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§vergen - Emit cargo instructions from a build script

vergen, when used in conjunction with cargo build scripts can emit the following:

• Will emit cargo:rustc-env=VAR=VALUE for each feature you have enabled. These can be referenced with the env! macro in your code.
• Will emit cargo:rerun-if-changed=.git/HEAD if the git feature is enabled. This is done to ensure any git instructions are regenerated when commits are made.
• Will emit cargo:rerun-if-changed=.git/<path_to_ref> if the git feature is enabled. This is done to ensure any git instructions are regenerated when commits are made.
• Can emit cargo:warning outputs if the fail_on_error feature is not enabled and the requested variable is defaulted through error or the idempotent flag.
• Will emit cargo:rerun-if-changed=build.rs to rerun instruction emission if the build.rs file changed.
• Will emit cargo:rerun-if-env-changed=VERGEN_IDEMPOTENT to rerun instruction emission if the VERGEN_IDEMPOTENT environment variable has changed.
• Will emit cargo:rerun-if-env-changed=SOURCE_DATE_EPOCH to rerun instruction emission if the SOURCE_DATE_EPOCH environment variable has changed.

§Usage

1. Ensure you have build scripts enabled via the build configuration in your Cargo.toml

[package]
#..
build = "build.rs"

2. Add vergen as a build dependency in Cargo.toml, specifying the features you wish to enable.

[dependencies]
#..

[build-dependencies]
# All features enabled
vergen = { version = "8.0.0", features = ["build", "cargo", "git", "gitcl", "rustc", "si"] }
# or
vergen = { version = "8.0.0", features = ["build", "git", "gitcl"] }
# if you wish to disable certain features

3. Create a build.rs file that uses vergen to emit cargo instructions. Configuration starts with EmitBuilder. Eventually you will call emit to output the cargo instructions. See the emit documentation for more robust examples.

§Generate all output

use anyhow::Result;
use vergen::EmitBuilder;

pub fn main() -> Result<()> {
    // NOTE: This will output everything, and requires all features enabled.
    // NOTE: See the EmitBuilder documentation for configuration options.
    EmitBuilder::builder()
        .all_build()
        .all_cargo()
        .all_git()
        .all_rustc()
        .all_sysinfo()
        .emit()?;
    Ok(())
}

§Generate specific output

use anyhow::Result;
use vergen::EmitBuilder;

pub fn main() -> Result<()> {
    // NOTE: This will output only a build timestamp and long SHA from git.
    // NOTE: This set requires the build and git features.
    // NOTE: See the EmitBuilder documentation for configuration options.
    EmitBuilder::builder()
        .build_timestamp()
        .git_sha(false)
        .emit()?;
    Ok(())
}

4. Use the env! macro in your code to read the environment variables.

println!("Build Timestamp: {}", env!("VERGEN_BUILD_TIMESTAMP"));
println!("git describe: {}", env!("VERGEN_GIT_DESCRIBE"));

§Features

vergen has five main feature toggles allowing you to customize your output. No features are enabled by default.
You must specifically enable the features you wish to use.

§Configuring the git feature

If you wish to use the git feature, you must also enable one of the git implementations. The gitcl features is lightweight, but depends on git being on the path. The other implementations allow for git instructions to be emitted without a reliance on the git binary. The git2 library are bindings over the libgit2 library, while gitoxide is entirely implemented in Rust.

NOTE - These 3 features are mutually exclusive. Only one can be chosen. If you select multiple, vergen intentionally will not compile.

A common configuration would be as follows:

[build-dependencies]
vergen = { version = "8.0.0", features = [ "build", "git", "gitcl" ]}
# ...

§Cargo feature unification for vergen versions prior to 8.3.0

When a dependency is used by multiple packages, Cargo will use the union of all features enabled on that dependency when building it. Prior to version 8.3.0, vergen had a set of mutually exclusive features gitcl, git2, and gitoxide to enable to specific git backend you wished to use. If your crate has a dependency on another crate that uses vergen, your crate may fail to compile if you select a different git backend then the crate you depend on. For example, your crate depends on fancy-lib.

