Load credentials for explicit members when lowering (#15844)

## Summary

If the target for `uv pip compile` is a `pyproject.toml` in a
subdirectory, we won't have loaded the credentials when we go to lower
(since it won't be loaded as part of "configuration discovery"). We now
add those indexes just-in-time.

Closes https://github.com/astral-sh/uv/issues/15362.

Cargo.lock

@@ -5624,6 +5624,7 @@ dependencies = [
  "toml",
  "tracing",
  "url",
+ "uv-auth",
  "uv-cache",
  "uv-cache-info",
  "uv-client",

crates/uv-distribution/Cargo.toml

@@ -16,6 +16,7 @@ doctest = false
 workspace = true
 
 [dependencies]
+uv-auth = { workspace = true }
 uv-cache = { workspace = true }
 uv-cache-info = { workspace = true }
 uv-client = { workspace = true }

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

@@ -1,6 +1,7 @@
 use std::collections::BTreeMap;
 use std::io;
 use std::path::{Path, PathBuf};
+use std::sync::Arc;
 
 use either::Either;
 use thiserror::Error;
@@ -222,20 +223,20 @@ impl LoweredRequirement {
                                 .find(|Index { name, .. }| {
                                     name.as_ref().is_some_and(|name| *name == index)
                                 })
-                                .map(
-                                    |Index {
-                                         url, format: kind, ..
-                                     }| IndexMetadata {
-                                        url: url.clone(),
-                                        format: *kind,
-                                    },
-                                )
                             else {
                                 return Err(LoweringError::MissingIndex(
                                     requirement.name.clone(),
                                     index,
                                 ));
                             };
+                            if let Some(credentials) = index.credentials() {
+                                let credentials = Arc::new(credentials);
+                                uv_auth::store_credentials(index.raw_url(), credentials);
+                            }
+                            let index = IndexMetadata {
+                                url: index.url.clone(),
+                                format: index.format,
+                            };
                             let conflict = project_name.and_then(|project_name| {
                                 if let Some(extra) = extra {
                                     Some(ConflictItem::from((project_name.clone(), extra)))
@@ -456,20 +457,20 @@ impl LoweredRequirement {
                                 .find(|Index { name, .. }| {
                                     name.as_ref().is_some_and(|name| *name == index)
                                 })
-                                .map(
-                                    |Index {
-                                         url, format: kind, ..
-                                     }| IndexMetadata {
-                                        url: url.clone(),
-                                        format: *kind,
-                                    },
-                                )
                             else {
                                 return Err(LoweringError::MissingIndex(
                                     requirement.name.clone(),
                                     index,
                                 ));
                             };
+                            if let Some(credentials) = index.credentials() {
+                                let credentials = Arc::new(credentials);
+                                uv_auth::store_credentials(index.raw_url(), credentials);
+                            }
+                            let index = IndexMetadata {
+                                url: index.url.clone(),
+                                format: index.format,
+                            };
                             let conflict = None;
                             let source = registry_source(&requirement, index, conflict);
                             (source, marker)

crates/uv/tests/it/pip_compile.rs

@@ -17784,3 +17784,69 @@ fn omit_python_patch_universal() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn credentials_from_subdirectory() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    // Create a local dependency in a subdirectory.
+    let pyproject_toml = context.temp_dir.child("foo").child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "foo"
+        version = "1.0.0"
+        dependencies = ["iniconfig"]
+
+        [build-system]
+        requires = ["hatchling"]
+        build-backend = "hatchling.build"
+
+        [tool.uv.sources]
+        iniconfig = { index = "internal" }
+
+        [[tool.uv.index]]
+        name = "internal"
+        url = "https://pypi-proxy.fly.dev/basic-auth/simple/"
+        explicit = true
+        "#,
+    )?;
+    context
+        .temp_dir
+        .child("foo")
+        .child("src")
+        .child("foo")
+        .child("__init__.py")
+        .touch()?;
+
+    uv_snapshot!(context.filters(), context
+        .pip_compile()
+        .arg("foo/pyproject.toml"), @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+
+    ----- stderr -----
+      × No solution found when resolving dependencies:
+      ╰─▶ Because iniconfig was not found in the package registry and foo depends on iniconfig, we can conclude that your requirements are unsatisfiable.
+    ");
+
+    uv_snapshot!(context.filters(), context
+        .pip_compile()
+        .arg("foo/pyproject.toml")
+        .env("UV_INDEX_INTERNAL_USERNAME", "public")
+        .env("UV_INDEX_INTERNAL_PASSWORD", "heron"), @r"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] foo/pyproject.toml
+    iniconfig==2.0.0
+        # via foo (foo/pyproject.toml)
+
+    ----- stderr -----
+    Resolved 1 package in [TIME]
+    ");
+
+    Ok(())
+}