Prevent insta-stable no alloc shim support

You will need to add the following as replacement for the old __rust_*
definitions when not using the alloc shim.

    #[no_mangle]
    static __rust_no_alloc_shim_is_unstable: u8 = 0;

compiler/rustc_ast/src/expand/allocator.rs

@@ -21,6 +21,8 @@ pub fn alloc_error_handler_name(alloc_error_handler_kind: AllocatorKind) -> &'st
     }
 }
 
+pub const NO_ALLOC_SHIM_IS_UNSTABLE: &str = "__rust_no_alloc_shim_is_unstable";
+
 pub enum AllocatorTy {
     Layout,
     Ptr,

compiler/rustc_codegen_cranelift/src/allocator.rs

@@ -5,7 +5,7 @@ use crate::prelude::*;
 
 use rustc_ast::expand::allocator::{
     alloc_error_handler_name, default_fn_name, global_fn_name, AllocatorKind, AllocatorTy,
-    ALLOCATOR_METHODS,
+    ALLOCATOR_METHODS, NO_ALLOC_SHIM_IS_UNSTABLE,
 };
 use rustc_codegen_ssa::base::allocator_kind_for_codegen;
 use rustc_session::config::OomStrategy;
@@ -94,4 +94,11 @@ fn codegen_inner(
     let val = oom_strategy.should_panic();
     data_ctx.define(Box::new([val]));
     module.define_data(data_id, &data_ctx).unwrap();
+
+    let data_id =
+        module.declare_data(NO_ALLOC_SHIM_IS_UNSTABLE, Linkage::Export, false, false).unwrap();
+    let mut data_ctx = DataContext::new();
+    data_ctx.set_align(1);
+    data_ctx.define(Box::new([0]));
+    module.define_data(data_id, &data_ctx).unwrap();
 }

compiler/rustc_codegen_gcc/src/allocator.rs

@@ -3,7 +3,7 @@ use gccjit::FnAttribute;
 use gccjit::{FunctionType, GlobalKind, ToRValue};
 use rustc_ast::expand::allocator::{
     alloc_error_handler_name, default_fn_name, global_fn_name, AllocatorKind, AllocatorTy,
-    ALLOCATOR_METHODS,
+    ALLOCATOR_METHODS, NO_ALLOC_SHIM_IS_UNSTABLE,
 };
 use rustc_middle::bug;
 use rustc_middle::ty::TyCtxt;
@@ -127,4 +127,9 @@ pub(crate) unsafe fn codegen(tcx: TyCtxt<'_>, mods: &mut GccContext, _module_nam
     let value = tcx.sess.opts.unstable_opts.oom.should_panic();
     let value = context.new_rvalue_from_int(i8, value as i32);
     global.global_set_initializer_rvalue(value);
+
+    let name = NO_ALLOC_SHIM_IS_UNSTABLE.to_string();
+    let global = context.new_global(None, GlobalKind::Exported, i8, name);
+    let value = context.new_rvalue_from_int(i8, 0);
+    global.global_set_initializer_rvalue(value);
 }

compiler/rustc_codegen_llvm/src/allocator.rs

@@ -2,7 +2,7 @@ use crate::attributes;
 use libc::c_uint;
 use rustc_ast::expand::allocator::{
     alloc_error_handler_name, default_fn_name, global_fn_name, AllocatorKind, AllocatorTy,
-    ALLOCATOR_METHODS,
+    ALLOCATOR_METHODS, NO_ALLOC_SHIM_IS_UNSTABLE,
 };
 use rustc_middle::bug;
 use rustc_middle::ty::TyCtxt;
@@ -161,6 +161,14 @@ pub(crate) unsafe fn codegen(
     let llval = llvm::LLVMConstInt(i8, val as u64, False);
     llvm::LLVMSetInitializer(ll_g, llval);
 
