Discuss optionally allowing cfg_alias at module scope

Author: tgross35

text/3804-cfg-alias.md

@@ -97,8 +97,13 @@ CfgAliasAttribute:
     cfg_alias(IDENTIFIER `=` ConfigurationPredicate)
 ```
 
-The identifier is added to the `cfg` namespace. It must not conflict with any
-builtin configuration names, or with those passed via `--cfg` or `--check-cfg`. [^check-cfg]
+The identifier is added to the `cfg` namespace. It must not conflict with:
+
+- Any builtin configuration names
+- Any configuration passed via `--cfg`
+- Any configuration passed with `--check-cfg`, since this indicates a possible
+  but omitted `--cfg` option
+- Other aliases that are in scope
 
 Once defined, the alias can be used as a regular predicate.
 
@@ -125,8 +130,38 @@ other?
 If we go with the first option, we should limit to a single expansion to avoid
 recursing (as is done for `#define` in C).
 
-[^check-cfg]: `--check-cfg` is included here because it indicates there may be a
-    corresponding `--cfg`.
+## Optional: `cfg_alias` in non-crate attributes
+
+At a language level, it should be possbile to allow using `cfg_alias` as a
+module-level attribute rather than crate-level:
+
+```rust
+#[cfg_alias(foo = bar)]
+mod uses_bar {
+    // Enabled/disabled based on `cfg(bar)`
+    #[cfg(foo)]
+    fn qux() { /* ... */ }
+}
+
+#[cfg_alias(foo = baz)]
+mod uses_baz {
+    // Enabled/disabled based on `cfg(baz)`
+    #[cfg(foo)]
+    fn qux() { /* ... */ }
+}
+```
+
+This has the advantage of keeping aliases in closer proximity to where they are
+used; if a configuration pattern is only used within a specific module, an alias
+can be added at the top of the file rather than making it crate-global.
+
+This RFC proposes that the use of `cfg_alias` on modules _should_ be included if
+possible. However, this may bring implementation complexity since (to the RFC
+author's knowledge), the rustc configuration system is not designed to allow
+scoped configuration.
+
+If implementation of module-level aliases turns out to be nontrivial, this
+portion of the feature may be deferred or dropped before stabilization.
 
 # Drawbacks