## TL;DR
cargo cannot fetch the dependency's source: a git URL it cannot reach, or a registry it cannot download from. Identify which source failed, verify it is reachable, then retry the fetch. The manifest entry is usually fine; the network path is not.

## Error

```text
cargo 'failed to load source for dependency X'  -  how to fix
```

## Steps

1. Read the error for the source kind: a git URL versus the registry.
   Expected output: You know whether to debug git access or registry access.
2. For a git dependency, verify the URL and rev: try fetching the repo with git directly.
   Expected output: Either git succeeds (transient cargo issue) or it fails the same way (bad URL, missing rev, auth).
3. For a registry dependency, check network access to the registry and index; retry cargo fetch.
   Expected output: cargo downloads the crate sources.
4. Clear a poisoned cache entry if one fetch corrupted it: remove the crate from the registry cache and fetch again.
   Expected output: A clean download replaces the bad cache entry.
5. Build to confirm: cargo build.
   Expected output: All sources load and the build proceeds.

## When to use

- cargo fails with failed to load source for dependency X
- A git dependency points at a moved repo, a deleted branch, or a private repo without credentials
- The registry is reachable for browsing but cargo cannot download

## When not to use

- cargo cannot find the package name at all; that is a resolution error, not a source error
- The download succeeds but the checksum fails; that is an integrity error
- The manifest has a typo in the version; fix the version instead

## Variant phrasings

### cargo failed to load source for a git dependency
The git shape: URL, rev, or credentials are the suspects.

### failed to load source from the registry behind a proxy
The proxy shape: cargo needs the proxy configured where git and curl already work.

### Intermittent failed to load source in CI
Transient network blips; retry the fetch step before re-resolving.

## Why it happens

cargo separates resolving (picking versions) from loading sources (downloading them). Resolution can succeed against the index while the actual download fails: the git remote moved, the rev was force-pushed away, the registry is down, or a proxy blocks cargo but not the browser. The error names the source so you debug the fetch, not the version pick.

## Edge cases

- SSH git URLs need keys available to the cargo process; HTTPS URLs need credential helpers in CI.
- A force-pushed branch moves the rev the lockfile recorded; pin tags or full SHAs for stability.
- Corporate proxies often require config in cargo's own config file, not just env vars.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_1EohZbCWrSsoFBjYT7B7Qw