Add extern "custom"

src/items/external-blocks.md

@@ -48,7 +48,7 @@ r[items.extern.fn.param-patterns]
 Patterns are not allowed in parameters, only [IDENTIFIER] or `_` may be used.
 
 r[items.extern.fn.qualifiers]
-The `safe` and `unsafe` function qualifiers are allowed, but other function qualifiers (e.g. `const`, `async`, `extern`) are not.
+The `safe` and `unsafe` function qualifiers are allowed, but other function qualifiers (e.g. `const`, `async`, `extern`) are not. The `safe` qualifier is rejected in `extern "custom"` blocks.
 
 r[items.extern.fn.foreign-abi]
 Functions within external blocks may be called by Rust code, just like functions defined in Rust. The Rust compiler automatically translates between the Rust ABI and the foreign ABI.
@@ -112,6 +112,9 @@ r[items.extern.abi.system]
 r[items.extern.abi.unwind]
 * `extern "C-unwind"` and `extern "system-unwind"` --- Identical to `"C"` and `"system"`, respectively, but with [different behavior][unwind-behavior] when the callee unwinds (by panicking or throwing a C++ style exception).
 
+r[items.extern.abi.custom]
+* `unsafe extern "custom"` --- A custom ABI that is not known to the rust compiler.
+
 r[items.extern.abi.platform]
 There are also some platform-specific ABI strings:
 

src/items/functions.md

@@ -239,6 +239,15 @@ With `panic=unwind`, when a `panic` is turned into an abort by a non-unwinding A
 
 For other considerations and limitations regarding unwinding across FFI boundaries, see the [relevant section in the Panic documentation][panic-ffi].
 
+r[items.fn.extern.custom]
+An `extern "custom"` function has an unknown, custom ABI. The only way to call such a function is via [inline assembly].
+
+r[items.fn.extern.custom.safety]
+An `extern "custom"` function must be `unsafe`.
+
+r[items.fn.extern.custom.naked]
+An `extern "custom"` function definition must be a [naked function].
+
 [forced-unwinding]: https://rust-lang.github.io/rfcs/2945-c-unwind-abi.html#forced-unwinding
 [panic handler]: ../panic.md#the-panic_handler-attribute
 [panic-ffi]: ../panic.md#unwinding-across-ffi-boundaries
@@ -411,6 +420,7 @@ fn foo_oof(#[some_inert_attribute] arg: u8) {
 [testing attributes]: ../attributes/testing.md
 [`cold`]: ../attributes/codegen.md#the-cold-attribute
 [`inline`]: ../attributes/codegen.md#the-inline-attribute
+[naked function]: ../attributes/codegen.md#the-naked-attribute
 [`deprecated`]: ../attributes/diagnostics.md#the-deprecated-attribute
 [`doc`]: ../../rustdoc/the-doc-attribute.html
 [`must_use`]: ../attributes/diagnostics.md#the-must_use-attribute
@@ -427,3 +437,4 @@ fn foo_oof(#[some_inert_attribute] arg: u8) {
 [variadic function]: external-blocks.md#variadic-functions
 [`extern` block]: external-blocks.md
 [zero-sized]: glossary.zst
+[inline assembly]: ../inline-assembly.md