docs(coercions): clarify LUB safety and target_feature rules

src/attributes.rst

@@ -540,6 +540,9 @@ Safe :t:`[function]s` that are annotated with :c:`[target_feature]s`
 can only be called without an :t:`unsafe block` by a caller that is within a function
 that enables all the :c:`[target_feature]s` that the callee enables.
 
+:dp:`fls_ReYuWzijQ1aL`
+A safe :t:`[function]` that is annotated with :c:`[target_feature]s` shall only be coerced to a :t:`function pointer type` that lacks :t:`keyword` ``unsafe`` at a :t:`coercion site` within a :t:`function` that enables all the :c:`[target_feature]s` that the source enables; otherwise it may only be coerced to an :t:`unsafe function pointer type`.
+
 :dp:`fls_agpkz1v3c281`
 :t:`Attribute` :dc:`target_feature` enables target architecture features for its
 related :t:`function`.
@@ -2373,4 +2376,3 @@ the exhaustiveness of :t:`[match arm]s`.
    struct struct_with_future_fields {
        field: u32
    }
-

src/changelog.rst

@@ -34,6 +34,13 @@ Language changes in Rust 1.93.0
 - `Stabilize asm_cfg <https://github.com/rust-lang/rust/pull/147736>`_
 - `During const-evaluation, support copying pointers byte-by-byte <https://github.com/rust-lang/rust/pull/148259>`_
 - `LUB coercions now correctly handle function item types, and functions with differing safeties <https://github.com/rust-lang/rust/pull/148602>`_
+
+  - New paragraphs:
+
+    - :p:`fls_ReYuWzijQ1aL`
+    - :p:`fls_yhtOWsxAcccM`
+    - :p:`fls_rRegjSIudDM1`
+
 - `Allow const items that contain mutable references to static (which is *very* unsafe, but not *always* UB) <https://github.com/rust-lang/rust/pull/148746>`_
 
   * Removed paragraph: :p:`fls_ooOYxhVh8hZo`

src/types-and-traits.rst

@@ -2050,6 +2050,9 @@ occur when:
   and the target :t:`type` is a :t:`trait object type` with some :t:`principal trait` ``U``,
   where ``U`` is a :t:`supertrait` of ``T``.
 
+:dp:`fls_yhtOWsxAcccM`
+A :t:`function signature` is a :t:`subtype` of another :t:`function signature` when they are identical except that the subtype lacks :t:`keyword` ``unsafe`` where the supertype uses :t:`keyword` ``unsafe`` in its :t:`function qualifier` list.
+
 :dp:`fls_iiiu2q7pym4p`
 An :t:`unsized coercion` is a :t:`type coercion` that converts a :t:`sized type`
 into an :t:`unsized type`. :t:`Unsized coercion` from a source :t:`type` to a
@@ -2148,6 +2151,9 @@ obtained as follows:
       that both ``TC`` and ``U`` can coerce to, make that
       :t:`function pointer type` be target :t:`type` ``U``.
 
+   #. :dp:`fls_rRegjSIudDM1`
+      When applying the previous step, select a :t:`function pointer type` without :t:`keyword` ``unsafe`` only when each source :t:`type` can coerce to a :t:`function pointer type` without :t:`keyword` ``unsafe`` at the :t:`coercion site`; otherwise select an :t:`unsafe function pointer type`.
+
    #. :dp:`fls_bWHQIL4DSN4S`
       Otherwise, no coercion is performed.