Rollup merge of #119365 - nbdd0121:asm-goto, r=Amanieu

Add asm goto support to `asm!`

Tracking issue: #119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? ``@Amanieu``
cc ``@ojeda``

1 files changed, 30 insertions, 0 deletions

diff --git a/src/doc/unstable-book/src/language-features/asm-goto.md b/src/doc/unstable-book/src/language-features/asm-goto.md
new file mode 100644
index 00000000000..d72eb7c0c6e
--- /dev/null
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@@ -0,0 +1,30 @@
+# `asm_goto`
+
+The tracking issue for this feature is: [#119364]
+
+[#119364]: https://github.com/rust-lang/rust/issues/119364
+
+------------------------
+
+This feature adds a `label <block>` operand type to `asm!`.
+
+Example:
+```rust,ignore (partial-example, x86-only)
+
+unsafe {
+    asm!(
+        "jmp {}",
+        label {
+            println!("Jumped from asm!");
+        }
+    );
+}
+```
+
+The block must have unit type or diverge.
+
+When `label <block>` is used together with `noreturn` option, it means that the
+assembly will not fallthrough. It's allowed to jump to a label within the
+assembly. In this case, the entire `asm!` expression will have an unit type as
+opposed to diverging, if not all label blocks diverge. The `asm!` expression
+still diverges if `noreturn` option is used and all label blocks diverge.