Copy musl startup objects before building std

They are required for linking it, even though it is a library, because
crtn.o in post_link_objects, as hardcoded in src/librustc_back/target/
linux_musl_base.rs, is added to the linker command line for both
executables and libraries.

Author: smaeul

src/bootstrap/compile.rs

@@ -77,6 +77,14 @@ impl Step for Std {
                 target,
             });
             println!("Uplifting stage1 std ({} -> {})", from.host, target);
+
+            // Even if we're not building std this stage, the new sysroot must
+            // still contain the musl startup objects.
+            if target.contains("musl") && !target.contains("mips") {
+                let libdir = builder.sysroot_libdir(compiler, target);
+                copy_musl_third_party_objects(build, target, &libdir);
+            }
+
             builder.ensure(StdLink {
                 compiler: from,
                 target_compiler: compiler,
@@ -89,6 +97,11 @@ impl Step for Std {
         println!("Building stage{} std artifacts ({} -> {})", compiler.stage,
                 &compiler.host, target);
 
+        if target.contains("musl") && !target.contains("mips") {
+            let libdir = builder.sysroot_libdir(compiler, target);
+            copy_musl_third_party_objects(build, target, &libdir);
+        }
+
         let out_dir = build.cargo_out(compiler, Mode::Libstd, target);
         build.clear_if_dirty(&out_dir, &builder.rustc(compiler));
         let mut cargo = builder.cargo(compiler, Mode::Libstd, target, "build");
@@ -105,6 +118,20 @@ impl Step for Std {
     }
 }
 
+/// Copies the crt(1,i,n).o startup objects
+///
+/// Since musl supports fully static linking, we can cross link for it even
+/// with a glibc-targeting toolchain, given we have the appropriate startup
+/// files. As those shipped with glibc won't work, copy the ones provided by
+/// musl so we have them on linux-gnu hosts.
+fn copy_musl_third_party_objects(build: &Build,
+                                 target: Interned<String>,
+                                 into: &Path) {
+    for &obj in &["crt1.o", "crti.o", "crtn.o"] {
+        copy(&build.musl_root(target).unwrap().join("lib").join(obj), &into.join(obj));
+    }
+}
+
 /// Configure cargo to compile the standard library, adding appropriate env vars
 /// and such.
 pub fn std_cargo(build: &Build,
@@ -189,10 +216,6 @@ impl Step for StdLink {
         let libdir = builder.sysroot_libdir(target_compiler, target);
         add_to_sysroot(&libdir, &libstd_stamp(build, compiler, target));
 
-        if target.contains("musl") && !target.contains("mips") {
-            copy_musl_third_party_objects(build, target, &libdir);
-        }
-
         if build.config.sanitizers && compiler.stage != 0 && target == "x86_64-apple-darwin" {
             // The sanitizers are only built in stage1 or above, so the dylibs will
             // be missing in stage0 and causes panic. See the `std()` function above
@@ -208,18 +231,6 @@ impl Step for StdLink {
     }
 }
 
-/// Copies the crt(1,i,n).o startup objects
-///
-/// Since musl supports fully static linking, we can cross link for it even
-/// with a glibc-targeting toolchain, given we have the appropriate startup
-/// files. As those shipped with glibc won't work, copy the ones provided by
-/// musl so we have them on linux-gnu hosts.
-fn copy_musl_third_party_objects(build: &Build, target: Interned<String>, into: &Path) {
-    for &obj in &["crt1.o", "crti.o", "crtn.o"] {
-        copy(&build.musl_root(target).unwrap().join("lib").join(obj), &into.join(obj));
-    }
-}
-
 fn copy_apple_sanitizer_dylibs(native_dir: &Path, platform: &str, into: &Path) {
     for &sanitizer in &["asan", "tsan"] {
         let filename = format!("libclang_rt.{}_{}_dynamic.dylib", sanitizer, platform);