cargo-rail 0.27.0

//! Deterministic Cargo feature reachability analysis.
//!
//! Builds roots from package visibility, defaults, resolved configurations,
//! workspace consumers, source cfgs, Cargo target requirements, and policy,
//! then follows member-local feature edges.
//!
//! This module distinguishes between:
//! - **Truly dead features**: unreachable empty no-ops in private packages
//! - **Optional features**: Enable deps/features but not currently used - NOT safe to remove
//!
//! Only features in `publish = false` packages outside that closure are
//! removable, and only when configuration declares the workspace to be the
//! complete consumer scope. Cycles terminate because each feature is visited once.

use cargo_metadata::Package;
use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet, VecDeque};
use std::path::Path;

use super::multi_target_metadata::MultiTargetMetadata;
use crate::compiler::cfg_eval::{TargetCfgSet, cfg_expression_may_apply};
use crate::error::{RailError, RailResult};
use crate::workspace::WorkspaceSnapshot;

/// Feature conditions derived from source bytes bound to one workspace capture.
pub(crate) struct CapturedSourceFeatures {
    /// Minimal feature selections that can make a parsed cfg expression true.
    pub(crate) selections: Vec<Vec<String>>,
    /// Every feature name referenced by captured source.
    pub(crate) referenced: BTreeSet<String>,
    /// Parsed cfg expressions retained for target applicability decisions.
    pub(crate) expressions: BTreeSet<String>,
    /// Identifiers mentioned by captured Rust source or the package README.
    ///
    /// These conservatively retain declarations when compiler evidence cannot
    /// distinguish a documented integration from an accidental unused entry.
    pub(crate) retention_identifiers: BTreeSet<String>,
}

/// Derive source feature evidence from one authoritative workspace capture.
pub(crate) fn scan_snapshot_source_features(
    snapshot: &WorkspaceSnapshot,
    package_root: &Path,
) -> RailResult<CapturedSourceFeatures> {
    let mut selections = BTreeSet::new();
    let mut referenced = BTreeSet::new();
    let mut expressions = BTreeSet::new();
    let mut retention_identifiers = BTreeSet::new();
    let entries = snapshot.source().tree().entries();
    let package_start = entries.partition_point(|entry| entry.path.as_path() < package_root);
    for entry in &entries[package_start..] {
        let Ok(package_path) = entry.path.as_path().strip_prefix(package_root) else {
            break;
        };
        let rust_source = analysis_rust_source(package_path);
        if !rust_source && package_path != Path::new("README.md") {
            continue;
        }
        let bytes = snapshot.read_source_file(&entry.path)?.ok_or_else(|| {
            RailError::message(format!(
                "captured analysis source '{}' is absent from its workspace snapshot",
                entry.path
            ))
        })?;
        let content = std::str::from_utf8(&bytes)
            .map_err(|_| RailError::message(format!("captured analysis source '{}' is not valid UTF-8", entry.path)))?;
        extract_identifiers(content, &mut retention_identifiers);
        if !rust_source {
            continue;
        }
        let mut file_features = HashSet::new();
        extract_cfg_features(content, &mut file_features);
        referenced.extend(file_features);
        for expression in extract_cfg_expressions(content) {
            selections.extend(crate::compiler::cfg_eval::feature_selections_for_cfg(expression));
            expressions.insert(expression.to_string());
        }
    }
    Ok(CapturedSourceFeatures {
        selections: selections
            .into_iter()
            .filter(|selection| !selection.is_empty())
            .collect(),
        referenced,
        expressions,
        retention_identifiers,
    })
}

fn extract_identifiers(content: &str, identifiers: &mut BTreeSet<String>) {
    identifiers.extend(
        content
            .split(|character: char| !(character.is_ascii_alphanumeric() || character == '_'))
            .filter(|identifier| !identifier.is_empty())
            .map(str::to_string),
    );
}

fn analysis_rust_source(package_path: &Path) -> bool {
    if package_path == Path::new("build.rs") {
        return true;
    }
    package_path.extension().and_then(std::ffi::OsStr::to_str) == Some("rs")
        && ["src", "tests", "benches", "examples"]
            .iter()
            .any(|root| package_path.starts_with(root))
}

