move abi checks to a feature and build the C code at build time

The abi tests are now conditional to the abi-tests feature,
when that is enabled, the C code for the abi tests is now built
at compile time, using the cc crate rather than having our own
Compiler abstraction and having to compile stuff at runtime and
deal with temp directories etc.

src/codegen/sys/build.rs

@@ -3,7 +3,7 @@ use crate::{codegen::general, env::Env, file_saver::save_to_file};
 use log::info;
 use std::io::{Result, Write};
 
-pub fn generate(env: &Env) {
+pub fn generate(env: &Env, has_abi_tests: bool) {
     info!(
         "Generating sys build script for {}",
         env.config.library_name
@@ -15,7 +15,7 @@ pub fn generate(env: &Env) {
     if !split_build_rs || !path.exists() {
         info!("Generating file {:?}", path);
         save_to_file(&path, env.config.make_backup, |w| {
-            generate_build_script(w, env, split_build_rs)
+            generate_build_script(w, env, split_build_rs, has_abi_tests)
         });
     }
 
@@ -29,11 +29,17 @@ pub fn generate(env: &Env) {
 }
 
 #[allow(clippy::write_literal)]
-fn generate_build_script(w: &mut dyn Write, env: &Env, split_build_rs: bool) -> Result<()> {
+fn generate_build_script(
+    w: &mut dyn Write,
+    env: &Env,
+    split_build_rs: bool,
+    has_abi_tests: bool,
+) -> Result<()> {
     if !split_build_rs {
         general::start_comments(w, &env.config)?;
         writeln!(w)?;
     }
+
     writeln!(
         w,
         "{}",
@@ -55,13 +61,42 @@ fn main() {} // prevent linking libraries to avoid documentation failure
 
 #[cfg(not(feature = "dox"))]
 fn main() {
-    if let Err(s) = system_deps::Config::new().probe() {
+    let deps = system_deps::Config::new().probe();
+    if let Err(s) = deps {
         println!("cargo:warning={}", s);
         process::exit(1);
+    }"##
+    )?;
+
+    if has_abi_tests {
+        write!(
+            w,
+            "{}",
+            r##"
+
+        #[cfg(feature = "abi-tests")]
+        {
+            let mut cc = cc::Build::new();
+
+            cc.flag_if_supported("-Wno-deprecated-declarations");
+            cc.flag_if_supported("-std=c11"); // for _Generic
+            cc.flag_if_supported("-std:c11"); // for _Generic (MSVC)
+
+            cc.file("tests/constant.c");
+            cc.file("tests/layout.c");
+
+            for i in deps.unwrap().all_include_paths() {
+                cc.include(i);
+            }
+
+            cc.compile("cabitests");
+        }
     }
-}
 "##
-    )
+        )
+    } else {
+        writeln!(w, "}}")
+    }
 }
 
 fn generate_build_version(w: &mut dyn Write, env: &Env) -> Result<()> {

src/codegen/sys/cargo_toml.rs

@@ -83,14 +83,13 @@ fn fill_in(root: &mut Table, env: &Env) {
 
     {
         let build_deps = upsert_table(root, "build-dependencies");
+        set_string(build_deps, "cc", "1.0");
         set_string(build_deps, "system-deps", "3");
     }
 
     {
         let dev_deps = upsert_table(root, "dev-dependencies");
         set_string(dev_deps, "shell-words", "1.0.0");
-        set_string(dev_deps, "tempfile", "3");
-        unset(dev_deps, "tempdir");
     }
 
     {
@@ -115,6 +114,7 @@ fn fill_in(root: &mut Table, env: &Env) {
                     .collect(),
             ),
         );
+        features.insert("abi-tests".to_string(), Value::Array(Vec::new()));
     }
 
     {

src/codegen/sys/mod.rs

@@ -13,9 +13,9 @@ mod tests;
 pub fn generate(env: &Env) {
     generate_single_version_file(env);
     lib_::generate(env);
-    build::generate(env);
     let crate_name = cargo_toml::generate(env);
-    tests::generate(env, &crate_name);
+    let has_abi_tests = tests::generate(env, &crate_name);
+    build::generate(env, has_abi_tests);
 }
 
 pub fn collect_versions(env: &Env) -> BTreeMap<Version, Version> {