Fix compiler panic with a large number of threads

Author: practicalrs

compiler/rustc_session/src/options.rs

@@ -663,7 +663,13 @@ mod parse {
                 true
             }
             Some(i) => {
-                *slot = i;
+                // We want to cap the number of threads here to avoid large numbers like 999999 and compiler panics.
+                // This solution was suggested here https://github.com/rust-lang/rust/issues/117638#issuecomment-1800925067
+                if i <= 256 {
+                    *slot = i;
+                } else {
+                    *slot = 256;
+                }
                 true
             }
             None => false,