You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: resolve.rs
    path of file: ./repos/cargo/src/cargo/ops
    
    the code of the file until where you have to start completion: //! High-level APIs for executing the resolver.
//!
//! This module provides functions for running the resolver given a workspace, including loading
//! the `Cargo.lock` file and checkinf if it needs updating.
//!
//! There are roughly 3 main func
    
fn master_branch_git_source(id: PackageId, resolve: &Resolve) -> Option<PackageId> {
    if resolve.version() <= ResolveVersion::V2 {
        let source = id.source_id();
        if let Some(GitReference::DefaultBranch) = source.git_reference() {
            let new_source =
                SourceId::for_git(source.url(), GitReference::Branch("master".to_string()))
                    .unwrap()
                    .with_precise_from(source);
            return Some(id.with_source_id(new_source));
        }
    }
    None
}