Add missing changes to the grammar

Unsafe extern blocks imply some changes to the grammar.  Let's make
those and note the edition differences.

Author: traviscross

src/items/external-blocks.md

@@ -2,16 +2,20 @@
 
 > **<sup>Syntax</sup>**\
 > _ExternBlock_ :\
-> &nbsp;&nbsp; `unsafe`<sup>?</sup> `extern` [_Abi_]<sup>?</sup> `{`\
+> &nbsp;&nbsp; `unsafe`[^unsafe-2024] `extern` [_Abi_]<sup>?</sup> `{`\
 > &nbsp;&nbsp; &nbsp;&nbsp; [_InnerAttribute_]<sup>\*</sup>\
 > &nbsp;&nbsp; &nbsp;&nbsp; _ExternalItem_<sup>\*</sup>\
 > &nbsp;&nbsp; `}`
 >
 > _ExternalItem_ :\
 > &nbsp;&nbsp; [_OuterAttribute_]<sup>\*</sup> (\
 > &nbsp;&nbsp; &nbsp;&nbsp; &nbsp;&nbsp; [_MacroInvocationSemi_]\
-> &nbsp;&nbsp; &nbsp;&nbsp; | ( [_Visibility_]<sup>?</sup> ( [_StaticItem_] | [_Function_] ) )\
+> &nbsp;&nbsp; &nbsp;&nbsp; | ( [_Visibility_]<sup>?</sup> ( (`safe` | `unsafe`)<sup>?</sup>[^static-qualifiers] [_StaticItem_] | [_Function_] ) )\
 > &nbsp;&nbsp; )
+>
+> [^unsafe-2024]: Prior to the 2024 edition, the `unsafe` keyword is optional.
+>
+> [^static-qualifiers]: *Relevant to editions earlier than Rust 2024*: The `safe` or `unsafe` qualifier is only allowed when the `extern` is qualified as `unsafe`.
 
 External blocks provide _declarations_ of items that are not _defined_ in the
 current crate and are the basis of Rust's foreign function interface. These are

src/items/functions.md

@@ -8,7 +8,7 @@
 >       ( [_BlockExpression_] | `;` )
 >
 > _FunctionQualifiers_ :\
-> &nbsp;&nbsp; `const`<sup>?</sup> `async`[^async-edition]<sup>?</sup> `unsafe`<sup>?</sup> (`extern` _Abi_<sup>?</sup>)<sup>?</sup>
+> &nbsp;&nbsp; `const`<sup>?</sup> `async`[^async-edition]<sup>?</sup> (`safe`[^extern-safe] | `unsafe`)<sup>?</sup>[^extern-qualifiers] (`extern` _Abi_<sup>?</sup>)<sup>?</sup>
 >
 > _Abi_ :\
 > &nbsp;&nbsp; [STRING_LITERAL] | [RAW_STRING_LITERAL]
@@ -39,6 +39,13 @@
 >
 > [^async-edition]: The `async` qualifier is not allowed in the 2015 edition.
 >
+> [^extern-safe]: The `safe` function qualifier is only allowed within
+>   `extern` blocks.
+>
+> [^extern-qualifiers]: *Relevant to editions earlier than Rust 2024*: Within
+>   `extern` blocks, the `safe` or `unsafe` function qualifier is only allowed
+>   when the `extern` is qualified as `unsafe`.
+>
 > [^fn-param-2015]: Function parameters with only a type are only allowed
 >   in an associated function of a [trait item] in the 2015 edition.