/// Whether a Cargo feature edge activates or configures a dependency alias.
pub(crate) fn feature_edge_references_dependency(edge: &str, dependency: &str) -> bool {
    edge == dependency
        || edge.strip_prefix("dep:") == Some(dependency)
        || edge
            .strip_prefix(dependency)
            .is_some_and(|suffix| suffix.starts_with('/') || suffix.starts_with("?/"))
}

fn insert_feature_roots<I, S>(roots: &mut BTreeMap<String, &'static str>, features: I, root_kind: &'static str)
where
    I: IntoIterator<Item = S>,
    S: AsRef<str>,
{
    for feature in features {
        roots.entry(feature.as_ref().to_string()).or_insert(root_kind);
    }
}

fn applicable_source_features(
    manifest: &crate::cargo::manifest_analyzer::ParsedManifest,
    targets: &[&str],
    cfg_sets: &HashMap<String, TargetCfgSet>,
) -> HashSet<String> {
    let mut applicable = HashSet::new();
    let mut expression_features = HashSet::new();
    for expression in &manifest.source_cfg_expressions {
        let mut features = HashSet::new();
        extract_cfg_features(expression, &mut features);
        expression_features.extend(features.iter().cloned());
        if cfg_expression_may_apply(expression, targets.iter().map(|target| cfg_sets.get(*target))) {
            applicable.extend(features);
        }
    }
    // Preserve references not attributable to a parsed cfg expression. This
    // includes malformed syntax, where absence is not proven.
    applicable.extend(
        manifest
            .source_cfg_features
            .iter()
            .filter(|feature| !expression_features.contains(*feature))
            .cloned(),
    );
    applicable
}

fn extract_cfg_expressions(content: &str) -> Vec<&str> {
    let mut expressions = Vec::new();
    let mut offset = 0;
    while let Some(relative) = content.get(offset..).and_then(|remaining| remaining.find("cfg(")) {
        let start = offset + relative + 4;
        let bytes = content.as_bytes();
        let mut depth = 1usize;
        let mut index = start;
        let mut quoted = false;
        while index < bytes.len() {
            match bytes[index] {
                b'"' if index == 0 || bytes[index - 1] != b'\\' => quoted = !quoted,
                b'(' if !quoted => depth += 1,
                b')' if !quoted => {
                    depth -= 1;
                    if depth == 0 {
                        if let Some(expression) = content.get(start..index) {
                            expressions.push(expression);
                        }
                        offset = index + 1;
                        break;
                    }
                }
                _ => {}
            }
            index += 1;
        }
        if depth != 0 {
            break;
        }
    }
    expressions
}

/// Extract feature names from `feature = "name"` patterns in source code
///
/// Handles patterns like:
/// - `#[cfg(feature = "foo")]`
/// - `#[cfg(not(feature = "foo"))]`
/// - `#[cfg_attr(feature = "foo", ...)]`
/// - `cfg!(feature = "foo")`
fn extract_cfg_features(content: &str, features: &mut HashSet<String>) {
    // Look for `feature = "` or `feature="` patterns
    let mut remaining = content;

    while let Some((_, after_feature)) = remaining.split_once("feature") {
        remaining = after_feature;

        // Skip whitespace
        let trimmed = remaining.trim_start();

        // Check for `=`
        let Some(after_eq) = trimmed.strip_prefix('=') else {
            continue;
        };
        let after_eq = after_eq.trim_start();

        // Check for opening quote
        let Some(after_quote) = after_eq.strip_prefix('"') else {
            continue;
        };

        // Find closing quote
        if let Some((feature_name, _)) = after_quote.split_once('"')
            && !feature_name.is_empty()
            && is_valid_feature_name(feature_name)
        {
            features.insert(feature_name.to_string());
        }
    }
}

/// Check if a string is a valid Cargo feature name
fn is_valid_feature_name(s: &str) -> bool {
    !s.is_empty() && s.chars().all(|c| c.is_ascii_alphanumeric() || c == '_' || c == '-')
}

/// Result of analyzing a crate's features
#[derive(Debug, Clone)]
pub struct FeatureScanResult {
    /// Crate name that was analyzed
    pub crate_name: String,
    /// Features declared in Cargo.toml
    pub declared_features: HashSet<String>,
    /// Features that are actually enabled in the resolved graph
    pub enabled_features: HashSet<String>,
    /// Features in `publish = false` packages unreachable from every root.
    pub dead_features: HashSet<String>,
    /// Inactive forwarding features retained as public API in published packages.
    pub optional_features: HashSet<String>,
    /// Deterministic root-to-feature path for every feature proven reachable.
    pub reachability_paths: BTreeMap<String, FeatureReachability>,
}

