Provide uuid macro without dependencies

Cargo.toml

@@ -54,7 +54,7 @@ repository = "uuid-rs/uuid"
 [features]
 default = ["std"]
 std = []
-macros = ["uuid_macro"]
+macro-diagnostics = ["uuid_macro"]
 
 v1 = ["atomic"]
 v3 = ["md-5"]

README.md

@@ -51,8 +51,10 @@ let my_uuid = Uuid::parse_str("67e55044-10b1-426f-9247-bb680e5fe0c8")?;
 assert_eq!(Some(Version::Random), my_uuid.get_version());
 ```
 
-If you add the `macros` feature then you can parse UUIDs at compile time
-instead of at runtime:
+You can parse UUIDs at compile time instead of at runtime.
+
+If you add the `macro-diagnostics` feature then you can see much better 
+error messages.
 
 ```rust
 #[macro_use]
@@ -78,7 +80,7 @@ various pieces of functionality:
   generate a `Uuid`.
 * `v5` - adds the `Uuid::new_v5` function and the ability to create a V5
   UUID based on the SHA1 hash of some data.
-* `macros` - adds the `uuid!` macro that can parse UUIDs at compile time.
+* `macro-diagnostics` - enhances the diagnostics of `uuid!` macro.
 * `serde` - adds the ability to serialize and deserialize a `Uuid` using the
   `serde` crate.
 * `arbitrary` - adds an `Arbitrary` trait implementation to `Uuid`.

examples/uuid_macro.rs

@@ -1,12 +1,13 @@
 //! Using the `uuid!` macro.
 //!
-//! If you enable the `macros` feature you can use the `uuid!` macro.
 //! `uuid!` will parse encoded UUIDs at compile time instead of at runtime.
 //! If you've got a fixed UUID string handy then consider using `uuid!` instead
 //! of `Uuid::parse_str` or `str::parse`.
+//!
+//! If you enable the `macro-diagnostics` feature, you can see much better
+//! error messages.
 
 #[test]
-#[cfg(feature = "macros")]
 fn parse_uuid_at_compile_time() {
     use uuid::uuid;
 

macros/src/lib.rs

@@ -1,11 +1,11 @@
 //! Implementation details for the `uuid!` macro.
 //!
 //! This crate is not meant to be used directly. Instead,
-//! you can use the `macros` feature of `uuid`:
+//! you can use the `macro-diagnostics` feature of `uuid`:
 //!
 //! ```toml
 //! [dependencies.uuid]
-//! features = ["macros"]
+//! features = ["macro-diagnostics"]
 //! ```
 
 use proc_macro::TokenStream;

src/lib.rs

@@ -70,7 +70,7 @@
 //!
 //! Other crate features can also be useful beyond the version support:
 //!
-//! * `macros` - adds the `uuid!` macro that can parse UUIDs from string literals at compile time.
+//! * `macro-diagnostics` - enhances the diagnostics of `uuid!` macro.
 //! * `serde` - adds the ability to serialize and deserialize a UUID using
 //!   `serde`.
 //! * `arbitrary` - adds an `Arbitrary` trait implementation to `Uuid` for
@@ -241,11 +241,11 @@ mod rng;
 
 mod external;
 
-#[cfg(feature = "macros")]
 #[macro_use]
 mod macros;
+
 #[doc(hidden)]
-#[cfg(feature = "macros")]
+#[cfg(feature = "macro-diagnostics")]
 pub extern crate uuid_macro;
 
 use crate::std::convert;

src/macros.rs

@@ -1,3 +1,37 @@
+macro_rules! define_uuid_macro {
+    {$(#[$doc:meta])*} => {
+        $(#[$doc])*
+        #[cfg(feature = "macro-diagnostics")]
+        #[macro_export]
+        macro_rules! uuid {
+            ($uuid:literal) => {{
+                $crate::Uuid::from_bytes($crate::uuid_macro::parse_lit!($uuid))
+            }};
+        }
+
+        $(#[$doc])*
+        #[cfg(not(feature = "macro-diagnostics"))]
+        #[macro_export]
+        macro_rules! uuid {
+            ($uuid:literal) => {{
+                const OUTPUT: $crate::Uuid = match $crate::Uuid::try_parse($uuid) {
+                    Ok(u) => u,
+                    Err(_) => {
+                        // here triggers const_err
+                        // const_panic requires 1.57
+                        #[allow(unconditional_panic)]
+                        let _ = ["invalid uuid representation"][1];
+
+                        loop {} // -> never type
+                    }
+                };
+                OUTPUT
+            }};
+        }
+    }
+}
+
+define_uuid_macro! {
 /// Parse [`Uuid`][uuid::Uuid]s from string literals at compile time.
 ///
 /// ## Usage
@@ -33,6 +67,8 @@
 /// let uuid: Uuid = uuid!("F9168C5E-ZEB2-4FAA-B6BF-329BF39FA1E4");
 /// ```
 ///
+/// Enable the feature `macro-diagnostics` to see the error messages below.
+///
 /// Provides the following compilation error:
 ///
 /// ```txt
@@ -60,9 +96,4 @@
 /// ```
 ///
 /// [uuid::Uuid]: https://docs.rs/uuid/*/uuid/struct.Uuid.html
-#[macro_export]
-macro_rules! uuid {
-    ($uuid:tt) => {{
-        $crate::Uuid::from_bytes($crate::uuid_macro::parse_lit!($uuid))
-    }};
 }

tests/macros.rs

@@ -1,4 +1,4 @@
-#[cfg(feature = "macros")]
+#[cfg(feature = "macro-diagnostics")]
 #[test]
 fn ui() {
     let t = trybuild::TestCases::new();