Warn when duplicate index names found in single file (#11824)

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## Summary

This pull request introduces validation for unique index names in the
`tool.uv.index` field and adds corresponding tests to ensure the
functionality. The most important changes include adding a custom
deserializer function, updating the `ToolUv` struct to use the new
deserializer, and adding tests to verify the behavior.

Validation and deserialization:

*
[`crates/uv-workspace/src/pyproject.rs`](diffhunk://#diff-e12cd255985adfd45ab06f398cb420d2f543841ccbeea4175ccf827aa9215b9dR283-R311):
Added a custom deserializer function `deserialize_index_vec` to validate
that index names in the `tool.uv.index` field are unique.
*
[`crates/uv-workspace/src/pyproject.rs`](diffhunk://#diff-e12cd255985adfd45ab06f398cb420d2f543841ccbeea4175ccf827aa9215b9dR374):
Updated the `ToolUv` struct to use the `deserialize_index_vec` function
for the `index` field.

Testing:

*
[`crates/uv/tests/it/lock.rs`](diffhunk://#diff-82edd36151736f44055f699a34c8b19a63ffc4cf3c86bf5fb34d69f8ac88a957R15336):
Added a test `lock_repeat_named_index` to verify that duplicate index
names result in an error.
[[1]](diffhunk://#diff-82edd36151736f44055f699a34c8b19a63ffc4cf3c86bf5fb34d69f8ac88a957R15336)
[[2]](diffhunk://#diff-82edd36151736f44055f699a34c8b19a63ffc4cf3c86bf5fb34d69f8ac88a957R15360-R15402)
*
[`crates/uv/tests/it/lock.rs`](diffhunk://#diff-82edd36151736f44055f699a34c8b19a63ffc4cf3c86bf5fb34d69f8ac88a957R15360-R15402):
Added a test `lock_unique_named_index` to verify that unique index names
result in successful lock file generation.

Schema update:

*
[`uv.schema.json`](diffhunk://#diff-c669473b258a19ba6d3557d0369126773b68b27171989f265333a77bc5cb935bR205):
Updated the schema to set the default value of the `index` field to
`null`.

Fixes #11804

## Test Plan
### Steps to reproduce and verify the fix:

1. Clone the repository and checkout the feature branch
   ```bash
   git clone https://github.com/astral-sh/uv.git
   cd uv
   git checkout feature/warn-duplicate-index-names
   ```

2. Build the modified binary
   ```bash
   cargo build
   ```

3. Create a test project using the system installed uv
   ```bash
   uv init uv-test
   cd uv-test
   ```

4. Manually edit pyproject.toml to add duplicate index names
   ```toml
   [[tool.uv.index]]
   name = "alpha_b"
   url = "<omitted>"

   [[tool.uv.index]]
   name = "alpha_b"
   url = "<omitted>"
   ```

5. Try to add a package using the modified binary
   ```bash
   ../target/debug/uv add numpy
   ```

### Results
Before: use release binary
![スクリーンショット 2025-02-27 15 52
28](https://github.com/user-attachments/assets/2823a4b4-b3ba-40aa-aa41-e593d35c3f3b)

After: use self build binary
![スクリーンショット 2025-02-27 15 51
58](https://github.com/user-attachments/assets/9ac773a9-58cd-4d4b-8685-148bf6dc85fb)

Now when attempting to use a pyproject.toml with duplicate index names,
the modified binary correctly detects the issue and produces an error
message:
```
error: Failed to parse: `pyproject.toml`
  Caused by: TOML parse error at line 9, column 1
  |
9 | [[tool.uv.index]]
  | ^^^^^^^^^^^^^^^^^
duplicate index name `alpha_b`
```

---------

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

crates/uv-workspace/src/pyproject.rs

@@ -14,6 +14,7 @@ use std::str::FromStr;
 
 use glob::Pattern;
 use owo_colors::OwoColorize;
+use rustc_hash::{FxBuildHasher, FxHashSet};
 use serde::{de::IntoDeserializer, de::SeqAccess, Deserialize, Deserializer, Serialize};
 use thiserror::Error;
 use url::Url;
@@ -280,6 +281,30 @@ pub struct Tool {
     pub uv: Option<ToolUv>,
 }
 
+/// Validates that index names in the `tool.uv.index` field are unique.
+///
+/// This custom deserializer function checks for duplicate index names
+/// and returns an error if any duplicates are found.
+fn deserialize_index_vec<'de, D>(deserializer: D) -> Result<Option<Vec<Index>>, D::Error>
+where
+    D: Deserializer<'de>,
+{
+    let indexes = Option::<Vec<Index>>::deserialize(deserializer)?;
+    if let Some(indexes) = indexes.as_ref() {
+        let mut seen_names = FxHashSet::with_capacity_and_hasher(indexes.len(), FxBuildHasher);
+        for index in indexes {
+            if let Some(name) = index.name.as_ref() {
+                if !seen_names.insert(name) {
+                    return Err(serde::de::Error::custom(format!(
+                        "duplicate index name `{name}`"
+                    )));
+                }
+            }
+        }
+    }
+    Ok(indexes)
+}
+
 // NOTE(charlie): When adding fields to this struct, mark them as ignored on `Options` in
 // `crates/uv-settings/src/settings.rs`.
 #[derive(Deserialize, OptionsMetadata, Debug, Clone, PartialEq, Eq)]
@@ -342,6 +367,7 @@ pub struct ToolUv {
             url = "https://download.pytorch.org/whl/cu121"
         "#
     )]
+    #[serde(deserialize_with = "deserialize_index_vec", default)]
     pub index: Option<Vec<Index>>,
 
     /// The workspace definition for the project, if any.

crates/uv/tests/it/lock.rs

@@ -15328,11 +15328,7 @@ fn lock_named_index_cli() -> Result<()> {
     Ok(())
 }
 
-/// If a name is reused, the higher-priority index should "overwrite" the lower-priority index.
-/// In other words, the lower-priority index should be ignored entirely during implicit resolution.
-///
-/// In this test, we should use PyPI (the default index) and ignore `https://example.com` entirely.
-/// (Querying `https://example.com` would fail with a 500.)
+/// If a name is reused, within a single file, we should raise an error.
 #[test]
 fn lock_repeat_named_index() -> Result<()> {
     let context = TestContext::new("3.12");
@@ -15356,6 +15352,46 @@ fn lock_repeat_named_index() -> Result<()> {
         "#,
     )?;
 
+    uv_snapshot!(context.filters(), context.lock(), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Failed to parse: `pyproject.toml`
+      Caused by: TOML parse error at line 8, column 9
+      |
+    8 |         [[tool.uv.index]]
+      |         ^^^^^^^^^^^^^^^^^
+    duplicate index name `pytorch`
+    "###);
+
+    Ok(())
+}
+
+#[test]
+fn lock_unique_named_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"]
+
+        [[tool.uv.index]]
+        name = "pytorch"
+        url = "https://astral-sh.github.io/pytorch-mirror/whl/cu121"
+
+        [[tool.uv.index]]
+        name = "example"
+        url = "https://example.com"
+        "#,
+    )?;
+
     // Fall back to PyPI, since `iniconfig` doesn't exist on the PyTorch index.
     uv_snapshot!(context.filters(), context.lock(), @r###"
     success: true