Remove unconstrained version error from requirements (#3443)

## Summary

It's not clear to me that this should exist at all, but it's causing
errors in projects that don't use `tool.uv.sources`, so we should
definitely remove it for now.

crates/uv-requirements/src/pyproject.rs

@@ -20,6 +20,7 @@ use thiserror::Error;
 use url::Url;
 
 use distribution_types::{ParsedUrlError, Requirement, RequirementSource, Requirements};
+use pep440_rs::VersionSpecifiers;
 use pep508_rs::{VerbatimUrl, VersionOrUrl};
 use uv_configuration::PreviewMode;
 use uv_fs::Simplified;
@@ -50,8 +51,6 @@ pub enum LoweringError {
     PathToUrl(PathBuf),
     #[error("Package is not included as workspace package in `tool.uv.workspace`")]
     UndeclaredWorkspacePackage,
-    #[error("Must specify a version constraint")]
-    UnconstrainedVersion,
     #[error("Can only specify one of rev, tag, or branch")]
     MoreThanOneGitRef,
     #[error("Unable to combine options in `tool.uv.sources`")]
@@ -283,7 +282,6 @@ impl Pep621Metadata {
         let requirements = lower_requirements(
             &project.dependencies.unwrap_or_default(),
             &project.optional_dependencies.unwrap_or_default(),
-            &project.name,
             project_dir,
             &project_sources.unwrap_or_default(),
             workspace_sources,
@@ -322,7 +320,6 @@ impl Pep621Metadata {
 pub(crate) fn lower_requirements(
     dependencies: &[String],
     optional_dependencies: &IndexMap<ExtraName, Vec<String>>,
-    project_name: &PackageName,
     project_dir: &Path,
     project_sources: &HashMap<PackageName, Source>,
     workspace_sources: &HashMap<PackageName, Source>,
@@ -336,7 +333,6 @@ pub(crate) fn lower_requirements(
             let name = requirement.name.clone();
             lower_requirement(
                 requirement,
-                project_name,
                 project_dir,
                 project_sources,
                 workspace_sources,
@@ -356,7 +352,6 @@ pub(crate) fn lower_requirements(
                     let name = requirement.name.clone();
                     lower_requirement(
                         requirement,
-                        project_name,
                         project_dir,
                         project_sources,
                         workspace_sources,
@@ -378,7 +373,6 @@ pub(crate) fn lower_requirements(
 /// Combine `project.dependencies` or `project.optional-dependencies` with `tool.uv.sources`.
 pub(crate) fn lower_requirement(
     requirement: pep508_rs::Requirement,
-    project_name: &PackageName,
     project_dir: &Path,
     project_sources: &HashMap<PackageName, Source>,
     workspace_sources: &HashMap<PackageName, Source>,
@@ -389,6 +383,7 @@ pub(crate) fn lower_requirement(
         .get(&requirement.name)
         .or(workspace_sources.get(&requirement.name))
         .cloned();
+
     if !matches!(
         source,
         Some(Source::Workspace {
@@ -403,12 +398,7 @@ pub(crate) fn lower_requirement(
 
     let Some(source) = source else {
         // Support recursive editable inclusions.
-        // TODO(konsti): This is a workspace feature.
-        return if requirement.version_or_url.is_none() && &requirement.name != project_name {
-            Err(LoweringError::UnconstrainedVersion)
-        } else {
-            Ok(Requirement::from_pep508(requirement)?)
-        };
+        return Ok(Requirement::from_pep508(requirement)?);
     };
 
     if preview.is_disabled() {
@@ -488,7 +478,10 @@ pub(crate) fn lower_requirement(
             path_source(path, project_dir, editable)?
         }
         Source::Registry { index } => match requirement.version_or_url {
-            None => return Err(LoweringError::UnconstrainedVersion),
+            None => RequirementSource::Registry {
+                specifier: VersionSpecifiers::empty(),
+                index: Some(index),
+            },
             Some(VersionOrUrl::VersionSpecifier(version)) => RequirementSource::Registry {
                 specifier: version,
                 index: Some(index),
@@ -765,24 +758,6 @@ mod test {
         "###);
     }
 
-    #[test]
-    fn missing_constraint() {
-        let input = indoc! {r#"
-            [project]
-            name = "foo"
-            version = "0.0.0"
-            dependencies = [
-              "tqdm",
-            ]
-        "#};
-
-        assert_snapshot!(format_err(input), @r###"
-        error: Failed to parse `pyproject.toml`
-          Caused by: Failed to parse entry for: `tqdm`
-          Caused by: Must specify a version constraint
-        "###);
-    }
-
     #[test]
     fn invalid_syntax() {
         let input = indoc! {r#"