An error occurred when compiling the binary using the libssh2-sys package, due to the lack of inclusion of OpenSSL header files.

[Kila2]

There is a geneate content.
Before building the libssh2-sys_build_script, we need to build the openssl-sys package first. The openssl-sys package will write the DEP_OPENSSL_INCLUDE variable to the depenv file. Additionally, the libssh2-sys_build_script should be sourced from the depenvs file.

###############################################################################
# @generated
# DO NOT MODIFY: This file is auto-generated by a crate_universe tool. To 
# regenerate this file, run the following:
#
#     bazel sync --only=crate_index
###############################################################################

load("@rules_rust//cargo:defs.bzl", "cargo_build_script")

load("@rules_rust//rust:defs.bzl", "rust_library")

# buildifier: disable=bzl-visibility
load("@rules_rust//crate_universe/private:selects.bzl", "selects")

package(default_visibility = ["//visibility:public"])

# licenses([
#     "TODO",  # MIT/Apache-2.0
# ])

rust_library(
    name = "libssh2_sys",
    deps = [
        "@crate_index__libc-0.2.137//:libc",
        "@crate_index__libssh2-sys-0.2.23//:build_script_build",
        "@crate_index__libz-sys-1.1.8//:libz_sys",
    ] + select({
        "@rules_rust//rust/platform:x86_64-apple-darwin": [
            "@crate_index__openssl-sys-0.9.78//:openssl_sys",  # cfg(unix)
        ],
        "//conditions:default": [],
    }),
    compile_data = glob(
        include = ["**"],
        exclude = [
            "**/* *",
            "BUILD",
            "BUILD.bazel",
            "WORKSPACE",
            "WORKSPACE.bazel",
        ],
    ),
    crate_root = "lib.rs",
    edition = "2015",
    rustc_flags = ["--cap-lints=allow"],
    srcs = glob(["**/*.rs"]),
    tags = [
        "cargo-bazel",
        "crate-name=libssh2-sys",
        "manual",
        "noclippy",
        "norustfmt",
    ],
    version = "0.2.23",
)

cargo_build_script(
    name = "libssh2-sys_build_script",
    crate_name = "build_script_build",
    crate_root = "build.rs",
    data = glob(
        include = ["**"],
        exclude = [
            "**/* *",
            "BUILD",
            "BUILD.bazel",
            "WORKSPACE",
            "WORKSPACE.bazel",
        ],
    ),
    deps = [
        "@crate_index__cc-1.0.77//:cc",
        "@crate_index__pkg-config-0.3.26//:pkg_config",
    ],
    edition = "2015",
    links = "ssh2",
    rustc_flags = [
        "--cap-lints=allow",
    ],
    srcs = glob(["**/*.rs"]),
    tags = [
        "cargo-bazel",
        "crate-name=libssh2-sys",
        "manual",
        "noclippy",
        "norustfmt",
    ],
    version = "0.2.23",
    visibility = ["//visibility:private"],
)

alias(
    name = "build_script_build",
    actual = "libssh2-sys_build_script",
    tags = ["manual"],
)

[UebelAndre]

Could you share your crates_repository definition and the root Cargo.toml if you're using Cargo manifests?

[Kila2]

I modify rules_rust/example/crate_universe/cargo_workspace/rng/BUILD.bazel and rules_rust/example/crate_universe/cargo_workspace/rng/Cargo.toml to reproduce issues.
The BUILD.bazel

load("@crate_index_cargo_workspace//:defs.bzl", "all_crate_deps")
load("@rules_rust//rust:defs.bzl", "rust_library")

package(default_visibility = ["//visibility:public"])

rust_library(
    name = "rng",
    srcs = ["src/lib.rs"],
    edition = "2018",
    deps = all_crate_deps(),
)

The Cargo.toml

[package]
name = "rng"
version = "0.1.0"
edition = "2018"

[dependencies]
rand = "0.7.3"
openssl = { version = "0.10", features = ["vendored"] }
git2 = "0.15"

[UebelAndre]

Ok, after reading the following
https://doc.rust-lang.org/cargo/reference/build-script-examples.html#using-another-sys-crate
https://doc.rust-lang.org/cargo/reference/build-scripts.html#build-dependencies

It seems this is a gap in the cargo_build_script functionality. I can't find anywhere where we're ensuring variables output by build scripts of dependency crates are available in one from the current crate. I'd be happy to review a pull request that added this functionality. I've added some testing in #1943 that shows the current expectations of parsing build script dependencies and what code would likely need to be updated and I would expect some changes to be made to https://github.com/bazelbuild/rules_rust/tree/0.21.1/cargo followed by tests at https://github.com/bazelbuild/rules_rust/tree/0.21.1/test/cargo_build_script. Again, I'm happy to review a PR or give further guidance 😄