/// Deterministic explanation for one reachable feature.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct FeatureReachability {
    /// Semantic root that starts the path.
    pub root_kind: &'static str,
    /// Member-local feature path, including root and destination.
    pub path: Vec<String>,
}

/// Analyzes workspace features using resolved cargo metadata
#[derive(Debug)]
pub struct FeatureScanner;

impl FeatureScanner {
    /// Analyze a single workspace member for dead and optional features
    ///
    /// Uses the resolved metadata across all targets to determine which
    /// declared features are never enabled.
    ///
    /// A feature is considered "alive" (not dead/optional) if:
    /// 1. It's enabled in the resolved graph for any target, OR
    /// 2. It's referenced by any workspace crate's feature definition (e.g., `dep/feature`), OR
    /// 3. It's referenced by another feature in the same crate (e.g., `production = ["api-key"]`), OR
    /// 4. It's used in source code via `#[cfg(feature = "...")]` (conditional compilation)
    ///
    /// For features that are NOT alive, we distinguish:
    /// - **Dead features**: Empty no-ops in private packages - safe to remove
    /// - **Optional features**: Enable something (deps, other features) - user-facing API, don't remove
    pub fn analyze_crate(
        pkg: &Package,
        metadata: &MultiTargetMetadata,
        referenced_features: &HashMap<String, HashSet<String>>,
        source_referenced: &HashSet<String>,
        preserved_features: &HashSet<String>,
        workspace_is_consumer_scope: bool,
    ) -> FeatureScanResult {
        let crate_name = pkg.name.to_string();

        // Get declared features from the package
        let declared_features: HashSet<String> = pkg.features.keys().cloned().collect();

        // Get enabled features across all targets from the resolved graph
        let enabled_across_targets = metadata.all_features(&crate_name);
        let enabled_features: HashSet<String> = enabled_across_targets.values().flatten().cloned().collect();

        // Get features referenced by other workspace crates (even if not currently enabled)
        let externally_referenced = referenced_features.get(&crate_name).cloned().unwrap_or_default();

        let cargo_target_required: HashSet<&str> = pkg
            .targets
            .iter()
            .flat_map(|target| target.required_features.iter().map(String::as_str))
            .collect();

        let private_package = pkg.publish.as_ref().is_some_and(Vec::is_empty);
        let removable_package = private_package && workspace_is_consumer_scope;
        let mut roots = BTreeMap::new();
        if removable_package {
            insert_feature_roots(&mut roots, enabled_features.iter(), "resolved");
            insert_feature_roots(&mut roots, externally_referenced.iter(), "workspace_consumer");
            insert_feature_roots(&mut roots, source_referenced, "source_cfg");
            insert_feature_roots(&mut roots, cargo_target_required, "cargo_target_required");
            insert_feature_roots(&mut roots, preserved_features.iter(), "preservation_policy");
            if declared_features.contains("default") {
                roots.entry("default".to_string()).or_insert("default");
            }
        } else {
            let root_kind = if private_package {
                "open_consumer_scope"
            } else {
                "published_api"
            };
            insert_feature_roots(&mut roots, declared_features.iter(), root_kind);
        }

        let reachability_paths = Self::reachable_feature_paths(pkg, &declared_features, roots);
        let dead_features = if removable_package {
            declared_features
                .iter()
                .filter(|feature| !reachability_paths.contains_key(*feature))
                .cloned()
                .collect()
        } else {
            HashSet::new()
        };

        // Kept for the reporting contract: non-private inactive forwarding
        // features are public API, not removal candidates.
        let optional_features = if removable_package {
            HashSet::new()
        } else {
            declared_features
                .difference(&enabled_features)
                .filter(|feature| pkg.features.get(*feature).is_some_and(|edges| !edges.is_empty()))
                .cloned()
                .collect()
        };

