Rollup merge of rust-lang#139490 - RalfJung:unstable-intrinsics-docs, r=oli-obk

Update some comment/docs related to "extern intrinsic" removal

Follow-up to rust-lang#139455.

r? `@oli-obk`

Author: Zalathar

Diff for: library/core/src/intrinsics/mod.rs

@@ -5,15 +5,11 @@
 //!
 //! # Const intrinsics
 //!
-//! Note: any changes to the constness of intrinsics should be discussed with the language team.
-//! This includes changes in the stability of the constness.
-//!
-//! //FIXME(#132735) "old" style intrinsics support has been removed
-//! In order to make an intrinsic usable at compile-time, it needs to be declared in the "new"
-//! style, i.e. as a `#[rustc_intrinsic]` function, not inside an `extern` block. Then copy the
-//! implementation from <https://github.com/rust-lang/miri/blob/master/src/intrinsics> to
+//! In order to make an intrinsic unstable usable at compile-time, copy the implementation from
+//! <https://github.com/rust-lang/miri/blob/master/src/intrinsics> to
 //! <https://github.com/rust-lang/rust/blob/master/compiler/rustc_const_eval/src/interpret/intrinsics.rs>
-//! and make the intrinsic declaration a `const fn`.
+//! and make the intrinsic declaration below a `const fn`. This should be done in coordination with
+//! wg-const-eval.
 //!
 //! If an intrinsic is supposed to be used from a `const fn` with a `rustc_const_stable` attribute,
 //! `#[rustc_intrinsic_const_stable_indirect]` needs to be added to the intrinsic. Such a change requires

Diff for: src/doc/unstable-book/src/language-features/intrinsics.md

@@ -53,7 +53,8 @@ Various intrinsics have native MIR operations that they correspond to. Instead o
 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. These intrinsics only make sense without a body, and can be declared as a `#[rustc_intrinsic]`.
-The body is never used, as calls to the intrinsic do not exist anymore after MIR analyses.
+The body is never used as the lowering pass implements support for all backends, so we never have to
+use the fallback logic.
 
 ## Intrinsics without fallback logic
 

Diff for: src/tools/rust-analyzer/crates/ide/src/hover/tests.rs

@@ -6882,85 +6882,14 @@ pub fn foo() {}
 
 #[test]
 fn hover_feature() {
-    check(
-        r#"#![feature(intrinsics$0)]"#,
-        expect![[r#"
-            *intrinsics*
-            ```
-            intrinsics
-            ```
-            ___
-
-            # `intrinsics`
-
-            The tracking issue for this feature is: None.
-
-            Intrinsics are rarely intended to be stable directly, but are usually
-            exported in some sort of stable manner. Prefer using the stable interfaces to
-            the intrinsic directly when you can.
-
-            ------------------------
-
-
-            ## Intrinsics with fallback logic
-
-            Many intrinsics can be written in pure rust, albeit inefficiently or without supporting
-            some features that only exist on some backends. Backends can simply not implement those
-            intrinsics without causing any code miscompilations or failures to compile.
-            All intrinsic fallback bodies are automatically made cross-crate inlineable (like `#[inline]`)
-            by the codegen backend, but not the MIR inliner.
-
-            ```rust
-            #![feature(intrinsics)]
-            #![allow(internal_features)]
-
-            #[rustc_intrinsic]
-            const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {}
-            ```
-
-            Since these are just regular functions, it is perfectly ok to create the intrinsic twice:
-
-            ```rust
-            #![feature(intrinsics)]
-            #![allow(internal_features)]
-
-            #[rustc_intrinsic]
-            const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {}
-
-            mod foo {
-                #[rustc_intrinsic]
-                const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize) {
-                    panic!("noisy const dealloc")
-                }
-            }
-
-            ```
-
-            The behaviour on backends that override the intrinsic is exactly the same. On other
-            backends, the intrinsic behaviour depends on which implementation is called, just like
-            with any regular function.
-
-            ## Intrinsics lowered to MIR instructions
-
-            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. These intrinsics only make sense without a body, and can be 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.
-
-        "#]],
-    )
+    let (analysis, position) = fixture::position(r#"#![feature(intrinsics$0)]"#);
+    analysis
+        .hover(
+            &HoverConfig { links_in_hover: true, ..HOVER_BASE_CONFIG },
+            FileRange { file_id: position.file_id, range: TextRange::empty(position.offset) },
+        )
+        .unwrap()
+        .unwrap();
 }
 
 #[test]