Improved compiler errors when feature flags are missing

garde/Cargo.toml

@@ -25,11 +25,11 @@ full = [
 
 serde = ["dep:serde", "compact_str/serde", "smallvec/serde"]
 derive = ["dep:garde_derive"]
-url = ["dep:url"]
+url = ["dep:url", "garde_derive?/url"]
 unicode = ["dep:unicode-segmentation"]
-credit-card = ["dep:card-validate"]
-phone-number = ["dep:phonenumber"]
-email = ["regex"]
+credit-card = ["dep:card-validate", "garde_derive?/credit-card"]
+phone-number = ["dep:phonenumber", "garde_derive?/phone-number"]
+email = ["regex", "garde_derive?/email"]
 email-idna = ["dep:idna"]
 regex = ["dep:regex", "dep:once_cell", "garde_derive?/regex"]
 pattern = ["regex"] # for backward compatibility with <0.14.0

garde_derive/Cargo.toml

@@ -14,11 +14,16 @@ proc-macro = true
 
 [features]
 regex = ["dep:regex"]
+email = []
+url = []
+credit-card = []
+phone-number = []
 
 [dependencies]
 syn = { version = "2", features = ["full", "derive"] }
 quote = { version = "1" }
 proc-macro2 = { version = "1" }
+proc-macro-error = "1"
 regex = { version = "1", default-features = false, features = [
   "std",
 ], optional = true }

garde_derive/src/lib.rs

@@ -5,9 +5,11 @@ mod syntax;
 mod util;
 
 use proc_macro::{Delimiter, Literal, Span, TokenStream, TokenTree};
+use proc_macro_error::proc_macro_error;
 use quote::quote;
 use syn::DeriveInput;
 
+#[proc_macro_error]
 #[proc_macro_derive(Validate, attributes(garde))]
 pub fn derive_validate(input: TokenStream) -> TokenStream {
     let input = syn::parse_macro_input!(input as DeriveInput);

garde_derive/src/syntax.rs

@@ -271,6 +271,41 @@ impl Parse for model::RawRule {
             };
         }
 
+        macro_rules! error_if_missing_feature {
+            ($rule:expr, $feature:expr) => {
+                #[cfg(not(feature = $feature))]
+                proc_macro_error::abort!(
+                    ident,
+                    concat!(
+                        "Validation rule `",
+                        $rule,
+                        "` not found. Did you forget to add the `",
+                        $feature,
+                        "` feature flag?"
+                    )
+                );
+            };
+        }
+
+        match ident.to_string().as_str() {
+            "email" => {
+                error_if_missing_feature!("email", "email");
+            }
+            "url" => {
+                error_if_missing_feature!("url", "url");
+            }
+            "credit_card" => {
+                error_if_missing_feature!("credit_card", "credit-card");
+            }
+            "phone_number" => {
+                error_if_missing_feature!("phone_number", "phone-number");
+            }
+            "regex" => {
+                error_if_missing_feature!("regex", "regex");
+            }
+            _ => {}
+        }
+
         rules! {
             (input, ident) {
                 "skip" => Skip,