From bc0170a43bd6e4f5beef12900d76a2aef89844af Mon Sep 17 00:00:00 2001
From: bjorn3 <17426603+bjorn3@users.noreply.github.com>
Date: Fri, 24 Jan 2025 12:15:11 +0000
Subject: [PATCH 1/2] Add a workaround for parallel rustc crashing when there
 are delayed bugs

This doesn't fix the root cause of this crash, but at least stops it
from happening for the time being.
---
 compiler/rustc_middle/src/ty/context.rs | 45 ++++++++++++++++---------
 1 file changed, 30 insertions(+), 15 deletions(-)

diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index 2d76f6ec7d690..00e5f4f3465df 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -1349,6 +1349,33 @@ pub struct GlobalCtxt<'tcx> {
 
     /// Stores memory for globals (statics/consts).
     pub(crate) alloc_map: Lock<interpret::AllocMap<'tcx>>,
+
+    current_gcx: CurrentGcx,
+}
+
+impl<'tcx> GlobalCtxt<'tcx> {
+    /// Installs `self` in a `TyCtxt` and `ImplicitCtxt` for the duration of
+    /// `f`.
+    pub fn enter<F, R>(&'tcx self, f: F) -> R
+    where
+        F: FnOnce(TyCtxt<'tcx>) -> R,
+    {
+        let icx = tls::ImplicitCtxt::new(self);
+
+        // Reset `current_gcx` to `None` when we exit.
+        let _on_drop = defer(move || {
+            *self.current_gcx.value.write() = None;
+        });
+
+        // Set this `GlobalCtxt` as the current one.
+        {
+            let mut guard = self.current_gcx.value.write();
+            assert!(guard.is_none(), "no `GlobalCtxt` is currently set");
+            *guard = Some(self as *const _ as *const ());
+        }
+
+        tls::enter_context(&icx, || f(icx.tcx))
+    }
 }
 
 /// This is used to get a reference to a `GlobalCtxt` if one is available.
@@ -1539,23 +1566,11 @@ impl<'tcx> TyCtxt<'tcx> {
             canonical_param_env_cache: Default::default(),
             data_layout,
             alloc_map: Lock::new(interpret::AllocMap::new()),
+            current_gcx,
         });
 
-        let icx = tls::ImplicitCtxt::new(&gcx);
-
-        // Reset `current_gcx` to `None` when we exit.
-        let _on_drop = defer(|| {
-            *current_gcx.value.write() = None;
-        });
-
-        // Set this `GlobalCtxt` as the current one.
-        {
-            let mut guard = current_gcx.value.write();
-            assert!(guard.is_none(), "no `GlobalCtxt` is currently set");
-            *guard = Some(&gcx as *const _ as *const ());
-        }
-
-        tls::enter_context(&icx, || f(icx.tcx))
+        // This is a separate function to work around a crash with parallel rustc (#135870)
+        gcx.enter(f)
     }
 
     /// Obtain all lang items of this crate and all dependencies (recursively)

From 6f543d5cebe220595174a135eca92765df8c97e3 Mon Sep 17 00:00:00 2001
From: bjorn3 <17426603+bjorn3@users.noreply.github.com>
Date: Fri, 24 Jan 2025 16:19:25 +0000
Subject: [PATCH 2/2] Add regression test

---
 tests/ui/parallel-rustc/cycle_crash.rs     |  5 +++++
 tests/ui/parallel-rustc/cycle_crash.stderr | 18 ++++++++++++++++++
 2 files changed, 23 insertions(+)
 create mode 100644 tests/ui/parallel-rustc/cycle_crash.rs
 create mode 100644 tests/ui/parallel-rustc/cycle_crash.stderr

diff --git a/tests/ui/parallel-rustc/cycle_crash.rs b/tests/ui/parallel-rustc/cycle_crash.rs
new file mode 100644
index 0000000000000..94ae11aef39d4
--- /dev/null
+++ b/tests/ui/parallel-rustc/cycle_crash.rs
@@ -0,0 +1,5 @@
+//@ compile-flags: -Z threads=2
+
+const FOO: usize = FOO; //~ERROR cycle detected when simplifying constant for the type system `FOO`
+
+fn main() {}
diff --git a/tests/ui/parallel-rustc/cycle_crash.stderr b/tests/ui/parallel-rustc/cycle_crash.stderr
new file mode 100644
index 0000000000000..7af3b8ee532c7
--- /dev/null
+++ b/tests/ui/parallel-rustc/cycle_crash.stderr
@@ -0,0 +1,18 @@
+error[E0391]: cycle detected when simplifying constant for the type system `FOO`
+  --> $DIR/cycle_crash.rs:3:1
+   |
+LL | const FOO: usize = FOO;
+   | ^^^^^^^^^^^^^^^^
+   |
+note: ...which requires const-evaluating + checking `FOO`...
+  --> $DIR/cycle_crash.rs:3:20
+   |
+LL | const FOO: usize = FOO;
+   |                    ^^^
+   = note: ...which again requires simplifying constant for the type system `FOO`, completing the cycle
+   = note: cycle used when running analysis passes on this crate
+   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0391`.