Rollup merge of #135988 - bjorn3:workaround_parallel_rustc_crash, r=lqd

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.

Workaround for https://github.com/rust-lang/rust/issues/135870

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)

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() {}

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`.