Rollup merge of rust-lang#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

alloc/src/alloc.rs

@@ -361,7 +361,7 @@ unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
 unsafe extern "Rust" {
     // This is the magic symbol to call the global alloc error handler. rustc generates
     // it to call `__rg_oom` if there is a `#[alloc_error_handler]`, or to call the
-    // default implementations below (`__rdl_oom`) otherwise.
+    // default implementations below (`__rdl_alloc_error_handler`) otherwise.
     #[rustc_std_internal_symbol]
     fn __rust_alloc_error_handler(size: usize, align: usize) -> !;
 }
@@ -425,7 +425,7 @@ pub mod __alloc_error_handler {
     // called via generated `__rust_alloc_error_handler` if there is no
     // `#[alloc_error_handler]`.
     #[rustc_std_internal_symbol]
-    pub unsafe fn __rdl_oom(size: usize, _align: usize) -> ! {
+    pub unsafe fn __rdl_alloc_error_handler(size: usize, _align: usize) -> ! {
         unsafe extern "Rust" {
             // This symbol is emitted by rustc next to __rust_alloc_error_handler.
             // Its value depends on the -Zoom={panic,abort} compiler option.

std/src/alloc.rs

@@ -358,9 +358,10 @@ fn default_alloc_error_hook(layout: Layout) {
         // This is the default path taken on OOM, and the only path taken on stable with std.
         // Crucially, it does *not* call any user-defined code, and therefore users do not have to
         // worry about allocation failure causing reentrancy issues. That makes it different from
-        // the default `__rdl_oom` defined in alloc (i.e., the default alloc error handler that is
-        // called when there is no `#[alloc_error_handler]`), which triggers a regular panic and
-        // thus can invoke a user-defined panic hook, executing arbitrary user-defined code.
+        // the default `__rdl_alloc_error_handler` defined in alloc (i.e., the default alloc error
+        // handler that is  called when there is no `#[alloc_error_handler]`), which triggers a
+        // regular panic and thus can invoke a user-defined panic hook, executing arbitrary
+        // user-defined code.
         rtprintpanic!("memory allocation of {} bytes failed\n", layout.size());
     }
 }