Rollup merge of #138783 - bjorn3:cache_current_dll_path, r=lqd

Cache current_dll_path output

Computing the current dll path is somewhat expensive relative to other work when compiling `fn main() {}` as `dladdr` needs to iterate over the symbol table of librustc_driver.so until it finds a match.

compiler/rustc_session/src/filesearch.rs

@@ -60,6 +60,14 @@ pub fn make_target_bin_path(sysroot: &Path, target_triple: &str) -> PathBuf {
 
 #[cfg(unix)]
 fn current_dll_path() -> Result<PathBuf, String> {
+    use std::sync::OnceLock;
+
+    // This is somewhat expensive relative to other work when compiling `fn main() {}` as `dladdr`
+    // needs to iterate over the symbol table of librustc_driver.so until it finds a match.
+    // As such cache this to avoid recomputing if we try to get the sysroot in multiple places.
+    static CURRENT_DLL_PATH: OnceLock<Result<PathBuf, String>> = OnceLock::new();
+    CURRENT_DLL_PATH
+        .get_or_init(|| {
             use std::ffi::{CStr, OsStr};
             use std::os::unix::prelude::*;
 
@@ -115,11 +123,13 @@ fn current_dll_path() -> Result<PathBuf, String> {
                     if (*current).ldinfo_next == 0 {
                         break;
                     }
-            current =
-                (current as *mut i8).offset((*current).ldinfo_next as isize) as *mut libc::ld_info;
+                    current = (current as *mut i8).offset((*current).ldinfo_next as isize)
+                        as *mut libc::ld_info;
                 }
                 return Err(format!("current dll's address {} is not in the load map", addr));
             }
+        })
+        .clone()
 }
 
 #[cfg(windows)]