Show ambiguous requirements when `uv add` failed (#12106)

Improves the error message.

Fixes #12105

Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>

crates/uv-workspace/src/pyproject_mut.rs

@@ -44,8 +44,11 @@ pub enum Error {
     MalformedWorkspace,
     #[error("Expected a dependency at index {0}")]
     MissingDependency(usize),
-    #[error("Cannot perform ambiguous update; found multiple entries with matching package names")]
-    Ambiguous,
+    #[error("Cannot perform ambiguous update; found multiple entries for `{}`:\n{}", package_name, requirements.iter().map(|requirement| format!("- `{requirement}`")).join("\n"))]
+    Ambiguous {
+        package_name: PackageName,
+        requirements: Vec<Requirement>,
+    },
 }
 
 /// The result of editing an array in a TOML document.
@@ -1124,7 +1127,13 @@ pub fn add_dependency(
             Ok(ArrayEdit::Update(i))
         }
         // Cannot perform ambiguous updates.
-        _ => Err(Error::Ambiguous),
+        _ => Err(Error::Ambiguous {
+            package_name: req.name.clone(),
+            requirements: to_replace
+                .into_iter()
+                .map(|(_, requirement)| requirement)
+                .collect(),
+        }),
     }
 }
 

crates/uv/tests/it/edit.rs

@@ -10389,3 +10389,50 @@ fn add_auth_policy_never_without_credentials() -> Result<()> {
     context.assert_command("import anyio").success();
     Ok(())
 }
+
+/// Test the error message when adding a package with multiple existing references in
+/// `pyproject.toml`.
+#[test]
+fn add_ambiguous() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    context
+        .temp_dir
+        .child("pyproject.toml")
+        .write_str(indoc! {r#"
+        [project]
+        name = "foo"
+        version = "0.1.0"
+        requires-python = ">=3.12.0"
+        dependencies = [
+            "anyio>=4.0.0",
+            "anyio>=4.1.0",
+        ]
+        [dependency-groups]
+        bar = ["anyio>=4.1.0", "anyio>=4.2.0"]
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.add().arg("anyio"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Cannot perform ambiguous update; found multiple entries for `anyio`:
+    - `anyio>=4.0.0`
+    - `anyio>=4.1.0`
+    "###);
+
+    uv_snapshot!(context.filters(), context.add().arg("--group").arg("bar").arg("anyio"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Cannot perform ambiguous update; found multiple entries for `anyio`:
+    - `anyio>=4.1.0`
+    - `anyio>=4.2.0`
+    "###);
+
+    Ok(())
+}