Remove flatten to improve deserialization error messages (#7598)

## Summary

`#[serde(flatten)]` has a disastrous effect on error messages: serde no
longer tells you which field errored, nor does it show it to you in the
diagnostic output.

Before:

```
warning: Failed to parse `pyproject.toml` during settings discovery:
  TOML parse error at line 9, column 1
    |
  9 | [tool.uv]
    | ^^^^^^^^^
  invalid type: string "foo", expected a sequence
```

After:

```
warning: Failed to parse `pyproject.toml` during settings discovery:
  TOML parse error at line 10, column 19
     |
  10 | extra-index-url = "foo"
     |                   ^^^^^
  invalid type: string "foo", expected a sequence
```

Closes #7113.

crates/uv-settings/src/settings.rs

@@ -33,13 +33,15 @@ pub(crate) struct Tools {
 /// A `[tool.uv]` section.
 #[allow(dead_code)]
 #[derive(Debug, Clone, Default, Deserialize, CombineOptions, OptionsMetadata)]
-#[serde(rename_all = "kebab-case", deny_unknown_fields)]
+#[serde(from = "OptionsWire", rename_all = "kebab-case")]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub struct Options {
     #[serde(flatten)]
     pub globals: GlobalOptions,
+
     #[serde(flatten)]
     pub top_level: ResolverInstallerOptions,
+
     #[option_group]
     pub pip: Option<PipOptions>,
 
@@ -79,7 +81,6 @@ pub struct Options {
             cache-keys = [{ file = "pyproject.toml" }, { file = "requirements.txt" }, { git = true }]
         "#
     )]
-    #[serde(default, skip_serializing)]
     cache_keys: Option<Vec<CacheKey>>,
 
     // NOTE(charlie): These fields are shared with `ToolUv` in
@@ -92,28 +93,6 @@ pub struct Options {
 
     #[cfg_attr(feature = "schemars", schemars(skip))]
     pub environments: Option<SupportedEnvironments>,
-
-    // NOTE(charlie): These fields should be kept in-sync with `ToolUv` in
-    // `crates/uv-workspace/src/pyproject.rs`.
-    #[serde(default, skip_serializing)]
-    #[cfg_attr(feature = "schemars", schemars(skip))]
-    workspace: serde::de::IgnoredAny,
-
-    #[serde(default, skip_serializing)]
-    #[cfg_attr(feature = "schemars", schemars(skip))]
-    sources: serde::de::IgnoredAny,
-
-    #[serde(default, skip_serializing)]
-    #[cfg_attr(feature = "schemars", schemars(skip))]
-    dev_dependencies: serde::de::IgnoredAny,
-
-    #[serde(default, skip_serializing)]
-    #[cfg_attr(feature = "schemars", schemars(skip))]
-    managed: serde::de::IgnoredAny,
-
-    #[serde(default, skip_serializing)]
-    #[cfg_attr(feature = "schemars", schemars(skip))]
-    r#package: serde::de::IgnoredAny,
 }
 