        FeatureScanResult {
            crate_name,
            declared_features,
            enabled_features,
            dead_features,
            optional_features,
            reachability_paths,
        }
    }

    fn reachable_feature_paths(
        pkg: &Package,
        declared_features: &HashSet<String>,
        roots: BTreeMap<String, &'static str>,
    ) -> BTreeMap<String, FeatureReachability> {
        let mut paths = BTreeMap::new();
        let mut queue = VecDeque::new();
        for (root, root_kind) in roots
            .into_iter()
            .filter(|(feature, _)| declared_features.contains(feature))
        {
            paths.insert(
                root.clone(),
                FeatureReachability {
                    root_kind,
                    path: vec![root.clone()],
                },
            );
            queue.push_back(root);
        }

        while let Some(feature) = queue.pop_front() {
            let Some(edges) = pkg.features.get(&feature) else {
                continue;
            };
            let mut internal: Vec<_> = edges
                .iter()
                .filter(|edge| !edge.contains('/') && !edge.starts_with("dep:"))
                .filter(|edge| declared_features.contains(*edge))
                .collect();
            internal.sort_unstable();
            for next in internal {
                if paths.contains_key(next) {
                    continue;
                }
                let Some(parent) = paths.get(&feature).cloned() else {
                    continue;
                };
                let mut path = parent.path;
                path.push(next.clone());
                paths.insert(
                    next.clone(),
                    FeatureReachability {
                        root_kind: parent.root_kind,
                        path,
                    },
                );
                queue.push_back(next.clone());
            }
        }
        paths
    }

    /// Analyze all workspace members for dead and optional features
    ///
    /// Returns results for crates that have dead or optional features.
    pub fn analyze_workspace(
        metadata: &MultiTargetMetadata,
        manifests: &crate::cargo::manifest_analyzer::ManifestAnalyzer,
        preserve: impl Fn(&str) -> bool,
        workspace_is_consumer_scope: bool,
        target_cfg_sets: &HashMap<String, TargetCfgSet>,
    ) -> Vec<FeatureScanResult> {
        // First, build a map of all features referenced by workspace crates
        // This catches conditional features like `dep/feature` in [features] tables
        let referenced_features = Self::build_referenced_features_map(metadata);
        let manifests_by_package = manifests
            .members
            .iter()
            .map(|manifest| (&manifest.package_id, manifest))
            .collect::<HashMap<_, _>>();

        let workspace_pkg_count = metadata.workspace_packages().len();
        let mut results = Vec::with_capacity(workspace_pkg_count);

        for pkg in metadata.workspace_packages() {
            // Skip crates with no declared features
            if pkg.features.is_empty() {
                continue;
            }

            let preserved_features = pkg
                .features
                .keys()
                .filter(|feature| preserve(feature))
                .cloned()
                .collect();
            let source_referenced = manifests_by_package
                .get(&pkg.id)
                .map(|manifest| applicable_source_features(manifest, &metadata.targets(), target_cfg_sets))
                .unwrap_or_default();
            let result = Self::analyze_crate(
                pkg,
                metadata,
                &referenced_features,
                &source_referenced,
                &preserved_features,
                workspace_is_consumer_scope,
            );

            results.push(result);
        }

        results
    }

    /// Build a map of features referenced by workspace crates' feature definitions
    ///
    /// Scans all workspace packages' `[features]` tables to find references like:
    /// - `dep/feature` (enables `feature` in dependency `dep`)
    /// - `dep?/feature` (optional dep feature)
    ///
    /// Returns: HashMap<crate_name, HashSet<feature_name>>
    /// where the features are ones referenced externally by other crates.
    fn build_referenced_features_map(metadata: &MultiTargetMetadata) -> HashMap<String, HashSet<String>> {
        let mut map: HashMap<String, HashSet<String>> = HashMap::new();

        for pkg in metadata.workspace_packages() {
            // Scan all feature definitions in this package
            for feature_deps in pkg.features.values() {
                for dep_str in feature_deps {
                    // Parse feature references like "dep/feature" or "dep?/feature"
                    if let Some((dep_name, feature_name)) = Self::parse_feature_reference(dep_str) {
                        map.entry(dep_name).or_default().insert(feature_name);
                    }
                }
            }
        }

        map
    }

    /// Parse a feature reference string to extract dep name and feature
    ///
    /// Handles formats:
    /// - `dep/feature` -> Some(("dep", "feature"))
    /// - `dep?/feature` -> Some(("dep", "feature"))
    /// - `dep:feature` (older format) -> Some(("dep", "feature"))
    /// - `feature` (just a feature name) -> None
    /// - `dep:dep` (enabling optional dep) -> None
    fn parse_feature_reference(s: &str) -> Option<(String, String)> {
        // Try slash format first (most common): dep/feature or dep?/feature
        if let Some((dep_part, feature)) = s.split_once('/') {
            // Remove trailing ? from optional deps
            let dep_name = dep_part.trim_end_matches('?');
            if !feature.is_empty() {
                return Some((dep_name.to_string(), feature.to_string()));
            }
        }

