Function fix_includes 

pub fn fix_includes<S: BuildHasher>(
    files: &mut HashMap<PathBuf, PreprocFile, S>,
    all_files: &HashMap<String, Vec<PathBuf>, S>,
) -> Vec<PreprocDiagnostic>

Constructs a dependency graph of the include directives in a C/C++ file.

The dependency graph is built using both preprocessor data and not extracted from the considered C/C++ files.

Best-effort include resolution emits non-fatal PreprocDiagnostics (self-inclusions, include cycles, non-UTF-8 paths, files referenced but never preprocessed) as the returned Vec rather than writing to stderr, so an embedder can capture or suppress them. The CLI prints them to stderr; callers that do not care may discard the result.

Panics

Panics if any of the lockstep invariants between the include graph g, the nodes map, and the scc_map is violated at runtime — specifically: an SCC component node missing from the graph, a graph node weight without a nodes map entry, a DFS-visited node without a stored weight, or an empty-path replacement node without a scc_map entry. These are built in lockstep here, so all four are unrecoverable programmer errors rather than reachable input failures. The last two live on the per-file fallback walk, which now runs only when the precomputed include closures could not order the graph.