+    let name = NO_ALLOC_SHIM_IS_UNSTABLE;
+    let ll_g = llvm::LLVMRustGetOrInsertGlobal(llmod, name.as_ptr().cast(), name.len(), i8);
+    if tcx.sess.target.default_hidden_visibility {
+        llvm::LLVMRustSetVisibility(ll_g, llvm::Visibility::Hidden);
+    }
+    let llval = llvm::LLVMConstInt(i8, 0, False);
+    llvm::LLVMSetInitializer(ll_g, llval);
+
     if tcx.sess.opts.debuginfo != DebugInfo::None {
         let dbg_cx = debuginfo::CodegenUnitDebugContext::new(llmod);
         debuginfo::metadata::build_compile_unit_di_node(tcx, module_name, &dbg_cx);

compiler/rustc_codegen_ssa/src/back/symbol_export.rs

@@ -2,7 +2,7 @@ use crate::base::allocator_kind_for_codegen;
 
 use std::collections::hash_map::Entry::*;
 
-use rustc_ast::expand::allocator::ALLOCATOR_METHODS;
+use rustc_ast::expand::allocator::{ALLOCATOR_METHODS, NO_ALLOC_SHIM_IS_UNSTABLE};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, LocalDefId, LOCAL_CRATE};
@@ -241,6 +241,17 @@ fn exported_symbols_provider_local(
                 used: false,
             },
         ));
+
+        let exported_symbol =
+            ExportedSymbol::NoDefId(SymbolName::new(tcx, NO_ALLOC_SHIM_IS_UNSTABLE));
+        symbols.push((
+            exported_symbol,
+            SymbolExportInfo {
+                level: SymbolExportLevel::Rust,
+                kind: SymbolExportKind::Data,
+                used: false,
+            },
+        ))
     }
 
     if tcx.sess.instrument_coverage() || tcx.sess.opts.cg.profile_generate.enabled() {

library/alloc/src/alloc.rs

@@ -37,6 +37,9 @@ extern "Rust" {
     #[rustc_allocator_zeroed]
     #[rustc_nounwind]
     fn __rust_alloc_zeroed(size: usize, align: usize) -> *mut u8;
+
+    #[cfg(not(bootstrap))]
+    static __rust_no_alloc_shim_is_unstable: u8;
 }
 
 /// The global memory allocator.
@@ -90,7 +93,14 @@ pub use std::alloc::Global;
 #[must_use = "losing the pointer will leak memory"]
 #[inline]
 pub unsafe fn alloc(layout: Layout) -> *mut u8 {
-    unsafe { __rust_alloc(layout.size(), layout.align()) }
+    unsafe {
+        // Make sure we don't accidentally allow omitting the allocator shim in
+        // stable code until it is actually stabilized.
+        #[cfg(not(bootstrap))]
+        core::ptr::read_volatile(&__rust_no_alloc_shim_is_unstable);
+
+        __rust_alloc(layout.size(), layout.align())
+    }
 }
 
 /// Deallocate memory with the global allocator.

tests/codegen/alloc-optimisation.rs

@@ -7,6 +7,7 @@
 pub fn alloc_test(data: u32) {
     // CHECK-LABEL: @alloc_test
     // CHECK-NEXT: start:
+    // CHECK-NEXT: {{.*}} load volatile i8, ptr @__rust_no_alloc_shim_is_unstable, align 1
     // CHECK-NEXT: ret void
     let x = Box::new(data);
     drop(x);

tests/codegen/vec-optimizes-away.rs

@@ -7,6 +7,7 @@
 pub fn sum_me() -> i32 {
     // CHECK-LABEL: @sum_me
     // CHECK-NEXT: {{^.*:$}}
+    // CHECK-NEXT: {{.*}} load volatile i8, ptr @__rust_no_alloc_shim_is_unstable, align 1
     // CHECK-NEXT: ret i32 6
     vec![1, 2, 3].iter().sum::<i32>()
 }