Assert that sysroot ABI version matches exactly

Otherwise, fall back to the multi ABI scheme, except in testing, where
it becomes a hard error.

This should make it possible to use a rustup-provided rust-analyzer with
proc macro dylibs compiled by older rustcs, and it'll also catch changes
to the format of `rustc --version` or the `.rustc` section that would
make them impossible to compare for equality.

crates/proc-macro-api/src/lib.rs

@@ -26,7 +26,7 @@ use crate::{
     process::ProcMacroProcessSrv,
 };
 
-pub use version::{read_dylib_info, RustCInfo};
+pub use version::{read_dylib_info, read_version, RustCInfo};
 
 #[derive(Copy, Clone, Eq, PartialEq, Debug, Serialize, Deserialize)]
 pub enum ProcMacroKind {

crates/proc-macro-api/src/version.rs

@@ -102,7 +102,7 @@ fn read_section<'a>(dylib_binary: &'a [u8], section_name: &str) -> io::Result<&'
 /// * [some more bytes that we don't really care but about still there] :-)
 /// Check this issue for more about the bytes layout:
 /// <https://github.com/rust-lang/rust-analyzer/issues/6174>
-fn read_version(dylib_path: &AbsPath) -> io::Result<String> {
+pub fn read_version(dylib_path: &AbsPath) -> io::Result<String> {
     let dylib_file = File::open(dylib_path)?;
     let dylib_mmaped = unsafe { Mmap::map(&dylib_file) }?;
 

crates/proc-macro-srv/build.rs

@@ -0,0 +1,25 @@
+use std::{env, fs::File, io::Write, path::PathBuf, process::Command};
+
+fn main() {
+    // Determine rustc version `proc-macro-srv` (and thus the sysroot ABI) is
+    // build with and make it accessible at runtime for ABI selection.
+
+    let mut path = PathBuf::from(env::var_os("OUT_DIR").unwrap());
+    path.push("rustc_version.rs");
+    let mut f = File::create(&path).unwrap();
+
+    let rustc = env::var("RUSTC").expect("proc-macro-srv's build script expects RUSTC to be set");
+    let output = Command::new(rustc).arg("--version").output().expect("rustc --version must run");
+    let version_string = std::str::from_utf8(&output.stdout[..])
+        .expect("rustc --version output must be UTF-8")
+        .trim();
+
+    write!(
+        f,
+        "
+    #[allow(dead_code)]
+    pub(crate) const RUSTC_VERSION_STRING: &str = {version_string:?};
+    "
+    )
+    .unwrap();
+}

crates/proc-macro-srv/src/abis/mod.rs

@@ -29,6 +29,9 @@ mod abi_1_64;
 #[cfg(feature = "sysroot-abi")]
 mod abi_sysroot;
 
+// see `build.rs`
+include!(concat!(env!("OUT_DIR"), "/rustc_version.rs"));
+
 // Used by `test/utils.rs`
 #[cfg(test)]
 pub(crate) use abi_1_64::TokenStream as TestTokenStream;
@@ -74,15 +77,39 @@ impl Abi {
         lib: &Library,
         symbol_name: String,
         info: RustCInfo,
+        #[cfg_attr(not(feature = "sysroot-abi"), allow(unused_variables))] version_string: String,
     ) -> Result<Abi, LoadProcMacroDylibError> {
-        // Gated behind an env var for now to avoid a change in behavior for
-        // rustup-installed rust-analyzer
+        // the sysroot ABI relies on `extern proc_macro` with unstable features,
+        // instead of a snapshot of the proc macro bridge's source code. it's only
+        // enabled if we have an exact version match.
         #[cfg(feature = "sysroot-abi")]
-        if std::env::var("PROC_MACRO_SRV_SYSROOT_ABI").is_ok() {
+        {
+            if version_string == RUSTC_VERSION_STRING {
                 let inner = unsafe { Abi_Sysroot::from_lib(lib, symbol_name) }?;
                 return Ok(Abi::AbiSysroot(inner));
             }
 
+            // if we reached this point, versions didn't match. in testing, we
+            // want that to panic - this could mean that the format of `rustc
+            // --version` no longer matches the format of the version string
+            // stored in the `.rustc` section, and we want to catch that in-tree
+            // with `x.py test`
+            #[cfg(test)]
+            {
+                let allow_mismatch = std::env::var("PROC_MACRO_SRV_ALLOW_SYSROOT_MISMATCH");
+                if let Ok("1") = allow_mismatch.as_deref() {
+                    // only used by rust-analyzer developers, when working on the
+                    // sysroot ABI from the rust-analyzer repository - which should
+                    // only happen pre-subtree. this can be removed later.
+                } else {
+                    panic!(
+                        "sysroot ABI mismatch: dylib rustc version (read from .rustc section): {:?} != proc-macro-srv version (read from 'rustc --version'): {:?}",
+                        version_string, RUSTC_VERSION_STRING
+                    );
+                }
+            }
+        }
+
         // FIXME: this should use exclusive ranges when they're stable
         // https://github.com/rust-lang/rust/issues/37854
         match (info.version.0, info.version.1) {

crates/proc-macro-srv/src/dylib.rs

@@ -12,7 +12,7 @@ use libloading::Library;
 use memmap2::Mmap;
 use object::Object;
 use paths::AbsPath;
-use proc_macro_api::{read_dylib_info, ProcMacroKind};
+use proc_macro_api::{read_dylib_info, read_version, ProcMacroKind};
 
 use super::abis::Abi;
 
@@ -122,9 +122,10 @@ impl ProcMacroLibraryLibloading {
             invalid_data_err(format!("expected an absolute path, got {}", file.display()))
         })?;
         let version_info = read_dylib_info(abs_file)?;
+        let version_string = read_version(abs_file)?;
 
         let lib = load_library(file).map_err(invalid_data_err)?;
-        let abi = Abi::from_lib(&lib, symbol_name, version_info)?;
+        let abi = Abi::from_lib(&lib, symbol_name, version_info, version_string)?;
         Ok(ProcMacroLibraryLibloading { _lib: lib, abi })
     }
 }