build: Further split up build script.

Split up build/build.rs into build/build/*.rs and split
build/cargo.rs into {build/main.rs, build/build/cargo.rs}.

```
git diff HEAD^1:build/build.rs build/build/c.rs
git diff HEAD^1:build/build.rs build/build/core.rs
git diff HEAD^1:build/build.rs build/build/target.rs
git diff HEAD^1:build/cargo.rs build/main.rs
git diff HEAD^1:build/cargo.rs build/build/cargo.rs
```

Author: briansmith

Cargo.toml

@@ -1,5 +1,5 @@
 [package]
-build = "build/cargo.rs"
+build = "build/main.rs"
 categories = ["cryptography", "no-std"]
 description = "An experiment."
 edition = "2024"

build/build/c.rs

@@ -0,0 +1,178 @@
+// Copyright 2015-2026 Brian Smith.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+//! A wrapper around cc-rs.
+
+// Avoid `std::env` here. All configuration should be done through `Target`,
+// `Profile`, and `Tools`.
+use super::target::*;
+use std::path::{Path, PathBuf};
+
+fn cpp_flags(compiler: &cc::Tool) -> &'static [&'static str] {
+    if !compiler.is_like_msvc() {
+        static NON_MSVC_FLAGS: &[&str] = &[
+            "-fvisibility=hidden",
+            "-std=c1x", // GCC 4.6 requires "c1x" instead of "c11"
+            "-Wall",
+            "-Wbad-function-cast",
+            "-Wcast-align",
+            "-Wcast-qual",
+            "-Wconversion",
+            "-Wmissing-field-initializers",
+            "-Wmissing-include-dirs",
+            "-Wnested-externs",
+            "-Wredundant-decls",
+            "-Wshadow",
+            "-Wsign-compare",
+            "-Wsign-conversion",
+            "-Wstrict-prototypes",
+            "-Wundef",
+            "-Wuninitialized",
+        ];
+        NON_MSVC_FLAGS
+    } else {
+        static MSVC_FLAGS: &[&str] = &[
+            "/Gy", // Enable function-level linking.
+            "/Zc:wchar_t",
+            "/Zc:forScope",
+            "/Zc:inline",
+            // Warnings.
+            "/W4",
+            "/wd4127", // C4127: conditional expression is constant
+            "/wd4464", // C4464: relative include path contains '..'
+            "/wd5045", /* C5045: Compiler will insert Spectre mitigation for memory load if
+                        * /Qspectre switch specified */
+        ];
+        MSVC_FLAGS
+    }
+}
+
+pub struct Profile {
+    /// Is this a debug build? This affects whether assertions might be enabled
+    /// in the C code. For packaged builds, this should always be `false`.
+    pub is_debug: bool,
+
+    /// true: Force warnings to be treated as errors.
+    /// false: Use the default behavior (perhaps determined by `$CFLAGS`, etc.)
+    pub force_warnings_into_errors: bool,
+
+    pub is_git: bool,
+}
+
+fn new_build(
+    target: &Target,
+    profile: &Profile,
+    c_root_dir: &Path,
+    include_dir: &Path,
+) -> cc::Build {
+    let mut b = cc::Build::new();
