Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="build.rs" line_range="23-27" />
<code_context>
-    use std::assert_matches;
+/// Location of assert_matches!() macro. Stabilisation was reverted at last minute
+/// on 2026-04-10, leaving the macro in the new planned location.
+enum AssertMatchesLocation {
+    /// Macro is at `std::assert_matches`
+    Root,
+    /// Macro is at `std::assert_matches::assert_matches`
+    Module,
+}

</code_context>
<issue_to_address>
**suggestion (bug_risk):** Avoid duplicating the `assert_matches_*` cfg key strings in multiple places.

The cfg values `"assert_matches_in_root"` and `"assert_matches_in_module"` are currently hard-coded in both `Display` and `emit_possibilities`, so a change in one place could silently desync the mapping between `emit_possibility` and `emit`. Please centralize these as `const` values or via an `AssertMatchesLocation` method that returns the cfg key, and use that in both locations.

Suggested implementation:

```rust
}

/// Location of assert_matches!() macro. Stabilisation was reverted at last minute
/// on 2026-04-10, leaving the macro in the new planned location.
enum AssertMatchesLocation {
    /// Macro is at `std::assert_matches`
    Root,
    /// Macro is at `std::assert_matches::assert_matches`
    Module,
}

impl AssertMatchesLocation {
    /// Returns the corresponding `cfg` key for this location.
    ///
    /// Centralizing these values here avoids having the string literals duplicated
    /// in multiple places (e.g. `Display` and `emit_possibilities`).
    const fn cfg_key(&self) -> &'static str {
        match self {
            AssertMatchesLocation::Root => "assert_matches_in_root",
            AssertMatchesLocation::Module => "assert_matches_in_module",
        }
    }
}

use autocfg::AutoCfg;

```

To fully implement your suggestion, any places that currently hard-code the cfg key strings must be updated to use the centralized source:

1. In the `impl Display for AssertMatchesLocation`:
   - Replace occurrences of `"assert_matches_in_root"` and `"assert_matches_in_module"` with `self.cfg_key()` (or with `AssertMatchesLocation::Root.cfg_key()` / `AssertMatchesLocation::Module.cfg_key()` if it’s not a method on `self` in that context).

2. In `emit_possibilities` (or similar helper that emits the cfg flags):
   - Where it currently does something like `println!("cargo:rustc-cfg=assert_matches_in_root")` or `..._in_module`, replace those string literals with the relevant `AssertMatchesLocation` variant’s `cfg_key()`:
     - `AssertMatchesLocation::Root.cfg_key()`
     - `AssertMatchesLocation::Module.cfg_key()`

3. If there are any other uses of these cfg strings elsewhere in `build.rs`, update them to also call `cfg_key()` on the appropriate `AssertMatchesLocation` rather than using raw string literals.

These changes will ensure that any future edits to the cfg names are made in a single location (`cfg_key`) and automatically applied everywhere else.
</issue_to_address>