Fix compiler panic with a large number of threads

Author: practicalrs

Diff for: compiler/rustc_session/src/config.rs

@@ -2464,6 +2464,10 @@ pub fn build_session_options(early_dcx: &mut EarlyDiagCtxt, matches: &getopts::M
         early_dcx.early_fatal("value for threads must be a positive non-zero integer");
     }
 
+    if unstable_opts.threads == parse::MAX_THREADS_CAP {
+        early_dcx.early_warn(format!("number of threads was capped at {}", parse::MAX_THREADS_CAP));
+    }
+
     let fuel = unstable_opts.fuel.is_some() || unstable_opts.print_fuel.is_some();
     if fuel && unstable_opts.threads > 1 {
         early_dcx.early_fatal("optimization fuel is incompatible with multiple threads");

Diff for: compiler/rustc_session/src/options.rs

@@ -457,10 +457,11 @@ mod desc {
         "either a boolean (`yes`, `no`, `on`, `off`, etc), or `nll` (default: `nll`)";
 }
 
-mod parse {
+pub mod parse {
     use std::str::FromStr;
 
     pub(crate) use super::*;
+    pub(crate) const MAX_THREADS_CAP: usize = 256;
 
     /// This is for boolean options that don't take a value and start with
     /// `no-`. This style of option is deprecated.
@@ -663,7 +664,9 @@ 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
+                *slot = i.min(MAX_THREADS_CAP);
                 true
             }
             None => false,