+    configure_cc(&mut b, target, profile, c_root_dir, include_dir);
+    b
+}
+
+pub fn build_library<'a>(
+    target: &Target,
+    profile: &Profile,
+    c_root_dir: &Path,
+    lib_name: &str,
+    srcs: impl Iterator<Item = &'a PathBuf>,
+    include_dir: &Path,
+    preassembled_objs: &[PathBuf],
+) {
+    let mut c = new_build(target, profile, c_root_dir, include_dir);
+
+    // Compile all the (dirty) source files into object files.
+    srcs.for_each(|src| {
+        c.file(c_root_dir.join(src));
+    });
+
+    preassembled_objs.iter().for_each(|obj| {
+        c.object(obj);
+    });
+
+    // Rebuild the library if necessary.
+    let lib_path = target.out_dir.join(format!("lib{lib_name}.a"));
+
+    // Handled below.
+    let _ = c.cargo_metadata(false);
+
+    c.compile(
+        lib_path
+            .file_name()
+            .and_then(|f| f.to_str())
+            .expect("No filename"),
+    );
+
+    // Link the library. This works even when the library doesn't need to be
+    // rebuilt.
+    println!("cargo:rustc-link-lib=static={lib_name}");
+    println!(
+        "cargo:rustc-link-search=native={}",
+        target.out_dir.to_str().expect("Invalid path")
+    );
+}
+
+fn configure_cc(
+    c: &mut cc::Build,
+    target: &Target,
+    profile: &Profile,
+    c_root_dir: &Path,
+    include_dir: &Path,
+) {
+    // FIXME: On Windows AArch64 we currently must use Clang to compile C code.
+    // clang-cl emulates the cl.exe command line, `$CFLAGS`, etc.
+    if target.os == WINDOWS && target.arch == AARCH64 {
+        let _: &_ = c.prefer_clang_cl_over_msvc(true);
+    };
+    let compiler = c.get_compiler();
+
+    let _ = c.include(c_root_dir.join("include"));
+    let _ = c.include(include_dir);
+    for f in cpp_flags(&compiler) {
+        let _ = c.flag(f);
+    }
+
+    if APPLE_ABI.contains(&target.os.as_str()) {
+        // ``-gfull`` is required for Darwin's |-dead_strip|.
+        let _ = c.flag("-gfull");
+    } else if !compiler.is_like_msvc() {
+        let _ = c.flag("-g3");
+    };
+
+    if !profile.is_debug {
+        let _ = c.define("NDEBUG", None);
+    }
+
+    if target.arch == X86 {
+        let is_msvc_not_clang_cl = compiler.is_like_msvc() && !compiler.is_like_clang_cl();
+        if !is_msvc_not_clang_cl {
+            let _ = c.flag("-msse2");
+        }
+    }
+
+    // Allow cross-compiling without a target sysroot for these targets.
+    if (target.arch == WASM32)
+        || (target.os == "linux" && target.env == "musl" && target.arch != X86_64)
+    {
+        // TODO: Expand this to non-clang compilers in 0.17.0 if practical.
+        if compiler.is_like_clang() {
+            let _ = c.flag("-nostdlibinc");
+            let _ = c.define("RING_CORE_NOSTDLIBINC", "1");
+        }
+    }
+
+    if profile.force_warnings_into_errors {
+        c.warnings_into_errors(true);
+    }
+}

