Auto merge of #120675 - oli-obk:intrinsics3.0, r=pnkfelix

Add a scheme for moving away from `extern "rust-intrinsic"` entirely

All `rust-intrinsic`s can become free functions now, either with a fallback body, or with a dummy body and an attribute, requiring backends to actually implement the intrinsic.

This PR demonstrates the dummy-body scheme with the `vtable_size` intrinsic.

cc https://github.com/rust-lang/rust/issues/63585

follow-up to #120500

MCP at https://github.com/rust-lang/compiler-team/issues/720

2 files changed, 13 insertions, 2 deletions

diff --git a/src/doc/unstable-book/src/language-features/intrinsics.md b/src/doc/unstable-book/src/language-features/intrinsics.md
index 1a8c1c0b36a..02a009d56d3 100644
--- a/src/doc/unstable-book/src/language-features/intrinsics.md
+++ b/src/doc/unstable-book/src/language-features/intrinsics.md
@@ -52,12 +52,23 @@ with any regular function.
 Various intrinsics have native MIR operations that they correspond to. Instead of requiring
 backends to implement both the intrinsic and the MIR operation, the `lower_intrinsics` pass
 will convert the calls to the MIR operation. Backends do not need to know about these intrinsics
-at all.
+at all. These intrinsics only make sense without a body, and can either be declared as a "rust-intrinsic"
+or as a `#[rustc_intrinsic]`. The body is never used, as calls to the intrinsic do not exist
+anymore after MIR analyses.
 
 ## Intrinsics without fallback logic
 
 These must be implemented by all backends.
 
+### `#[rustc_intrinsic]` declarations
+
+These are written like intrinsics with fallback bodies, but the body is irrelevant.
+Use `loop {}` for the body or call the intrinsic recursively and add
+`#[rustc_intrinsic_must_be_overridden]` to the function to ensure that backends don't
+invoke the body.
+
+### Legacy extern ABI based intrinsics
+
 These are imported as if they were FFI functions, with the special
 `rust-intrinsic` ABI. For example, if one was in a freestanding
 context, but wished to be able to `transmute` between types, and
diff --git a/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs b/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
index 183dbe3aecc..e369cb9d0a4 100644
--- a/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
+++ b/src/tools/clippy/clippy_utils/src/qualify_min_const_fn.rs
@@ -334,7 +334,7 @@ fn check_terminator<'tcx>(
                 // within const fns. `transmute` is allowed in all other const contexts.
                 // This won't really scale to more intrinsics or functions. Let's allow const
                 // transmutes in const fn before we add more hacks to this.
-                if matches!(tcx.intrinsic(fn_def_id), Some(sym::transmute)) {
+                if tcx.is_intrinsic(fn_def_id, sym::transmute) {
                     return Err((
                         span,
                         "can only call `transmute` from const items, not `const fn`".into(),