Error when an explicit index is unnamed (#17777)

Since you cannot reference these indexes, I don't think we need a
warning and deprecation period.

Co-authored-by: Claude <noreply@anthropic.com>

crates/uv-distribution-types/src/index.rs

@@ -51,7 +51,7 @@ impl IndexCacheControl {
     }
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Clone, Serialize)]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 #[serde(rename_all = "kebab-case")]
 pub struct Index {
@@ -545,6 +545,56 @@ impl<'a> From<&'a IndexUrl> for IndexMetadataRef<'a> {
     }
 }
 
+/// Wire type for deserializing an [`Index`] with validation.
+#[derive(Deserialize)]
+#[serde(rename_all = "kebab-case")]
+struct IndexWire {
+    name: Option<IndexName>,
+    url: IndexUrl,
+    #[serde(default)]
+    explicit: bool,
+    #[serde(default)]
+    default: bool,
+    #[serde(default)]
+    format: IndexFormat,
+    publish_url: Option<DisplaySafeUrl>,
+    #[serde(default)]
+    authenticate: AuthPolicy,
+    #[serde(default)]
+    ignore_error_codes: Option<Vec<SerializableStatusCode>>,
+    #[serde(default)]
+    cache_control: Option<IndexCacheControl>,
+}
+
+impl<'de> Deserialize<'de> for Index {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let wire = IndexWire::deserialize(deserializer)?;
+
+        if wire.explicit && wire.name.is_none() {
+            return Err(serde::de::Error::custom(format!(
+                "An index with `explicit = true` requires a `name`: {}",
+                wire.url
+            )));
+        }
+
+        Ok(Self {
+            name: wire.name,
+            url: wire.url,
+            explicit: wire.explicit,
+            default: wire.default,
+            origin: None,
+            format: wire.format,
+            publish_url: wire.publish_url,
+            authenticate: wire.authenticate,
+            ignore_error_codes: wire.ignore_error_codes,
+            cache_control: wire.cache_control,
+        })
+    }
+}
+
 /// An error that can occur when parsing an [`Index`].
 #[derive(Error, Debug)]
 pub enum IndexSourceError {

crates/uv/tests/it/lock.rs

@@ -18910,6 +18910,50 @@ fn lock_explicit_default_index() -> Result<()> {
     Ok(())
 }
 
+/// Error when an explicit index does not have a name.
+#[test]
+fn lock_unnamed_explicit_index() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["iniconfig==2.0.0"]
+
+        [[tool.uv.index]]
+        url = "https://test.pypi.org/simple"
+        explicit = true
+        "#,
+    )?;
+
+    uv_snapshot!(context.filters(), context.lock(), @r#"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: Failed to parse `pyproject.toml` during settings discovery:
+      TOML parse error at line 8, column 9
+        |
+      8 |         [[tool.uv.index]]
+        |         ^^^^^^^^^^^^^^^^^
+      An index with `explicit = true` requires a `name`: https://test.pypi.org/simple
+
+    error: Failed to parse: `pyproject.toml`
+      Caused by: TOML parse error at line 8, column 9
+      |
+    8 |         [[tool.uv.index]]
+      |         ^^^^^^^^^^^^^^^^^
+    An index with `explicit = true` requires a `name`: https://test.pypi.org/simple
+    "#);
+
+    Ok(())
+}
+
 #[test]
 fn lock_named_index() -> Result<()> {
     let context = TestContext::new("3.12");