in 0:01:47
[00:04:40] Makefile:79: recipe for target 'tidy' failed
[00:04:40] make: *** [tidy] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:04c5039e
$ 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)

[petrochenkov]

Couple of tests?
One for a module, one for a macro and one for nested module/macro inside some non-module/macro item like fn f() { mod m { ... } }.
r=me after that

[alexcrichton]

@bors: r=petrochenkov

[bors]

📌 Commit 81fc850 has been approved by petrochenkov

[kennytm]

@bors r-

run-pass-fulldeps/macro-quote-test.rs is missing a #![feature(proc_macro_gen)]. (Log: https://travis-ci.org/rust-lang/rust/jobs/380273579)

[01:13:54] ---- [run-pass] run-pass-fulldeps/macro-quote-test.rs stdout ----
[01:13:54] 	
[01:13:54] error: compilation failed!
[01:13:54] status: exit code: 101
[01:13:54] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/src/test/run-pass-fulldeps/macro-quote-test.rs" "--target=x86_64-unknown-linux-gnu" "-C" "prefer-dynamic" "-o" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass-fulldeps/macro-quote-test/a" "-Crpath" "-O" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass-fulldeps/macro-quote-test/auxiliary"
[01:13:54] stdout:
[01:13:54] ------------------------------------------
[01:13:54] 
[01:13:54] ------------------------------------------
[01:13:54] stderr:
[01:13:54] ------------------------------------------
[01:13:54] error[E0658]: procedural macros cannot expand to macro definitions
[01:13:54]   --> /checkout/src/test/run-pass-fulldeps/macro-quote-test.rs:21:5
[01:13:54]    |
[01:13:54] 21 |     hello_macro::hello!();
[01:13:54]    |     ^^^^^^^^^^^^^^^^^^^^^^
[01:13:54]    |
[01:13:54]    = help: add #![feature(proc_macro_gen)] to the crate attributes to enable
[01:13:54] 
[01:13:54] error: aborting due to previous error
[01:13:54] 
[01:13:54] For more information about this error, try `rustc --explain E0658`.
[01:13:54] 
[01:13:54] ------------------------------------------
[01:13:54] 
[01:13:54] thread '[run-pass] run-pass-fulldeps/macro-quote-test.rs' panicked at 'explicit panic', tools/compiletest/src/runtest.rs:3014:9
[01:13:54] note: Run with `RUST_BACKTRACE=1` for a backtrace.
[01:13:54] 
[01:13:54] 
[01:13:54] failures:
[01:13:54]     [run-pass] run-pass-fulldeps/macro-quote-test.rs
[01:13:54] 
[01:13:54] test result: FAILED. 89 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

[alexcrichton]

@bors: r=petrochenkov

[bors]

📌 Commit 5e4bac3 has been approved by petrochenkov

[bors]

⌛ Testing commit 5e4bac3 with merge ccb5e97...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: petrochenkov
Pushing ccb5e97 to master...

[sgrif]

Since #51952 landed, do we still need this feature gate? It looks like the underlying problem has now been fixed.

[petrochenkov]

@sgrif
#53459 partially stabilized proc_macro_gen recently (the module part).

[sgrif]

Thanks