 impl Options {
@@ -1472,3 +1451,171 @@ impl From<ToolOptions> for ResolverInstallerOptions {
         }
     }
 }
+
+/// Like [`Options]`, but with any `#[serde(flatten)]` fields inlined. This leads to far, far
+/// better error messages when deserializing.
+#[derive(Debug, Clone, Default, Deserialize)]
+#[serde(rename_all = "kebab-case", deny_unknown_fields)]
+pub struct OptionsWire {
+    // #[serde(flatten)]
+    // globals: GlobalOptions,
+    native_tls: Option<bool>,
+    offline: Option<bool>,
+    no_cache: Option<bool>,
+    cache_dir: Option<PathBuf>,
+    preview: Option<bool>,
+    python_preference: Option<PythonPreference>,
+    python_downloads: Option<PythonDownloads>,
+    concurrent_downloads: Option<NonZeroUsize>,
+    concurrent_builds: Option<NonZeroUsize>,
+    concurrent_installs: Option<NonZeroUsize>,
+
+    // #[serde(flatten)]
+    // top_level: ResolverInstallerOptions,
+    index_url: Option<IndexUrl>,
+    extra_index_url: Option<Vec<IndexUrl>>,
+    no_index: Option<bool>,
+    find_links: Option<Vec<FlatIndexLocation>>,
+    index_strategy: Option<IndexStrategy>,
+    keyring_provider: Option<KeyringProviderType>,
+    allow_insecure_host: Option<Vec<TrustedHost>>,
+    resolution: Option<ResolutionMode>,
+    prerelease: Option<PrereleaseMode>,
+    dependency_metadata: Option<Vec<StaticMetadata>>,
+    config_settings: Option<ConfigSettings>,
+    no_build_isolation: Option<bool>,
+    no_build_isolation_package: Option<Vec<PackageName>>,
+    exclude_newer: Option<ExcludeNewer>,
+    link_mode: Option<LinkMode>,
+    compile_bytecode: Option<bool>,
+    no_sources: Option<bool>,
+    upgrade: Option<bool>,
+    upgrade_package: Option<Vec<Requirement<VerbatimParsedUrl>>>,
+    reinstall: Option<bool>,
+    reinstall_package: Option<Vec<PackageName>>,
+    no_build: Option<bool>,
+    no_build_package: Option<Vec<PackageName>>,
+    no_binary: Option<bool>,
+    no_binary_package: Option<Vec<PackageName>>,
+
+    pip: Option<PipOptions>,
+    cache_keys: Option<Vec<CacheKey>>,
+
+    // NOTE(charlie): These fields are shared with `ToolUv` in
+    // `crates/uv-workspace/src/pyproject.rs`, and the documentation lives on that struct.
+    override_dependencies: Option<Vec<Requirement<VerbatimParsedUrl>>>,
+    constraint_dependencies: Option<Vec<Requirement<VerbatimParsedUrl>>>,
+    environments: Option<SupportedEnvironments>,
+
+    // NOTE(charlie): These fields should be kept in-sync with `ToolUv` in
+    // `crates/uv-workspace/src/pyproject.rs`.
+    #[allow(dead_code)]
+    workspace: Option<serde::de::IgnoredAny>,
+    #[allow(dead_code)]
+    sources: Option<serde::de::IgnoredAny>,
+    #[allow(dead_code)]
+    dev_dependencies: Option<serde::de::IgnoredAny>,
+    #[allow(dead_code)]
+    managed: Option<serde::de::IgnoredAny>,
+    #[allow(dead_code)]
+    r#package: Option<serde::de::IgnoredAny>,
+}
+
+impl From<OptionsWire> for Options {
+    fn from(value: OptionsWire) -> Self {
+        let OptionsWire {
+            native_tls,
+            offline,
+            no_cache,
+            cache_dir,
+            preview,
+            python_preference,
+            python_downloads,
+            concurrent_downloads,
+            concurrent_builds,
+            concurrent_installs,
+            index_url,
+            extra_index_url,
+            no_index,
+            find_links,
+            index_strategy,
+            keyring_provider,
+            allow_insecure_host,
+            resolution,
+            prerelease,
+            dependency_metadata,
+            config_settings,
+            no_build_isolation,
+            no_build_isolation_package,
+            exclude_newer,
+            link_mode,
+            compile_bytecode,
+            no_sources,
+            upgrade,
+            upgrade_package,
+            reinstall,
+            reinstall_package,
+            no_build,
+            no_build_package,
+            no_binary,
+            no_binary_package,
+            pip,
+            cache_keys,
+            override_dependencies,
+            constraint_dependencies,
+            environments,
+            workspace: _,
+            sources: _,
+            dev_dependencies: _,
+            managed: _,
+            package: _,
+        } = value;
+
+        Self {
+            globals: GlobalOptions {
+                native_tls,
+                offline,
+                no_cache,
+                cache_dir,
+                preview,
+                python_preference,
+                python_downloads,
+                concurrent_downloads,
+                concurrent_builds,
+                concurrent_installs,
+            },
+            top_level: ResolverInstallerOptions {
+                index_url,
+                extra_index_url,
+                no_index,
+                find_links,
+                index_strategy,
+                keyring_provider,
+                allow_insecure_host,
+                resolution,
+                prerelease,
+                dependency_metadata,
+                config_settings,
+                no_build_isolation,
+                no_build_isolation_package,
+                exclude_newer,
+                link_mode,
+                compile_bytecode,
+                no_sources,
+                upgrade,
+                upgrade_package,
+                reinstall,
+                reinstall_package,
+                no_build,
+                no_build_package,
+                no_binary,
+                no_binary_package,
+            },
+            pip,
+            cache_keys,
+            override_dependencies,
+            constraint_dependencies,
+            environments,
+        }
+    }
+}

