Add PROC_MACRO_TEST_TOOLCHAIN environment variable
This allows overriding the toolchain used to run `proc-macro-srv` tests.
---
Sample usage.
Testing the current ABI (variable unset/empty):
```shell
amos@tails ~/bearcove/rust-analyzer/crates/proc-macro-srv proc-macro-test-toolchain*
❯ PROC_MACRO_TEST_TOOLCHAIN="" cargo test --quiet
running 16 tests
................
test result: ok. 16 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s
```
Testing an older ABI:
```shell
amos@tails ~/bearcove/rust-analyzer/crates/proc-macro-srv proc-macro-test-toolchain*
❯ PROC_MACRO_TEST_TOOLCHAIN="1.58" cargo test --quiet
running 16 tests
................
test result: ok. 16 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s
```
Testing current nightly ABI:
```shell
❯ rustc +nightly --version
rustc 1.64.0-nightly (f8588549c 2022-07-18)
❯ PROC_MACRO_TEST_TOOLCHAIN="nightly" cargo test --quiet
running 16 tests
................
test result: ok. 16 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s
```
Testing future ABI (`rust-lang/rust` master):
```shell
amos@tails ~/bearcove/rust-analyzer/crates/proc-macro-srv proc-macro-test-toolchain
❯ PROC_MACRO_TEST_TOOLCHAIN="stage1" cargo test --quiet
running 16 tests
..........thread '<unnamed>' panicked at 'range end index 216221164920373249 out of range for slice of length 18', library/core/src/slice/index.rs:73:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
....F.
failures:
---- tests::test_fn_like_macro2 stdout ----
thread 'tests::test_fn_like_macro2' panicked at 'called `Result::unwrap()` on an `Err` value: "range end index 216221164920373249 out of range for slice of length 18"', crates/proc-macro-srv/src/tests/utils.rs:38:83
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failures:
tests::test_fn_like_macro2
test result: FAILED. 15 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
error: test failed, to rerun pass '--lib
```
---
Tagging `@jonas-schievink:` this might be helpful when updating versioned ABIs later on.
Add proc-macro-srv integration test that clones literals
This exercises some of the upcoming proc_macro bridge changes. It should also pass for all supported ABIs, with the older-style bridge.
This changed is tracked in:
* https://github.com/rust-lang/rust-analyzer/issues/12818
Building it in-place fails in rust CI because the source directory
is read-only. This changes `proc-macro-test`'s build script to first
copy `imp` under `OUT_DIR` (which is read-write).
It also prints stdout/stderr for the nested cargo invocation, should
it fail. (I've seen failures in rust CI that I couldn't explain, and
when they take 25 minutes to reproduce, you want to have that info)
