Rollup merge of rust-lang#39892 - petrochenkov:rt, r=alexcrichton

Fix test caching on Windows/GNU

Addresses rust-lang#36385 (comment)

Previously the sysroot directory was purged on every build and mingw startup objects were rebuilt unconditionally and always triggered test reruns.
Now the sysroot directory is reused and mingw startup objects are rebuilt only when necessary, so test caching works.

src/bootstrap/compile.rs

@@ -21,7 +21,7 @@ use std::fs::{self, File};
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
-use build_helper::{output, mtime};
+use build_helper::{output, mtime, up_to_date};
 use filetime::FileTime;
 
 use util::{exe, libdir, is_dylib, copy};
@@ -132,21 +132,29 @@ pub fn build_startup_objects(build: &Build, for_compiler: &Compiler, target: &st
 
     let compiler = Compiler::new(0, &build.config.build);
     let compiler_path = build.compiler_path(&compiler);
-    let into = build.sysroot_libdir(for_compiler, target);
-    t!(fs::create_dir_all(&into));
-
-    for file in t!(fs::read_dir(build.src.join("src/rtstartup"))) {
-        let file = t!(file);
-        let mut cmd = Command::new(&compiler_path);
-        build.run(cmd.env("RUSTC_BOOTSTRAP", "1")
-                     .arg("--target").arg(target)
-                     .arg("--emit=obj")
-                     .arg("--out-dir").arg(&into)
-                     .arg(file.path()));
+    let src_dir = &build.src.join("src/rtstartup");
+    let dst_dir = &build.native_dir(target).join("rtstartup");
+    let sysroot_dir = &build.sysroot_libdir(for_compiler, target);
+    t!(fs::create_dir_all(dst_dir));
+    t!(fs::create_dir_all(sysroot_dir));
+
+    for file in &["rsbegin", "rsend"] {
+        let src_file = &src_dir.join(file.to_string() + ".rs");
+        let dst_file = &dst_dir.join(file.to_string() + ".o");
+        if !up_to_date(src_file, dst_file) {
+            let mut cmd = Command::new(&compiler_path);
+            build.run(cmd.env("RUSTC_BOOTSTRAP", "1")
+                        .arg("--target").arg(target)
+                        .arg("--emit=obj")
+                        .arg("--out-dir").arg(dst_dir)
+                        .arg(src_file));
+        }
+
+        copy(dst_file, &sysroot_dir.join(file.to_string() + ".o"));
     }
 
     for obj in ["crt2.o", "dllcrt2.o"].iter() {
-        copy(&compiler_file(build.cc(target), obj), &into.join(obj));
+        copy(&compiler_file(build.cc(target), obj), &sysroot_dir.join(obj));
     }
 }