Introduce RUSTC_STATIC_CLANG_RT_PATH and RUSTC_STATIC_UNWIND_PATH envs

The goal of this envirements variable is easy statically link
`libclang_rt` or `libunwind` into rust.

It also introduces a way to easy extend rust build in a way which allows
to inject more static builds for some rare system if it required.

It haven't been documented because it seems as very deep hack and the
user who needs it will discover this hack by reading sources ;)

Author: catap

Cargo.lock

@@ -4995,6 +4995,7 @@ version = "0.0.0"
 dependencies = [
  "addr2line",
  "alloc",
+ "build_helper",
  "cfg-if 0.1.10",
  "compiler_builtins",
  "core",
@@ -5662,6 +5663,7 @@ dependencies = [
 name = "unwind"
 version = "0.0.0"
 dependencies = [
+ "build_helper",
  "cc",
  "cfg-if 0.1.10",
  "compiler_builtins",

compiler/rustc_llvm/build.rs

@@ -2,7 +2,7 @@ use std::env;
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
-use build_helper::{output, tracked_env_var_os};
+use build_helper::{maybe_static_library, output, tracked_env_var_os};
 
 fn detect_llvm_link() -> (&'static str, &'static str) {
     // Force the link mode we want, preferring static by default, but
@@ -307,4 +307,6 @@ fn main() {
     if target.contains("windows-gnu") {
         println!("cargo:rustc-link-lib=static:-bundle=pthread");
     }
+
+    maybe_static_library("RUSTC_STATIC_CLANG_RT_PATH", "clang_rt");
 }

library/std/Cargo.toml

@@ -84,3 +84,6 @@ heap_size = 0x8000000
 name = "stdbenches"
 path = "benches/lib.rs"
 test = true
+
+[build-dependencies]
+build_helper = { path = "../../src/build_helper" }

library/std/build.rs

@@ -1,5 +1,7 @@
 use std::env;
 
+use build_helper::maybe_static_library;
+
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     let target = env::var("TARGET").expect("TARGET was not set");
@@ -47,4 +49,6 @@ fn main() {
     }
     println!("cargo:rustc-env=STD_ENV_ARCH={}", env::var("CARGO_CFG_TARGET_ARCH").unwrap());
     println!("cargo:rustc-cfg=backtrace_in_libstd");
+
+    maybe_static_library("RUSTC_STATIC_CLANG_RT_PATH", "clang_rt");
 }

library/unwind/Cargo.toml

@@ -20,6 +20,7 @@ compiler_builtins = "0.1.0"
 cfg-if = "0.1.8"
 
 [build-dependencies]
+build_helper = { path = "../../src/build_helper" }
 cc = "1.0.69"
 
 [features]

library/unwind/build.rs

@@ -1,9 +1,15 @@
 use std::env;
 
+use build_helper::maybe_static_library;
+
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
     let target = env::var("TARGET").expect("TARGET was not set");
 
+    if maybe_static_library("RUSTC_STATIC_UNWIND_PATH", "unwind") {
+        return;
+    }
+
     if target.contains("android") {
         let build = cc::Build::new();
 

src/build_helper/lib.rs

@@ -203,3 +203,25 @@ fn fail(s: &str) -> ! {
     println!("\n\n{}\n\n", s);
     std::process::exit(1);
 }
+
+/// if you need for some reason to statically inject some library, like clang_rt
+/// here a good place. Anyway, you should use only thin .a file on macOS.
+/// You may extract it like:
+///   lipo -thin x86_64 -output libclang_rt.a /path/to/llvm/lib/../libclang_rt.osx.a
+///
+/// It returns true, when it had injected static library.
+pub fn maybe_static_library(env_path_name: &str, library_name: &str) -> bool {
+    println!("cargo:rerun-if-env-changed={}", env_path_name);
+
+    if let Ok(path) = env::var(env_path_name) {
+        let target = env::var("TARGET").expect("TARGET was not set");
+        println!("cargo:rerun-if-env-changed=TARGET");
+
+        println!("cargo:rustc-link-search=native={}", path);
+        println!("cargo:rustc-link-search=native={}/{}", path, target);
+        println!("cargo:rustc-link-lib=static={}", library_name);
+        return true;
+    }
+
+    return false;
+}