Implement Make handle_alloc_error default to panic (for no_std + liballoc)

Related: https://github.com/rust-lang/rust/issues/66741

Guarded with `#![feature(default_alloc_error_handler)]` a default
`alloc_error_handler` is called, if a custom allocator is used and no
other custom `#[alloc_error_handler]` is defined.

The panic message does not contain the size anymore, because it would
pull in the fmt machinery, which would blow up the code size
significantly.

compiler/rustc_codegen_llvm/src/allocator.rs

@@ -3,11 +3,17 @@ use libc::c_uint;
 use rustc_ast::expand::allocator::{AllocatorKind, AllocatorTy, ALLOCATOR_METHODS};
 use rustc_middle::bug;
 use rustc_middle::ty::TyCtxt;
+use rustc_span::symbol::sym;
 
 use crate::llvm::{self, False, True};
 use crate::ModuleLlvm;
 
-pub(crate) unsafe fn codegen(tcx: TyCtxt<'_>, mods: &mut ModuleLlvm, kind: AllocatorKind) {
+pub(crate) unsafe fn codegen(
+    tcx: TyCtxt<'_>,
+    mods: &mut ModuleLlvm,
+    kind: AllocatorKind,
+    has_alloc_error_handler: bool,
+) {
     let llcx = &*mods.llcx;
     let llmod = mods.llmod();
     let usize = match &tcx.sess.target.target.target_pointer_width[..] {
@@ -82,4 +88,41 @@ pub(crate) unsafe fn codegen(tcx: TyCtxt<'_>, mods: &mut ModuleLlvm, kind: Alloc
         }
         llvm::LLVMDisposeBuilder(llbuilder);
     }
+
+    // rust alloc error handler
+    let args = [usize, usize]; // size, align
+
+    let ty = llvm::LLVMFunctionType(void, args.as_ptr(), args.len() as c_uint, False);
+    let name = format!("__rust_alloc_error_handler");
+    let llfn = llvm::LLVMRustGetOrInsertFunction(llmod, name.as_ptr().cast(), name.len(), ty);
+    // -> ! DIFlagNoReturn
+    llvm::Attribute::NoReturn.apply_llfn(llvm::AttributePlace::Function, llfn);
+
+    if tcx.sess.target.target.options.default_hidden_visibility {
+        llvm::LLVMRustSetVisibility(llfn, llvm::Visibility::Hidden);
+    }
+    if tcx.sess.must_emit_unwind_tables() {
+        attributes::emit_uwtable(llfn, true);
+    }
+
+    let kind = if has_alloc_error_handler { AllocatorKind::Global } else { AllocatorKind::Default };
+    let callee = kind.fn_name(sym::oom);
+    let callee = llvm::LLVMRustGetOrInsertFunction(llmod, callee.as_ptr().cast(), callee.len(), ty);
+    // -> ! DIFlagNoReturn
+    llvm::Attribute::NoReturn.apply_llfn(llvm::AttributePlace::Function, callee);
+    llvm::LLVMRustSetVisibility(callee, llvm::Visibility::Hidden);
+
+    let llbb = llvm::LLVMAppendBasicBlockInContext(llcx, llfn, "entry\0".as_ptr().cast());
+
+    let llbuilder = llvm::LLVMCreateBuilderInContext(llcx);
+    llvm::LLVMPositionBuilderAtEnd(llbuilder, llbb);
+    let args = args
+        .iter()
+        .enumerate()
+        .map(|(i, _)| llvm::LLVMGetParam(llfn, i as c_uint))
+        .collect::<Vec<_>>();
+    let ret = llvm::LLVMRustBuildCall(llbuilder, callee, args.as_ptr(), args.len() as c_uint, None);
+    llvm::LLVMSetTailCall(ret, True);
+    llvm::LLVMBuildRetVoid(llbuilder);
+    llvm::LLVMDisposeBuilder(llbuilder);
 }

compiler/rustc_codegen_llvm/src/lib.rs

@@ -95,8 +95,9 @@ impl ExtraBackendMethods for LlvmCodegenBackend {
         tcx: TyCtxt<'tcx>,
         mods: &mut ModuleLlvm,
         kind: AllocatorKind,
+        has_alloc_error_handler: bool,
     ) {
-        unsafe { allocator::codegen(tcx, mods, kind) }
+        unsafe { allocator::codegen(tcx, mods, kind, has_alloc_error_handler) }
     }
     fn compile_codegen_unit(
         &self,

compiler/rustc_codegen_ssa/src/base.rs

@@ -538,8 +538,9 @@ pub fn codegen_crate<B: ExtraBackendMethods>(
         let llmod_id =
             cgu_name_builder.build_cgu_name(LOCAL_CRATE, &["crate"], Some("allocator")).to_string();
         let mut modules = backend.new_metadata(tcx, &llmod_id);
-        tcx.sess
-            .time("write_allocator_module", || backend.codegen_allocator(tcx, &mut modules, kind));
+        tcx.sess.time("write_allocator_module", || {
+            backend.codegen_allocator(tcx, &mut modules, kind, tcx.lang_items().oom().is_some())
+        });
 
         Some(ModuleCodegen { name: llmod_id, module_llvm: modules, kind: ModuleKind::Allocator })
     } else {

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -109,6 +109,7 @@ pub trait ExtraBackendMethods: CodegenBackend + WriteBackendMethods + Sized + Se
         tcx: TyCtxt<'tcx>,
         mods: &mut Self::Module,
         kind: AllocatorKind,
+        has_alloc_error_handler: bool,
     );
     /// This generates the codegen unit and returns it along with
     /// a `u64` giving an estimate of the unit's processing cost.

compiler/rustc_feature/src/active.rs

@@ -593,6 +593,9 @@ declare_features! (
     /// Allows to use the `#[cmse_nonsecure_entry]` attribute.
     (active, cmse_nonsecure_entry, "1.48.0", Some(75835), None),
 
+    /// Allows rustc to inject a default alloc_error_handler
+    (active, default_alloc_error_handler, "1.48.0", Some(66741), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_passes/src/weak_lang_items.rs

@@ -64,7 +64,10 @@ fn verify<'tcx>(tcx: TyCtxt<'tcx>, items: &lang_items::LanguageItems) {
             if item == LangItem::PanicImpl {
                 tcx.sess.err("`#[panic_handler]` function required, but not found");
             } else if item == LangItem::Oom {
-                tcx.sess.err("`#[alloc_error_handler]` function required, but not found");
+                if !tcx.features().default_alloc_error_handler {
+                    tcx.sess.err("`#[alloc_error_handler]` function required, but not found.");
+                    tcx.sess.note_without_error("Use `#![feature(default_alloc_error_handler)]` for a default error handler.");
+                }
             } else {
                 tcx.sess.err(&format!("language item required, but not found: `{}`", name));
             }

compiler/rustc_span/src/symbol.rs

@@ -415,6 +415,7 @@ symbols! {
         decl_macro,
         declare_lint_pass,
         decode,
+        default_alloc_error_handler,
         default_lib_allocator,
         default_type_parameter_fallback,
         default_type_params,