§fancy-lib Cargo.toml

[build-dependencies]
vergen = { version = "8.2.10", features = ["git","gitcl"] }

§your crate Cargo.toml

[dependencies]
fancy-lib = "0.1.0"

[build-dependencies]
vergen = { version = "8.2.10", features = ["git","gitoxide"] }

Your crate will fail to compile because cargo unifies this to

vergen = { version = "8.2.10", features = ["git","gitcl","gitoxide"] }

and prior to 8.3.0 vergen will not compile with both gitcl and gitoxide as features.

As a workaround, you can use cargo tree -f "{p} {f}" | grep vergen to determine the feature list cargo has set for vergen. If a git backend has already been determined you will be able to use that without declaring those features in your dependency list. This is not perfect as this leaves you at the mercy of your dependency and the git feature they selected, but it’s a workaround until version 9 comes out.

§fancy-lib Cargo.toml

[build-dependencies]
vergen = { version = "8.2.10", features = ["git","gitcl"] }

§your crate Cargo.toml

[dependencies]
fancy-lib = "0.1.0"

[build-dependencies]
vergen = "8.2.10"

§Unified

vergen = { version = "8.2.10", features = ["git","gitcl"] }

§Cargo feature unification for vergen versions 8.3.0 and beyond

vergen will accept gitcl,git2, and gitoxide as features. If more than one of them is included, vergen will select gitcl before git2 and git2 before gitoxide.

§Environment Variables

vergen currently recognizes the following environment variables

§Goals

I initially wrote vergen (version generator, so original) so I could embed a some git information in my personal projects. Now, usage has grown to the point that vergen needs to fit better in the rust ecosystem.

The current goals are as follows:

§Minimize the tool footprint

• Adopt an opt-in, rather than opt-out strategy for the features. The default feature set is empty and no instructions will be emitted.
• The instructions you have configured will be emitted. If there are errors or idempotentcy has been configured, some of those instructions may be defaulted.
• Allow overriding configurtion set in build.rs through environment variables. This will allow package maintainers to force sane defaults when packaging rust binaries for distribution.

§Minimize the compile time impact

• git2 and gitoxide are large features. These are opt-in now. I’ve also added back support for generating git instructions via the git binary.
• I’ve removed some extraneous libraries. Any libraries added in the future will be checked against the current standard compile times to ensure the impact is not too great.
• vergen should compile and test from a source tarball.

§Support deterministic output

Compilations run from the same source oftentimes need to generate identical binaries. vergen now supports this determinism in a few ways.

• An idempotent configuration option has been added. When this is enabled in a build script, each build via cargo against the same source code should generate identical binaries. Instructions that output information that may change between builds (i.e. timestamps, sysinfo) will be defaulted.
• Recognize common environment variables that support deterministic builds (i.e. SOURCE_DATE_EPOCH)
• Allow build.rs configuration overrides though environment variables to allow users building a binary, but not controlling the source to generate deterministic binaries.

§Use Cases

I generally use vergen for the following two cases

1. Generating verbose output describing a command line application.

~/p/r/app λ app -vv
app 0.1.0

Build Timestamp:     2021-02-23T20:14:46.558472672+00:00
Describe:            0.1.0-9-g46f83e1
Commit SHA:          46f83e112520533338245862d366f6a02cef07d4
Commit Date:         2021-02-23T08:08:02-05:00
Commit Branch:       master
rustc Version:       1.52.0-nightly
rustc Channel:       nightly
rustc Host Triple:   x86_64-unknown-linux-gnu
rustc Commit SHA:    3f5aee2d5241139d808f4fdece0026603489afd1
cargo Target Triple: x86_64-unknown-linux-musl
cargo Profile:       release

2. Information endpoints in web apis

~/p/r/app λ curl https://some.app.com/info | jq
{
  "build_timestamp": "2021-02-19T21:32:22.932833758+00:00",
  "git_describe": "0.0.0-7-gc96c096",
  "git_sha": "c96c0961c3b7b749eab92f6f588b67915889c4cd",
  "git_commit_date": "2021-02-19T16:29:06-05:00",
  "git_branch": "master",
  "rustc_semver": "1.52.0-nightly",
  "rustc_channel": "nightly",
  "rustc_host_triple": "x86_64-unknown-linux-gnu",
  "rustc_commit_sha": "3f5aee2d5241139d808f4fdece0026603489afd1",
  "cargo_target_triple": "x86_64-unknown-linux-musl",
  "cargo_profile": "release"
}