cache shared_helpers::parse_rustc_verbose result

Signed-off-by: onur-ozkan <[email protected]>

Author: onur-ozkan

src/bootstrap/src/utils/shared_helpers.rs

@@ -12,6 +12,7 @@ use std::fs::OpenOptions;
 use std::io::Write;
 use std::process::Command;
 use std::str::FromStr;
+use std::sync::Once;
 
 #[cfg(test)]
 mod tests;
@@ -65,9 +66,21 @@ pub fn exe(name: &str, target: &str) -> String {
 /// Panics if "RUSTC_VERBOSE" is defined with the value that is not an unsigned integer.
 #[inline(always)]
 pub fn parse_rustc_verbose() -> usize {
-    match env::var("RUSTC_VERBOSE") {
-        Ok(s) => usize::from_str(&s).expect("RUSTC_VERBOSE should be an integer"),
-        Err(_) => 0,
+    static mut VERBOSITY: usize = 0;
+    static INIT: Once = Once::new();
+
+    // SAFETY: Write only happens once.
+    unsafe {
+        // Result of this function never changes, so cache it to avoid recomputing
+        // it repeatedly as it’s called thousands of times from the rustc shim.
+        INIT.call_once(|| {
+            VERBOSITY = match env::var("RUSTC_VERBOSE") {
+                Ok(s) => usize::from_str(&s).expect("RUSTC_VERBOSE should be an integer"),
+                Err(_) => 0,
+            };
+        });
+
+        VERBOSITY
     }
 }