Add -Zannotate-moves for profiler visibility of move/copy operations

This implements a new unstable compiler flag `-Zannotate-moves` that makes
move and copy operations visible in profilers by creating synthetic debug
information. This is achieved with zero runtime cost by manipulating debug
info scopes to make moves/copies appear as calls to `compiler_move<T, SIZE>`
and `compiler_copy<T, SIZE>` marker functions in profiling tools.

This allows developers to identify expensive move/copy operations in their
code using standard profiling tools, without requiring specialized tooling
or runtime instrumentation.

The implementation works at codegen time. When processing MIR operands
(`Operand::Move` and `Operand::Copy`), the codegen creates an `OperandRef`
with an optional `move_annotation` field containing an `Instance` of the
appropriate profiling marker function. When storing the operand,
`store_with_annotation()` wraps the store operation in a synthetic debug
scope that makes it appear inlined from the marker.

Two marker functions (`compiler_move` and `compiler_copy`) are defined
in `library/core/src/profiling.rs`. These are never actually called -
they exist solely as debug info anchors.

Operations are only annotated if the type:
   - Meets the size threshold (default: 65 bytes, configurable via
     `-Zannotate-moves=SIZE`)
   - Has a non-scalar backend representation (scalars use registers,
     not memcpy)

This has a very small size impact on object file size. With the default
limit it's well under 0.1%, and even with a very small limit of 8 bytes
it's still ~1.5%. This could be enabled by default.

Author: jsgf

core/src/lib.rs

@@ -282,6 +282,8 @@ pub mod num;
 pub mod hint;
 pub mod intrinsics;
 pub mod mem;
+#[unstable(feature = "profiling_marker_api", issue = "148197")]
+pub mod profiling;
 pub mod ptr;
 #[unstable(feature = "ub_checks", issue = "none")]
 pub mod ub_checks;

core/src/profiling.rs

@@ -0,0 +1,33 @@
+//! Profiling markers for compiler instrumentation.
+
+/// Profiling marker for move operations.
+///
+/// This function is never called at runtime. When `-Z annotate-moves` is enabled,
+/// the compiler creates synthetic debug info that makes move operations appear as
+/// calls to this function in profilers.
+///
+/// The `SIZE` parameter encodes the size of the type being copied. It's the same as
+/// `size_of::<T>()`, and is only present for convenience.
+#[unstable(feature = "profiling_marker_api", issue = "148197")]
+#[lang = "compiler_move"]
+pub fn compiler_move<T, const SIZE: usize>(_src: *const T, _dst: *mut T) {
+    unreachable!(
+        "compiler_move marks where the compiler-generated a memcpy for moves. It is never actually called."
+    )
+}
+
+/// Profiling marker for copy operations.
+///
+/// This function is never called at runtime. When `-Z annotate-moves` is enabled,
+/// the compiler creates synthetic debug info that makes copy operations appear as
+/// calls to this function in profilers.
+///
+/// The `SIZE` parameter encodes the size of the type being copied. It's the same as
+/// `size_of::<T>()`, and is only present for convenience.
+#[unstable(feature = "profiling_marker_api", issue = "148197")]
+#[lang = "compiler_copy"]
+pub fn compiler_copy<T, const SIZE: usize>(_src: *const T, _dst: *mut T) {
+    unreachable!(
+        "compiler_copy marks where the compiler-generated a memcpy for Copies. It is never actually called."
+    )
+}