"Fix" intellisense for the f! macro in rust-analyzer.

libs/pavex_builder/src/reflection/callable/mod.rs

@@ -2,9 +2,24 @@ pub use identifiers::{RawCallable, RawCallableIdentifiers};
 
 mod identifiers;
 
+// The `pavex_ide_hint`-let binding is a hack to "nudge" 
+// rust-analyzer into parsing the macro input
+// as an expression path, therefore enabling auto-completion and 
+// go-to-definition for the path that's passed as input.
+//
+// Rust-analyzer doesn't do this by default because it can't infer
+// from the macro-generated code what the macro input "looks like".
+// `stringify` accepts anything as input, so that's not enough.
+//
+// The perma-disabled `let` binding lets us workaround the issue
+// that an _actual_ `let` binding would cause: it would fail to 
+// compile if the callable is generic, because the compiler would
+// demand to know the type of each generic parameter without a default.
 #[macro_export]
 macro_rules! f {
     ($p:expr) => {{
+        #[cfg(pavex_ide_hint)]
+        let _ = $p;
         $crate::reflection::RawCallable {
             import_path: stringify!($p),
             registered_at: ::std::env!("CARGO_PKG_NAME", "Failed to load the CARGO_PKG_NAME environment variable. Are you using a custom build system?")