Auto merge of #51335 - mark-i-m:allocator, r=oli-obk

Prohibit `global_allocator` in submodules

Background: #44113 is caused by weird interactions with hygiene. Hygiene is hard. After a lot of playing around, we decided that the best path forward would be to prohibit `global_allocator`s from being in submodules for now. When somebody gets it working, we can re-enable it.

This PR contains the following
- Some hygiene "fixes" -- things I suspect are the correct thing to do that will make life easier in the future. This includes using call_site hygiene for the generated module and passing the correct crate name to the expansion config.
- Comments and minor formatting fixes
- Some debugging code
- Code to prohibit `global_allocator` in submodules
- Test checking that the proper error occurs.

cc #44113 #49320 #51241

r? @alexcrichton

src/test/ui/allocator-submodule.rs

@@ -0,0 +1,40 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Tests that it is possible to create a global allocator in a submodule, rather than in the crate
+// root.
+
+#![feature(alloc, allocator_api, global_allocator)]
+
+extern crate alloc;
+
+use std::{
+    alloc::{GlobalAlloc, Layout},
+    ptr,
+};
+
+struct MyAlloc;
+
+unsafe impl GlobalAlloc for MyAlloc {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        ptr::null_mut()
+    }
+
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {}
+}
+
+mod submod {
+    use super::MyAlloc;
+
+    #[global_allocator]
+    static MY_HEAP: MyAlloc = MyAlloc; //~ ERROR global_allocator
+}
+
+fn main() {}

src/test/ui/allocator-submodule.stderr

@@ -0,0 +1,8 @@
+error: `global_allocator` cannot be used in submodules
+  --> $DIR/allocator-submodule.rs:37:5
+   |
+LL |     static MY_HEAP: MyAlloc = MyAlloc; //~ ERROR global_allocator
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+