crates/uv/tests/pip_install.rs

@@ -113,7 +113,7 @@ fn invalid_pyproject_toml_syntax() -> Result<()> {
 }
 
 #[test]
-fn invalid_pyproject_toml_schema() -> Result<()> {
+fn invalid_pyproject_toml_project_schema() -> Result<()> {
     let context = TestContext::new("3.12");
     let pyproject_toml = context.temp_dir.child("pyproject.toml");
     pyproject_toml.write_str("[project]")?;
@@ -139,6 +139,71 @@ fn invalid_pyproject_toml_schema() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn invalid_pyproject_toml_option_schema() -> Result<()> {
+    let context = TestContext::new("3.12");
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r"
+        [tool.uv]
+        index-url = true
+    "})?;
+
+    uv_snapshot!(context.pip_install()
+        .arg("iniconfig"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: Failed to parse `pyproject.toml` during settings discovery:
+      TOML parse error at line 2, column 13
+        |
+      2 | index-url = true
+        |             ^^^^
+      invalid type: boolean `true`, expected a string
+
+    Resolved 1 package in [TIME]
+    Prepared 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + iniconfig==2.0.0
+    "###
+    );
+
+    Ok(())
+}
+
+#[test]
+fn invalid_pyproject_toml_option_unknown_field() -> Result<()> {
+    let context = TestContext::new("3.12");
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [tool.uv]
+        unknown = "field"
+    "#})?;
+
+    uv_snapshot!(context.pip_install()
+        .arg("-r")
+        .arg("pyproject.toml"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    warning: Failed to parse `pyproject.toml` during settings discovery:
+      TOML parse error at line 2, column 1
+        |
+      2 | unknown = "field"
+        | ^^^^^^^
+      unknown field `unknown`, expected one of `native-tls`, `offline`, `no-cache`, `cache-dir`, `preview`, `python-preference`, `python-downloads`, `concurrent-downloads`, `concurrent-builds`, `concurrent-installs`, `index-url`, `extra-index-url`, `no-index`, `find-links`, `index-strategy`, `keyring-provider`, `allow-insecure-host`, `resolution`, `prerelease`, `dependency-metadata`, `config-settings`, `no-build-isolation`, `no-build-isolation-package`, `exclude-newer`, `link-mode`, `compile-bytecode`, `no-sources`, `upgrade`, `upgrade-package`, `reinstall`, `reinstall-package`, `no-build`, `no-build-package`, `no-binary`, `no-binary-package`, `pip`, `cache-keys`, `override-dependencies`, `constraint-dependencies`, `environments`, `workspace`, `sources`, `dev-dependencies`, `managed`, `package`
+
+    Resolved in [TIME]
+    Audited in [TIME]
+    "###
+    );
+
+    Ok(())
+}
+
 /// For indirect, non-user controlled pyproject.toml, we don't enforce correctness.
 ///
 /// If we fail to extract the PEP 621 metadata, we fall back to treating it as a source

crates/uv/tests/show_settings.rs

@@ -3146,11 +3146,11 @@ fn resolve_config_file() -> anyhow::Result<()> {
 
     ----- stderr -----
     error: Failed to parse: `[CACHE_DIR]/uv.toml`
-      Caused by: TOML parse error at line 1, column 1
+      Caused by: TOML parse error at line 1, column 2
       |
     1 | [project]
-      | ^
-    unknown field `project`
+      |  ^^^^^^^
+    unknown field `project`, expected one of `native-tls`, `offline`, `no-cache`, `cache-dir`, `preview`, `python-preference`, `python-downloads`, `concurrent-downloads`, `concurrent-builds`, `concurrent-installs`, `index-url`, `extra-index-url`, `no-index`, `find-links`, `index-strategy`, `keyring-provider`, `allow-insecure-host`, `resolution`, `prerelease`, `dependency-metadata`, `config-settings`, `no-build-isolation`, `no-build-isolation-package`, `exclude-newer`, `link-mode`, `compile-bytecode`, `no-sources`, `upgrade`, `upgrade-package`, `reinstall`, `reinstall-package`, `no-build`, `no-build-package`, `no-binary`, `no-binary-package`, `pip`, `cache-keys`, `override-dependencies`, `constraint-dependencies`, `environments`, `workspace`, `sources`, `dev-dependencies`, `managed`, `package`
 
     "###
     );