2024: Add page for missing_fragment_specifier

src/SUMMARY.md

@@ -49,6 +49,7 @@
   - [Rustfmt: Combine all delimited exprs as last argument](rust-2024/rustfmt-overflow-delimited-expr.md)
   - [`gen` keyword](rust-2024/gen-keyword.md)
   - [Macro fragment specifiers](rust-2024/macro-fragment-specifiers.md)
+  - [Missing macro fragment specifiers](rust-2024/missing-macro-fragment-specifiers.md)
   - [Never type fallback change](rust-2024/never-type-fallback.md)
   - [`unsafe extern` blocks](rust-2024/unsafe-extern.md)
   - [Unsafe attributes](rust-2024/unsafe-attributes.md)

src/rust-2024/missing-macro-fragment-specifiers.md

@@ -0,0 +1,39 @@
+# Missing macro fragment specifiers
+
+🚧 The 2024 Edition has not yet been released and hence this section is still "under construction".
+More information may be found in the tracking issue at <https://github.com/rust-lang/rust/issues/128143>.
+
+## Summary
+
+- The [`missing_fragment_specifier`] lint is now a hard error.
+
+[`missing_fragment_specifier`]: ../../rustc/lints/listing/deny-by-default.html#missing-fragment-specifier
+
+## Details
+
+The [`missing_fragment_specifier`] lint detects a situation when an **unused** pattern in a `macro_rules!` macro definition has a meta-variable (e.g. `$e`) that is not followed by a fragment specifier (e.g. `:expr`). This is now a hard error in the 2024 Edition.
+
+```rust,compile_fail
+macro_rules! foo {
+   () => {};
+   ($name) => { }; // ERROR: missing fragment specifier
+}
+
+fn main() {
+   foo!();
+}
+```
+
+If you ever try to call the macro with arguments that would match a rule with a missing specifier, it is a hard error in all editions (for example, calling `foo!($name)` in the example above). However, this check was previously only performed when calling the macro, not when it was defined. A lint was added in Rust 1.17 to look at the *definition* for these missing specifiers.
+
+It was determined that it would cause too much breakage in the ecosystem to make this a hard error in all editions.[^future-incompat]
+
+[^future-incompat]: The lint is marked as a "future-incompatible" warning. It may become a hard error in all editions in a future release. See [#40107] for more information.
+
+[#40107]: https://github.com/rust-lang/rust/issues/40107
+
+## Migration
+
+To migrate your code to the 2024 Edition, remove the unused matcher rule from the macro. The [`missing_fragment_specifier`] lint is on by default in all editions, and should alert you to macros with this issue.
+
+There is no automatic migration for this change. It is expected that this style of macro is extremely rare. The lint has been a future-incompatible lint since Rust 1.17, a deny-by-default lint since Rust 1.20, and warns about dependencies using this pattern since Rust 1.82.