Default to MSVC on Windows

Instead of picking MSVC/GNU based on detection, just default to MSVC.
During install, if MSVC is not detected provide guidance.

This is more predictable and easier to explain.

Cargo.toml

@@ -57,6 +57,7 @@ winapi = "0.2.8"
 winreg = "0.3.2"
 user32-sys = "0.1.2"
 kernel32-sys = "0.2.1"
+gcc = "0.3.28"
 
 [dev-dependencies]
 rustup-mock = { path = "src/rustup-mock", version = "0.6.5" }

src/rustup-cli/main.rs

@@ -21,6 +21,8 @@ extern crate sha2;
 extern crate markdown;
 extern crate toml;
 
+#[cfg(windows)]
+extern crate gcc;
 #[cfg(windows)]
 extern crate winapi;
 #[cfg(windows)]

src/rustup-cli/self_update.rs

@@ -167,6 +167,30 @@ This will uninstall all Rust toolchains and data, and remove
     }
 }
 
+static MSVC_MESSAGE: &'static str =
+r#"# Rust Visual C++ prerequisites 
+
+Rust requires the Microsoft C++ build tools for Visual Studio 2013 or
+later, but they don't seem to be installed.
+
+The easiest way to acquire the build tools is by installing Microsoft
+Visual C++ Build Tools 2015 which provides just the Visual C++ build
+tools:
+
+    http://landinghub.visualstudio.com/visual-cpp-build-tools
+
+Alternately, you can install Visual Studio 2015 or Visual
+Studio 2013 and during install select the "C++ tools":
+
+    https://www.visualstudio.com/downloads/
+
+_Install the C++ build tools before proceeding_.
+
+If you will be targetting the GNU ABI or otherwise know what you are
+doing then it is fine to continue installation without the build
+tools, but otherwise, install the C++ build tools before proceeding.
+"#;
+
 static TOOLS: &'static [&'static str]
     = &["rustc", "rustdoc", "cargo", "rust-lldb", "rust-gdb"];
 
@@ -200,6 +224,11 @@ pub fn install(no_prompt: bool, verbose: bool,
     // miscompiled on the deployed builds. I am very confused.
     //try!(do_anti_sudo_check(no_prompt));
 
+    if !try!(do_msvc_check(&opts)) {
+        info!("aborting installation");
+        return Ok(());
+    }
+
     if !no_prompt {
         let ref msg = try!(pre_install_msg(opts.no_modify_path));
 
@@ -423,6 +452,25 @@ fn do_anti_sudo_check(no_prompt: bool) -> Result<()> {
     Ok(())
 }
 
+// Provide guidance about setting up MSVC if it doesn't appear to be
+// installed
+fn do_msvc_check(opts: &InstallOpts) -> Result<bool> {
+    if cfg!(windows) {
+        use gcc::windows_registry;
+        let installing_msvc = opts.default_host_triple.contains("msvc");
+        let have_msvc = windows_registry::find_tool(&opts.default_host_triple, "cl.exe").is_some();
+        let have_msvc = false;
+        if installing_msvc && !have_msvc {
+            term2::stdout().md(MSVC_MESSAGE);
+            if !try!(common::confirm("\nContinue? (Y/n)", true)) {
+                return Ok(false);
+            }
+        }
+    }
+
+    Ok(true)
+}
+
 fn pre_install_msg(no_modify_path: bool) -> Result<String> {
     let cargo_home = try!(utils::cargo_home());
     let cargo_home_bin = cargo_home.join("bin");

src/rustup-dist/Cargo.toml

@@ -32,7 +32,6 @@ winapi = "0.2.8"
 winreg = "0.3.2"
 user32-sys = "0.1.2"
 kernel32-sys = "0.2.1"
-gcc = "0.3.28"
 
 [target."cfg(not(windows))".dependencies]
 libc = "0.2.0"

src/rustup-dist/src/dist.rs

@@ -116,7 +116,6 @@ impl TargetTriple {
     pub fn from_host() -> Option<Self> {
         #[cfg(windows)]
         fn inner() -> Option<TargetTriple> {
-            use gcc::windows_registry;
             use kernel32::GetNativeSystemInfo;
             use std::mem;
 
@@ -136,16 +135,9 @@ impl TargetTriple {
                 _ => return None,
             };
 
-            // Now try to find an installation of msvc, using the gcc crate to do the hard work
+            // Default to msvc
             let msvc_triple = format!("{}-pc-windows-msvc", arch);
-            let gnu_triple = format!("{}-pc-windows-gnu", arch);
-            if let Some(_) = windows_registry::find_tool(&msvc_triple, "cl.exe") {
-                // Found msvc, so default to the msvc triple
-                Some(TargetTriple(msvc_triple))
-            } else {
-                // No msvc found, so use gnu triple as a fallback
-                Some(TargetTriple(gnu_triple))
-            }
+            Some(TargetTriple(msvc_triple))
         }
 
         #[cfg(not(windows))]

src/rustup-dist/src/lib.rs

@@ -21,8 +21,6 @@ extern crate winreg;
 extern crate user32;
 #[cfg(windows)]
 extern crate kernel32;
-#[cfg(windows)]
-extern crate gcc;
 #[cfg(not(windows))]
 extern crate libc;