        None
    }

    /// Get total count of dead features across workspace
    pub fn count_dead_features(results: &[FeatureScanResult]) -> usize {
        results.iter().map(|r| r.dead_features.len()).sum()
    }

    /// Get total count of optional features across workspace
    pub fn count_optional_features(results: &[FeatureScanResult]) -> usize {
        results.iter().map(|r| r.optional_features.len()).sum()
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_feature_scan_result_dead_features() {
        let mut declared = HashSet::new();
        declared.insert("foo".to_string());
        declared.insert("bar".to_string());
        declared.insert("default".to_string());

        let mut enabled = HashSet::new();
        enabled.insert("foo".to_string());

        // bar is dead (declared but not enabled)
        // default is NOT dead (special case)
        let dead: HashSet<String> = declared
            .difference(&enabled)
            .filter(|f| *f != "default")
            .cloned()
            .collect();

        assert!(dead.contains("bar"));
        assert!(!dead.contains("default"));
        assert!(!dead.contains("foo"));
        assert_eq!(dead.len(), 1);
    }

    #[test]
    fn test_parse_feature_reference() {
        // Standard dep/feature format
        assert_eq!(
            FeatureScanner::parse_feature_reference("tikv_alloc/mimalloc"),
            Some(("tikv_alloc".to_string(), "mimalloc".to_string()))
        );

        // Optional dep format: dep?/feature
        assert_eq!(
            FeatureScanner::parse_feature_reference("serde?/derive"),
            Some(("serde".to_string(), "derive".to_string()))
        );

        // Just a feature name (no dep reference)
        assert_eq!(FeatureScanner::parse_feature_reference("std"), None);

        // dep:dep format (enabling optional dep, not a feature)
        assert_eq!(FeatureScanner::parse_feature_reference("dep:serde"), None);

        // Empty feature after slash
        assert_eq!(FeatureScanner::parse_feature_reference("dep/"), None);
    }

    #[test]
    fn test_internally_referenced_filter() {
        // Test the filter logic for internally referenced features
        // This simulates what build_internally_referenced_features does

        let feature_deps = vec![
            // production = ["api-key", "server"]
            vec!["api-key".to_string(), "server".to_string()],
            // server = ["build"]
            vec!["build".to_string()],
            // build = ["dep/feature"] (external ref)
            vec!["dep/feature".to_string()],
            // with-serde = ["dep:serde"] (dep: syntax)
            vec!["dep:serde".to_string()],
        ];

        let mut referenced = HashSet::new();
        for deps in &feature_deps {
            for dep_str in deps {
                if dep_str.contains('/') || dep_str.starts_with("dep:") {
                    continue;
                }
                referenced.insert(dep_str.clone());
            }
        }

        // api-key is referenced by production
        assert!(
            referenced.contains("api-key"),
            "api-key should be internally referenced"
        );
        // server is referenced by production
        assert!(referenced.contains("server"), "server should be internally referenced");
        // build is referenced by server
        assert!(referenced.contains("build"), "build should be internally referenced");
        // dep/feature is NOT a plain feature reference (has slash)
        assert!(!referenced.contains("dep/feature"), "dep/feature should not be in set");
        // dep:serde is NOT a plain feature reference (has dep:)
        assert!(!referenced.contains("dep:serde"), "dep:serde should not be in set");
    }

    #[test]
    fn test_extract_cfg_features_basic_patterns() {
        let mut features = HashSet::new();

        // Standard cfg attribute
        extract_cfg_features(r#"#[cfg(feature = "foo")]"#, &mut features);
        assert!(features.contains("foo"), "should detect cfg(feature = \"foo\")");

        // No spaces around =
        features.clear();
        extract_cfg_features(r#"#[cfg(feature="bar")]"#, &mut features);
        assert!(features.contains("bar"), "should detect feature=\"bar\"");

        // Extra spaces
        features.clear();
        extract_cfg_features(r#"#[cfg(feature   =   "baz")]"#, &mut features);
        assert!(features.contains("baz"), "should detect with extra spaces");

        // cfg! macro
        features.clear();
        extract_cfg_features(r#"if cfg!(feature = "qux") {"#, &mut features);
        assert!(features.contains("qux"), "should detect cfg! macro");
    }

    #[test]
    fn test_extract_cfg_features_compound_patterns() {
        let mut features = HashSet::new();

