Support empty requires

crates/uv-distribution/src/distribution_database.rs

@@ -695,7 +695,7 @@ impl<'a, Context: BuildContext> DistributionDatabase<'a, Context> {
             } else {
                 Err(Error::VariantLockMissing {
                     variant_lock: PathBuf::from(variant_lock_path),
-                    requires: provider.requires.clone(),
+                    requires: provider.requires.clone().unwrap_or_default(),
                     plugin_api: provider.plugin_api.clone().unwrap_or_default().clone(),
                 })
             }
@@ -1548,7 +1548,10 @@ fn satisfies_provider_requires(
     {
         return false;
     }
-    requested_provider.requires.iter().all(|requested| {
+    let Some(requires) = &requested_provider.requires else {
+        return true;
+    };
+    requires.iter().all(|requested| {
         static_provider.resolved.iter().any(|resolved| {
             // We ignore extras for simplicity.
             &requested.name == resolved.name()

crates/uv-variant-frontend/src/error.rs

@@ -23,6 +23,8 @@ pub enum Error {
     RequirementsInstall(&'static str, #[source] AnyErrorBuild),
     #[error("Failed to create temporary virtualenv")]
     Virtualenv(#[from] uv_virtualenv::Error),
+    #[error("Provider plugin `{0}` is missing `requires`")]
+    MissingRequires(String),
     #[error("Failed to run `{0}`")]
     CommandFailed(PathBuf, #[source] io::Error),
     #[error("The build backend returned an error")]

crates/uv-variant-frontend/src/lib.rs

@@ -85,6 +85,8 @@ impl VariantBuild {
         // Resolve and install the provider requirements.
         let requirements = backend
             .requires
+            .as_ref()
+            .ok_or_else(|| Error::MissingRequires(backend_name.clone()))?
             .iter()
             .cloned()
             .map(Requirement::from)
@@ -158,6 +160,8 @@ impl VariantBuild {
             let requires = self
                 .backend
                 .requires
+                .as_ref()
+                .ok_or_else(|| Error::MissingRequires(self.backend_name.clone()))?
                 .iter()
                 .filter(|requirement| {
                     requirement.evaluate_markers(

crates/uv-variants/src/variants_json.rs

@@ -178,7 +178,7 @@ pub struct Provider {
     )]
     pub enable_if: MarkerTree,
     /// Dependency specifiers for how to install the plugin
-    pub requires: Vec<Requirement<VerbatimParsedUrl>>,
+    pub requires: Option<Vec<Requirement<VerbatimParsedUrl>>>,
     /// Whether this plugin is run at install time.
     pub plugin_use: Option<PluginUse>,
 }