Revert back to single iter

crates/uv-distribution/src/metadata/lowering.rs

@@ -34,10 +34,7 @@ impl LoweredRequirement {
         project_dir: &'data Path,
         project_sources: &'data BTreeMap<PackageName, Sources>,
         workspace: &'data Workspace,
-    ) -> Result<
-        impl Iterator<Item = Result<LoweredRequirement, LoweringError>> + 'data,
-        LoweringError,
-    > {
+    ) -> impl Iterator<Item = Result<LoweredRequirement, LoweringError>> + 'data {
         let (source, origin) = if let Some(source) = project_sources.get(&requirement.name) {
             (Some(source), Origin::Project)
         } else if let Some(source) = workspace.sources().get(&requirement.name) {
@@ -51,14 +48,16 @@ impl LoweredRequirement {
             // We require that when you use a package that's part of the workspace, ...
             !workspace.packages().contains_key(&requirement.name)
                 // ... it must be declared as a workspace dependency (`workspace = true`), ...
-                || source.as_ref().is_some_and(|source| source.iter().all(|source| {
+                || source.as_ref().filter(|sources| !sources.is_empty()).is_some_and(|source| source.iter().all(|source| {
                     matches!(source, Source::Workspace { workspace: true, .. })
                 }))
                 // ... except for recursive self-inclusion (extras that activate other extras), e.g.
                 // `framework[machine_learning]` depends on `framework[cuda]`.
                 || &requirement.name == project_name;
         if !workspace_package_declared {
-            return Err(LoweringError::UndeclaredWorkspacePackage);
+            return Either::Left(std::iter::once(Err(
+                LoweringError::UndeclaredWorkspacePackage,
+            )));
         }
 
         let Some(source) = source else {
@@ -73,9 +72,7 @@ impl LoweredRequirement {
                     requirement.name
                 );
             }
-            return Ok(Either::Left(std::iter::once(Ok(Self(Requirement::from(
-                requirement,
-            ))))));
+            return Either::Left(std::iter::once(Ok(Self(Requirement::from(requirement)))));
         };
 
         // Determine whether the markers cover the full space for the requirement. If not, fill the
@@ -97,7 +94,7 @@ impl LoweredRequirement {
             })
         };
 
-        Ok(Either::Right(
+        Either::Right(
             source
                 .into_iter()
                 .map(move |source| {
@@ -233,7 +230,7 @@ impl LoweredRequirement {
                     Ok(requirement) => !requirement.0.marker.is_false(),
                     Err(_) => true,
                 }),
-        ))
+        )
     }
 
     /// Lower a [`pep508_rs::Requirement`] in a non-workspace setting (for example, in a PEP 723

crates/uv-distribution/src/metadata/mod.rs

@@ -22,6 +22,8 @@ pub enum MetadataError {
     Workspace(#[from] WorkspaceError),
     #[error("Failed to parse entry for: `{0}`")]
     LoweringError(PackageName, #[source] LoweringError),
+    #[error(transparent)]
+    Lower(#[from] LoweringError),
 }
 
 #[derive(Debug, Clone)]

crates/uv-distribution/src/metadata/requires_dist.rs

@@ -1,6 +1,6 @@
 use crate::metadata::{LoweredRequirement, MetadataError};
 use crate::Metadata;
-use pypi_types::Requirement;
+
 use std::collections::BTreeMap;
 use std::path::Path;
 use uv_configuration::SourceStrategy;
@@ -93,8 +93,6 @@ impl RequiresDist {
                             sources,
                             project_workspace.workspace(),
                         )
-                        .into_iter()
-                        .flatten()
                         .map(move |requirement| match requirement {
                             Ok(requirement) => Ok(requirement.into_inner()),
                             Err(err) => {
@@ -105,7 +103,7 @@ impl RequiresDist {
                     .collect::<Result<Vec<_>, _>>()?,
                 SourceStrategy::Disabled => dev_dependencies
                     .into_iter()
-                    .map(Requirement::from)
+                    .map(pypi_types::Requirement::from)
                     .collect(),
             };
             if dev_dependencies.is_empty() {
@@ -127,17 +125,18 @@ impl RequiresDist {
                         sources,
                         project_workspace.workspace(),
                     )
-                    .into_iter()
-                    .flatten()
                     .map(move |requirement| match requirement {
                         Ok(requirement) => Ok(requirement.into_inner()),
                         Err(err) => {
                             Err(MetadataError::LoweringError(requirement_name.clone(), err))
                         }
                     })
                 })
-                .collect::<Result<_, _>>()?,
-            SourceStrategy::Disabled => requires_dist.into_iter().map(Requirement::from).collect(),
+                .collect::<Result<Vec<_>, _>>()?,
+            SourceStrategy::Disabled => requires_dist
+                .into_iter()
+                .map(pypi_types::Requirement::from)
+                .collect(),
         };
 
         Ok(Self {

crates/uv-workspace/src/pyproject.rs

@@ -423,6 +423,16 @@ impl Sources {
     pub fn iter(&self) -> impl Iterator<Item = &Source> {
         self.0.iter()
     }
+
+    /// Returns `true` if the sources list is empty.
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    /// Returns the number of sources in the list.
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
 }
 
 impl IntoIterator for Sources {

crates/uv-workspace/src/workspace.rs

@@ -1757,8 +1757,7 @@ mod tests {
                 "sources": {
                   "bird-feeder": [
                     {
-                      "workspace": true,
-                      "marker": null
+                      "workspace": true
                     }
                   ]
                 },
@@ -1778,8 +1777,7 @@ mod tests {
                       "sources": {
                         "bird-feeder": [
                           {
-                            "workspace": true,
-                            "marker": null
+                            "workspace": true
                           }
                         ]
                       },