Add the rust lib dir (containing std-<hash>.dll) to the path on windows

Adds sysroot/bin to the path on Windows which fixes finding libstd in
dynamic builds. This fixes a load of dylib tests in the cargo testsuite
on Windows. This will result in rustc.exe being directly in the path
although it is after CARGO_HOME/bin so the shim should be found first.

src/rustup/env_var.rs

@@ -1,6 +1,5 @@
-use std::ffi::OsString;
 use std::env;
-use std::path::{Path, PathBuf};
+use std::path::PathBuf;
 use std::process::Command;
 
 pub const RUST_RECURSION_COUNT_MAX: u32 = 5;
@@ -20,15 +19,19 @@ pub fn append_path(name: &str, value: Vec<PathBuf>, cmd: &mut Command) {
     }
 }
 
-pub fn prepend_path(name: &str, value: &Path, cmd: &mut Command) {
+pub fn prepend_path(name: &str, value: Vec<PathBuf>, cmd: &mut Command) {
     let old_value = env::var_os(name);
-    let mut parts = vec![value.to_owned()];
+    let mut parts: Vec<PathBuf>;
     if let Some(ref v) = old_value {
-        parts.extend(env::split_paths(v));
+        parts = value;
+        parts.extend(env::split_paths(v).collect::<Vec<_>>());
+    } else {
+        parts = value;
     }
-    let new_value = env::join_paths(parts).unwrap_or_else(|_| OsString::from(value));
 
-    cmd.env(name, new_value);
+    if let Ok(new_value) = env::join_paths(parts) {
+        cmd.env(name, new_value);
+    }
 }
 
 pub fn inc(name: &str, cmd: &mut Command) {

src/rustup/toolchain.rs

@@ -397,15 +397,22 @@ impl<'a> Toolchain<'a> {
         mod sysenv {
             pub const LOADER_PATH: &'static str = "DYLD_LIBRARY_PATH";
         }
-        env_var::prepend_path(sysenv::LOADER_PATH, &new_path, cmd);
+        env_var::prepend_path(sysenv::LOADER_PATH, vec![new_path.clone()], cmd);
 
         // Prepend CARGO_HOME/bin to the PATH variable so that we're sure to run
         // cargo/rustc via the proxy bins. There is no fallback case for if the
         // proxy bins don't exist. We'll just be running whatever happens to
         // be on the PATH.
+        let mut path_entries = vec![];
         if let Ok(cargo_home) = utils::cargo_home() {
-            env_var::prepend_path("PATH", &cargo_home.join("bin"), cmd);
+            path_entries.push(cargo_home.join("bin").to_path_buf());
         }
+
+        if cfg!(target_os = "windows") {
+            path_entries.push(self.path.join("bin"));
+        }
+
+        env_var::prepend_path("PATH", path_entries, cmd);
     }
 
     pub fn doc_path(&self, relative: &str) -> Result<PathBuf> {