Auto merge of #31064 - retep998:msvc-host-dlls, r=alexcrichton

Fixes https://github.com/rust-lang/rust/issues/31063

r? @alexcrichton

src/librustc_trans/back/link.rs

@@ -396,6 +396,9 @@ fn command_path(sess: &Session) -> OsString {
     if let Some(path) = env::var_os("PATH") {
         new_path.extend(env::split_paths(&path));
     }
+    if sess.target.target.options.is_like_msvc {
+        new_path.extend(msvc::host_dll_path());
+    }
     env::join_paths(new_path).unwrap()
 }
 

src/librustc_trans/back/msvc/mod.rs

@@ -31,20 +31,31 @@
 //! paths/files is based on Microsoft's logic in their vcvars bat files, but
 //! comments can also be found below leading through the various code paths.
 
-use std::process::Command;
-use session::Session;
-
 #[cfg(windows)]
 mod registry;
 
 #[cfg(windows)]
-pub fn link_exe_cmd(sess: &Session) -> Command {
+mod platform {
     use std::env;
     use std::ffi::OsString;
     use std::fs;
     use std::path::{Path, PathBuf};
-    use self::registry::{LOCAL_MACHINE};
+    use std::process::Command;
+    use session::Session;
+    use super::registry::{LOCAL_MACHINE};
 
+    // Cross toolchains depend on dlls from the host toolchain
+    // We can't just add it to the Command's PATH in `link_exe_cmd` because it
+    // is later overridden so we publicly expose it here instead
+    pub fn host_dll_path() -> Option<PathBuf> {
+        get_vc_dir().and_then(|(_, vcdir)| {
+            host_dll_subdir().map(|sub| {
+                vcdir.join("bin").join(sub)
+            })
+        })
+    }
+
+    pub fn link_exe_cmd(sess: &Session) -> Command {
         let arch = &sess.target.target.arch;
         let (binsub, libsub, vclibsub) =
             match (bin_subdir(arch), lib_subdir(arch), vc_lib_subdir(arch)) {
@@ -86,10 +97,7 @@ pub fn link_exe_cmd(sess: &Session) -> Command {
         }).or_else(|| {
             get_vc_dir().and_then(|(ver, vcdir)| {
                 debug!("Found VC installation directory {:?}", vcdir);
-            let mut linker = vcdir.clone();
-            linker.push("bin");
-            linker.push(binsub);
-            linker.push("link.exe");
+                let linker = vcdir.clone().join("bin").join(binsub).join("link.exe");
                 if !linker.is_file() { return None }
                 let mut cmd = Command::new(linker);
                 add_lib(&mut cmd, &vcdir.join("lib").join(vclibsub));
@@ -122,7 +130,7 @@ pub fn link_exe_cmd(sess: &Session) -> Command {
             debug!("Failed to locate linker.");
             Command::new("link.exe")
         });
-
+    }
     // A convenience function to make the above code simpler
     fn add_lib(cmd: &mut Command, lib: &Path) {
         let mut arg: OsString = "/LIBPATH:".into();
@@ -257,11 +265,23 @@ fn vc_lib_subdir(arch: &str) -> Option<&'static str> {
             _ => None,
         }
     }
+    fn host_dll_subdir() -> Option<&'static str> {
+        if cfg!(target_arch = "x86_64") { Some("amd64") }
+        else if cfg!(target_arch = "x86") { Some("") }
+        else { None }
+    }
 }
 
 // If we're not on Windows, then there's no registry to search through and MSVC
 // wouldn't be able to run, so we just call `link.exe` and hope for the best.
 #[cfg(not(windows))]
+mod platform {
+    use std::path::PathBuf;
+    use std::process::Command;
+    use session::Session;
     pub fn link_exe_cmd(_sess: &Session) -> Command {
         Command::new("link.exe")
     }
+    pub fn host_dll_path() -> Option<PathBuf> { None }
+}
+pub use self::platform::*;