        // not(feature = "...")
        extract_cfg_features(r#"#[cfg(not(feature = "disabled"))]"#, &mut features);
        assert!(features.contains("disabled"), "should detect not(feature)");

        // all(..., feature = "...")
        features.clear();
        extract_cfg_features(r#"#[cfg(all(unix, feature = "unix-ext"))]"#, &mut features);
        assert!(features.contains("unix-ext"), "should detect in all()");

        // any(..., feature = "...")
        features.clear();
        extract_cfg_features(r#"#[cfg(any(feature = "a", feature = "b"))]"#, &mut features);
        assert!(features.contains("a"), "should detect first in any()");
        assert!(features.contains("b"), "should detect second in any()");

        // cfg_attr
        features.clear();
        extract_cfg_features(r#"#[cfg_attr(feature = "serde", derive(Serialize))]"#, &mut features);
        assert!(features.contains("serde"), "should detect in cfg_attr");
    }

    #[test]
    fn test_extract_cfg_expressions_balances_nested_conditions() {
        assert_eq!(
            extract_cfg_expressions(r#"#[cfg(all(feature = "a", not(feature = "b")))] fn selected() {}"#),
            vec![r#"all(feature = "a", not(feature = "b"))"#]
        );
    }

    #[test]
    fn test_extract_cfg_features_real_world() {
        let mut features = HashSet::new();

        // Real example from helix-db
        let helix_code = r#"
      #[cfg(feature = "api-key")]
      match headers.get("x-api-key") {
          Some(key) => validate_key(key),
          None => return Err(AuthError),
      }
      #[cfg(not(feature = "api-key"))]
      Ok(())
    "#;
        extract_cfg_features(helix_code, &mut features);
        assert!(
            features.contains("api-key"),
            "should detect api-key from helix-db pattern"
        );

        // Real example from polars
        features.clear();
        let polars_code = r#"
      #[cfg(feature = "gather")]
      pub fn gather(&self, indices: &IdxCa) -> PolarsResult<Self> {
          // ...
      }
    "#;
        extract_cfg_features(polars_code, &mut features);
        assert!(features.contains("gather"), "should detect gather from polars pattern");

        // Real example from ruff
        features.clear();
        let ruff_code = r#"
      #[cfg(feature = "singlethreaded")]
      fn run_single() { }
      #[cfg(not(feature = "singlethreaded"))]
      fn run_parallel() { }
    "#;
        extract_cfg_features(ruff_code, &mut features);
        assert!(
            features.contains("singlethreaded"),
            "should detect singlethreaded from ruff pattern"
        );
    }

    #[test]
    fn test_extract_cfg_features_edge_cases() {
        let mut features = HashSet::new();

        // Feature with hyphen
        extract_cfg_features(r#"#[cfg(feature = "my-feature")]"#, &mut features);
        assert!(features.contains("my-feature"), "should handle hyphens");

        // Feature with underscore
        features.clear();
        extract_cfg_features(r#"#[cfg(feature = "my_feature")]"#, &mut features);
        assert!(features.contains("my_feature"), "should handle underscores");

        // Variable assignment like `let feature = "value"` technically matches our pattern
        // This is a safe false positive - we'll keep the feature rather than prune it
        // The alternative would require a full Rust parser, which is overkill
        features.clear();
        extract_cfg_features(r#"let feature = "not-a-cfg";"#, &mut features);
        // False positives are acceptable - they just prevent pruning a dead feature

        // Should NOT match: feature in a string literal
        features.clear();
        extract_cfg_features(r#"println!("feature = \"quoted\"");"#, &mut features);
        // This might match - that's acceptable, false positives are safe (we keep the feature)

        // Should NOT match: invalid feature names
        features.clear();
        extract_cfg_features(r#"#[cfg(feature = "")]"#, &mut features);
        assert!(features.is_empty(), "should not match empty feature name");
    }

    #[test]
    fn test_is_valid_feature_name() {
        // Valid names
        assert!(is_valid_feature_name("foo"));
        assert!(is_valid_feature_name("foo-bar"));
        assert!(is_valid_feature_name("foo_bar"));
        assert!(is_valid_feature_name("foo123"));
        assert!(is_valid_feature_name("FOO"));

        // Invalid names
        assert!(!is_valid_feature_name(""));
        assert!(!is_valid_feature_name("foo bar"));
        assert!(!is_valid_feature_name("foo/bar"));
        assert!(!is_valid_feature_name("foo:bar"));
    }
}