Auto merge of #43589 - aidanhs:aphs-fix-system-malloc, r=alexcrichton

Make a disable-jemalloc build work

Fixes #43510. I've tested this up to building a stage1 compiler.

r? @alexcrichton

cc @cuviper @vorner

@cuviper your fix was almost correct, you just had a stray `!` in there which caused the second error you saw.

src/libstd/lib.rs

@@ -318,6 +318,22 @@
 
 #![default_lib_allocator]
 
+// Always use alloc_system during stage0 since we don't know if the alloc_*
+// crate the stage0 compiler will pick by default is enabled (e.g.
+// if the user has disabled jemalloc in `./configure`).
+// `force_alloc_system` is *only* intended as a workaround for local rebuilds
+// with a rustc without jemalloc.
+// The not(stage0+msvc) gates will only last until the next stage0 bump
+#![cfg_attr(all(
+        not(all(stage0, target_env = "msvc")),
+        any(stage0, feature = "force_alloc_system")),
+    feature(global_allocator))]
+#[cfg(all(
+    not(all(stage0, target_env = "msvc")),
+    any(stage0, feature = "force_alloc_system")))]
+#[global_allocator]
+static ALLOC: alloc_system::System = alloc_system::System;
+
 // Explicitly import the prelude. The compiler uses this same unstable attribute
 // to import the prelude implicitly when building crates that depend on std.
 #[prelude_import]