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2nd chunk of `src/backend/libs-and-metadata.md`
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* The [Strict Version Hash](#strict-version-hash) (SVH). This helps ensure the
  correct dependency is loaded.
* The [Stable Crate Id](#stable-crate-id). This is a hash used
  to identify crates.
* Information about all the source files in the library. This can be used for
  a variety of things, such as diagnostics pointing to sources in a
  dependency.
* Information about exported macros, traits, types, and items. Generally,
  anything that's needed to be known when a path references something inside a
  crate dependency.
* Encoded [MIR]. This is optional, and only encoded if needed for code
  generation. `cargo check` skips this for performance reasons.


### Strict Version Hash

The Strict Version Hash ([SVH], also known as the "crate hash") is a 64-bit
hash that is used to ensure that the correct crate dependencies are loaded. It
is possible for a directory to contain multiple copies of the same dependency
built with different settings, or built from different sources. The crate
loader will skip any crates that have the wrong SVH.

The SVH is also used for the [incremental compilation] session filename,
though that usage is mostly historic.

The hash includes a variety of elements:

* Hashes of the HIR nodes.
* All of the upstream crate hashes.
* All of the source filenames.
* Hashes of certain command-line flags (like `-C metadata` via the [Stable
  Crate Id](#stable-crate-id), and all CLI options marked with `[TRACKED]`).

See [`compute_hir_hash`] for where the hash is actually computed.


### Stable Crate Id

The [`StableCrateId`] is a 64-bit hash used to identify different crates with
potentially the same name. It is a hash of the crate name and all the
[`-C metadata`] CLI options computed in [`StableCrateId::new`]. It is
used in a variety of places, such as symbol name mangling, crate loading, and
much more.

By default, all Rust symbols are mangled and incorporate the stable crate id.
This allows multiple versions of the same crate to be included together. Cargo
automatically generates `-C metadata` hashes based on a variety of factors, like
the package version, source, and target kind (a lib and test can have the same
crate name, so they need to be disambiguated).


## Crate loading

Crate loading can have quite a few subtle complexities. During [name
resolution], when an external crate is referenced (via an `extern crate` or
Title: Strict Version Hash, Stable Crate ID, and Crate Loading in Rust
Summary
This section details the Strict Version Hash (SVH) and Stable Crate ID used in Rust for ensuring correct crate dependencies and identifying crates. The SVH is a 64-bit hash including HIR nodes, upstream crate hashes, source filenames, and certain command-line flags. The Stable Crate ID is a 64-bit hash of the crate name and `-C metadata` CLI options, used in symbol mangling and crate loading. By default, Rust symbols are mangled and incorporate the stable crate id, allowing multiple versions of the same crate to be included together. The section ends by introducing the complexities of crate loading during name resolution.