build/build/cargo.rs

@@ -0,0 +1,162 @@
+// Copyright 2015-2016 Brian Smith.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+
+//! Cargo-specific build logic that isn't specific to this project.
+
+use super::{Profile, Target, target::Endian};
+// Avoid `std::env` here; use `self::env` instead.
+use std::{
+    fs::{self},
+    path::{Path, PathBuf},
+};
+
+pub mod env {
+    use std::ffi::OsString;
+
+    macro_rules! define_env {
+        { $vis:vis $NAME:ident : $ty:ident } => {
+            $vis const $NAME: EnvVar = EnvVar {
+                name: stringify!($NAME),
+                ty: EnvVarTy::$ty,
+            };
+        };
+    }
+
+    enum EnvVarTy {
+        RerunIfChanged,
+        SetByCargo,
+    }
+
+    pub struct EnvVar {
+        pub name: &'static str,
+        ty: EnvVarTy,
+    }
+
+    /// Read an environment variable and optionally tell Cargo that we depend on it.
+    ///
+    /// The env var is static since we intend to only read a static set of environment
+    /// variables.
+    pub fn var_os(env_var: &'static EnvVar) -> Option<OsString> {
+        match env_var.ty {
+            EnvVarTy::RerunIfChanged => {
+                println!("cargo:rerun-if-env-changed={}", env_var.name);
+            }
+            EnvVarTy::SetByCargo => {}
+        }
+        std::env::var_os(env_var.name)
+    }
+
+    pub fn var(env_var: &'static EnvVar) -> Option<String> {
+        var_os(env_var).and_then(|value| value.into_string().ok())
+    }
+
+    // In alphabetical order
+    define_env! { pub(super) CARGO_CFG_TARGET_ARCH: SetByCargo }
+    define_env! { pub(super) CARGO_CFG_TARGET_ENDIAN: SetByCargo }
+    define_env! { pub(super) CARGO_CFG_TARGET_ENV: SetByCargo }
+    define_env! { pub(super) CARGO_CFG_TARGET_OS: SetByCargo }
+    define_env! { pub(super) CARGO_MANIFEST_DIR: SetByCargo }
+    define_env! { pub(super) CARGO_MANIFEST_LINKS: SetByCargo }
+    define_env! { pub(super) CARGO_PKG_NAME: SetByCargo }
+    define_env! { pub(super) CARGO_PKG_VERSION_MAJOR: SetByCargo }
+    define_env! { pub(super) CARGO_PKG_VERSION_MINOR: SetByCargo }
+    define_env! { pub(super) CARGO_PKG_VERSION_PATCH: SetByCargo }
+    define_env! { pub(super) CARGO_PKG_VERSION_PRE: SetByCargo }
+    define_env! { pub(super) DEBUG: SetByCargo }
+    define_env! { pub(super) OUT_DIR: SetByCargo }
+
+    // XXX: These don't belong here.
+    define_env! { pub PERL_EXECUTABLE: RerunIfChanged }
+    define_env! { pub RING_PREGENERATE_ASM: RerunIfChanged }
+}
+
+pub fn root_dir() -> PathBuf {
+    // Avoid assuming the working directory is the same is the $CARGO_MANIFEST_DIR so that toolchains
+    // which may assume other working directories can still build this code.
+    PathBuf::from(
+        env::var_os(&env::CARGO_MANIFEST_DIR).expect("CARGO_MANIFEST_DIR should always be set"),
+    )
+}
+
+pub fn extern_c_prefix(component: &str) -> String {
+    // Keep in sync with `core_name_and_version!` in prefixed.rs.
+    let core_name_and_version = [
+        &env::var(&env::CARGO_PKG_NAME).unwrap(),
+        component,
+        &env::var(&env::CARGO_PKG_VERSION_MAJOR).unwrap(),
+        &env::var(&env::CARGO_PKG_VERSION_MINOR).unwrap(),
+        &env::var(&env::CARGO_PKG_VERSION_PATCH).unwrap(),
+        &env::var(&env::CARGO_PKG_VERSION_PRE).unwrap(), // Often empty
+    ]
+    .join("_");
+    // Ensure `links` in Cargo.toml is consistent with the version.
+    assert_eq!(
+        &env::var(&env::CARGO_MANIFEST_LINKS).unwrap(),
+        &core_name_and_version
+    );
+    core_name_and_version
+}
+
+impl Target {
+    pub fn new_from_env() -> Self {
+        let arch = env::var(&env::CARGO_CFG_TARGET_ARCH).unwrap();
+        let os = env::var(&env::CARGO_CFG_TARGET_OS).unwrap();
+        let env = env::var(&env::CARGO_CFG_TARGET_ENV).unwrap();
+        let endian = env::var(&env::CARGO_CFG_TARGET_ENDIAN).unwrap();
+        let endian = if endian == "little" {
+            Endian::Little
+        } else {
+            Endian::Other
+        };
+
+        let out_dir = PathBuf::from(env::var_os(&env::OUT_DIR).unwrap());
+
+        Self {
+            arch,
+            os,
+            env,
+            endian,
+            out_dir,
+        }
+    }
+}
+
+impl Profile {
+    pub fn new_from_env(root_dir: &Path) -> Self {
+        let is_git = fs::metadata(root_dir.join(".git")).is_ok();
+
+        // Published builds are always built in release mode.
+        let is_debug = is_git && env::var(&env::DEBUG).unwrap() != "false";
+
+        // During local development, force warnings in non-Rust code to be treated
+        // as errors. Since warnings are highly compiler-dependent and compilers
+        // don't maintain backward compatibility w.r.t. which warnings they issue,
+        // don't do this for packaged builds.
+        let force_warnings_into_errors = is_git;
+
+        Self {
+            is_git,
+            is_debug,
+            force_warnings_into_errors,
+        }
+    }
+}
+
+// TODO: We should emit `cargo:rerun-if-changed-env` for the various
+// environment variables that affect the build.
+pub fn emit_rerun_if_changed<'a>(paths: impl Iterator<Item = &'a Path>) {
+    paths.for_each(|path| {
+        println!("cargo:rerun-if-changed={}", path.to_str().unwrap());
+    })
+}