From 496056fb26fec158af8c045c0aefd813d82b9f9f Mon Sep 17 00:00:00 2001
From: Vadim Petrochenkov <vadim.petrochenkov@gmail.com>
Date: Mon, 3 Aug 2026 11:33:41 +0300
Subject: [PATCH] Use `thread::available_parallelism` as the default limit for
 backend parallelism

---
 compiler/rustc_interface/src/interface.rs |  6 ++---
 compiler/rustc_session/src/config.rs      | 29 ++++-------------------
 2 files changed, 7 insertions(+), 28 deletions(-)

diff --git a/compiler/rustc_interface/src/interface.rs b/compiler/rustc_interface/src/interface.rs
index 2ad6fb6450a16..18f869d24cbfb 100644
--- a/compiler/rustc_interface/src/interface.rs
+++ b/compiler/rustc_interface/src/interface.rs
@@ -15,7 +15,7 @@ use rustc_parse::lexer::StripTokens;
 use rustc_parse::new_parser_from_source_str;
 use rustc_parse::parser::Recovery;
 use rustc_query_impl::print_query_stack;
-use rustc_session::config::{self, BackendJobs, Cfg, CheckCfg, ExpectedValues, Input, OutFileName};
+use rustc_session::config::{self, Cfg, CheckCfg, ExpectedValues, Input, OutFileName};
 use rustc_session::parse::ParseSess;
 use rustc_session::{CompilerIO, EarlyDiagCtxt, Session, lint};
 use rustc_span::source_map::{FileLoader, RealFileLoader, SourceMapInputs};
@@ -375,9 +375,7 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
 
     // Initialize jobserver as early as possible.
     let early_dcx = EarlyDiagCtxt::new(config.opts.error_format);
-    if let Some(limit) =
-        config.opts.jobs.frontend.max(config.opts.jobs.backend.map(BackendJobs::value))
-    {
+    if let Some(limit) = config.opts.jobs.frontend.max(config.opts.jobs.backend) {
         jobserver::initialize(limit.get(), |err| {
             let note = "the build environment is likely misconfigured";
             early_dcx.early_struct_warn(err).with_note(note).emit()
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 022784b56d4ce..1cfd03288a417 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -1649,26 +1649,6 @@ impl PointerAuthOption {
     }
 }
 
-#[derive(Clone, Copy)]
-pub enum BackendJobs {
-    /// The number of backend jobs has a static limit.
-    Limited(NonZero<usize>),
-    /// The number of backend jobs is either unlimited if there's an inherited jobserver,
-    /// or limited to 32 if there's no inherited jobserver.
-    /// This variant exists only to preserve the historical behavior.
-    /// FIXME: Just use `thread::available_parallelism` as the default static limit.
-    UnlimitedOr32,
-}
-
-impl BackendJobs {
-    pub fn value(self) -> NonZero<usize> {
-        match self {
-            BackendJobs::Limited(n) => n,
-            BackendJobs::UnlimitedOr32 => NonZero::new(32).unwrap(),
-        }
-    }
-}
-
 #[derive(Clone, Copy)]
 pub enum LinkerJobs {
     /// Do not pass anything to the linker, use it's default behavior.
@@ -1682,7 +1662,7 @@ pub enum LinkerJobs {
 #[derive(Clone, Copy)]
 pub struct Jobs {
     pub frontend: Option<NonZero<usize>>,
-    pub backend: Option<BackendJobs>,
+    pub backend: Option<NonZero<usize>>,
     pub linker: LinkerJobs,
 }
 
@@ -1735,11 +1715,12 @@ fn parse_jobs_all(
             let backend =
                 parse_jobs_one(early_dcx, opt_name, &jobs_backend, unstable, &mut available);
             check_upper_limit(backend, opt_name);
-            backend.map(BackendJobs::Limited)
+            backend
         }
         None => match jobs {
-            Some(n) => n.map(BackendJobs::Limited),
-            None => Some(BackendJobs::UnlimitedOr32),
+            Some(n) => n,
+            // Use all available parallelism as the default.
+            None => parse_jobs_one(early_dcx, "", "0", unstable, &mut available),
         },
     };
     let linker = match matches.opt_str("jobs-linker") {