Rollup merge of #147526 - bjorn3:alloc_shim_weak_shape, r=petrochenkov,RalfJung

Move computation of allocator shim contents to cg_ssa

In the future this should make it easier to use weak symbols for the allocator shim on platforms that properly support weak symbols. And it would allow reusing the allocator shim code for handling default implementations of the upcoming externally implementable items feature on platforms that don't properly support weak symbols.

In addition to make this possible, the alloc error handler is now handled in a way such that it is possible to avoid using the allocator shim when liballoc is compiled without `no_global_oom_handling` if you use `#[alloc_error_handler]`. Previously this was only possible if you avoided liballoc entirely or compiled it with `no_global_oom_handling`. You still need to avoid libstd and to define the symbol that indicates that avoiding the allocator shim is unstable.

Author: matthiaskrgr

src/shims/foreign_items.rs

@@ -3,7 +3,7 @@ use std::io::Write;
 use std::path::Path;
 
 use rustc_abi::{Align, AlignFromBytesError, CanonAbi, Size};
-use rustc_ast::expand::allocator::alloc_error_handler_name;
+use rustc_ast::expand::allocator::AllocatorKind;
 use rustc_hir::attrs::Linkage;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::CrateNum;
@@ -52,6 +52,9 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
 
         // Some shims forward to other MIR bodies.
         match link_name.as_str() {
+            // This allocator function has forwarding shims synthesized during normal codegen
+            // (see `allocator_shim_contents`); this is where we emulate that behavior.
+            // FIXME should use global_fn_name, but mangle_internal_symbol requires a static str.
             name if name == this.mangle_internal_symbol("__rust_alloc_error_handler") => {
                 // Forward to the right symbol that implements this function.
                 let Some(handler_kind) = this.tcx.alloc_error_handler_kind(()) else {
@@ -60,12 +63,16 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                         "`__rust_alloc_error_handler` cannot be called when no alloc error handler is set"
                     );
                 };
-                let name = Symbol::intern(
-                    this.mangle_internal_symbol(alloc_error_handler_name(handler_kind)),
-                );
-                let handler =
-                    this.lookup_exported_symbol(name)?.expect("missing alloc error handler symbol");
-                return interp_ok(Some(handler));
+                if handler_kind == AllocatorKind::Default {
+                    let name =
+                        Symbol::intern(this.mangle_internal_symbol("__rdl_alloc_error_handler"));
+                    let handler = this
+                        .lookup_exported_symbol(name)?
+                        .expect("missing alloc error handler symbol");
+                    return interp_ok(Some(handler));
+                }
+                // Fall through to the `lookup_exported_symbol` below which should find
+                // a `__rust_alloc_error_handler`.
             }
             _ => {}
         }

tests/fail/alloc/alloc_error_handler.stderr

@@ -7,7 +7,7 @@ LL |     crate::process::abort()
    |
    = note: BACKTRACE:
    = note: inside `std::alloc::rust_oom` at RUSTLIB/std/src/alloc.rs:LL:CC
-   = note: inside `std::alloc::_::__rg_oom` at RUSTLIB/std/src/alloc.rs:LL:CC
+   = note: inside `std::alloc::_::__rust_alloc_error_handler` at RUSTLIB/std/src/alloc.rs:LL:CC
    = note: inside `std::alloc::handle_alloc_error::rt_error` at RUSTLIB/alloc/src/alloc.rs:LL:CC
    = note: inside `std::alloc::handle_alloc_error` at RUSTLIB/alloc/src/alloc.rs:LL:CC
 note: inside `main`

tests/fail/alloc/alloc_error_handler_custom.stderr

@@ -7,7 +7,7 @@ LL |     core::intrinsics::abort();
    |
    = note: BACKTRACE:
    = note: inside `alloc_error_handler` at tests/fail/alloc/alloc_error_handler_custom.rs:LL:CC
-note: inside `_::__rg_oom`
+note: inside `_::__rust_alloc_error_handler`
   --> tests/fail/alloc/alloc_error_handler_custom.rs:LL:CC
    |
 LL | #[alloc_error_handler]

tests/fail/alloc/alloc_error_handler_no_std.stderr

@@ -9,7 +9,7 @@ LL |     core::intrinsics::abort();
    |
    = note: BACKTRACE:
    = note: inside `panic_handler` at tests/fail/alloc/alloc_error_handler_no_std.rs:LL:CC
-   = note: inside `alloc::alloc::__alloc_error_handler::__rdl_oom` at RUSTLIB/alloc/src/alloc.rs:LL:CC
+   = note: inside `alloc::alloc::__alloc_error_handler::__rdl_alloc_error_handler` at RUSTLIB/alloc/src/alloc.rs:LL:CC
    = note: inside `alloc::alloc::handle_alloc_error::rt_error` at RUSTLIB/alloc/src/alloc.rs:LL:CC
    = note: inside `alloc::alloc::handle_alloc_error` at RUSTLIB/alloc/src/alloc.rs:LL:CC
 note: inside `miri_start`