Auto merge of rust-lang#12834 - fasterthanlime:proc-macro-test-toolchain, r=Veykril

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 (f858854 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.

Author: bors

crates/proc-macro-test/build.rs

@@ -2,6 +2,10 @@
 //! `OUT_DIR`.
 //!
 //! `proc-macro-test` itself contains only a path to that artifact.
+//!
+//! The `PROC_MACRO_TEST_TOOLCHAIN` environment variable can be exported to use
+//! a specific rustup toolchain: this allows testing against older ABIs (e.g.
+//! 1.58) and future ABIs (stage1, nightly)
 
 use std::{
     env, fs,
@@ -13,6 +17,7 @@ use cargo_metadata::Message;
 
 fn main() {
     println!("cargo:rerun-if-changed=imp");
+    println!("cargo:rerun-if-env-changed=PROC_MACRO_TEST_TOOLCHAIN");
 
     let out_dir = env::var_os("OUT_DIR").unwrap();
     let out_dir = Path::new(&out_dir);
@@ -47,7 +52,17 @@ fn main() {
     }
 
     let target_dir = out_dir.join("target");
-    let output = Command::new(toolchain::cargo())
+
+    let mut cmd = if let Ok(toolchain) = std::env::var("PROC_MACRO_TEST_TOOLCHAIN") {
+        // leverage rustup to find user-specific toolchain
+        let mut cmd = Command::new("cargo");
+        cmd.arg(format!("+{toolchain}"));
+        cmd
+    } else {
+        Command::new(toolchain::cargo())
+    };
+
+    let output = cmd
         .current_dir(&staging_dir)
         .args(&["build", "-p", "proc-macro-test-impl", "--message-format", "json"])
         // Explicit override the target directory to avoid using the same one which the parent