Automatically install override toolchain when missing.

A typical scenario is:

* I work on a repository that uses `rust-toolchain` to pin to a specific Nightly version
* I run `git pull`, `rust-toolchain` has been changed to update to a new Rust version
* I run `cargo build`

Result before this PR (typically): rustup fails with an error like:

```
error: override toolchain 'nightly-2017-08-31' is not installed
info: caused by: the toolchain file at '/home/simon/projects/servo/rust-toolchain' specifies an uninstalled toolchain
```

A better result would be to install toolchains as needed.

Closes #1218

src/rustup/config.rs

@@ -259,16 +259,20 @@ impl Cfg {
                 }
             };
 
-            match self.verify_toolchain(&name) {
-                Ok(t) => {
-                    Ok(Some((t, reason)))
+            match self.get_toolchain(&name, false) {
+                Ok(toolchain) => {
+                    if toolchain.exists() {
+                        Ok(Some((toolchain, reason)))
+                    } else if toolchain.is_custom() {
+                        // Strip the confusing NotADirectory error and only mention that the override
+                        // toolchain is not installed.
+                        Err(Error::from(reason_err))
+                            .chain_err(|| ErrorKind::OverrideToolchainNotInstalled(name.to_string()))
+                    } else {
+                        try!(toolchain.install_from_dist());
+                        Ok(Some((toolchain, reason)))
+                    }
                 }
-                Err(Error(ErrorKind::Utils(::rustup_utils::ErrorKind::NotADirectory { .. }), _)) => {
-                    // Strip the confusing NotADirectory error and only mention that the override
-                    // toolchain is not installed.
-                    Err(Error::from(reason_err))
-                        .chain_err(|| ErrorKind::OverrideToolchainNotInstalled(name.to_string()))
-                },
                 Err(e) => {
                     Err(e)
                         .chain_err(|| Error::from(reason_err))

tests/cli-rustup.rs

@@ -4,7 +4,6 @@ extern crate rustup_dist;
 extern crate rustup_utils;
 extern crate rustup_mock;
 extern crate tempdir;
-extern crate regex;
 
 use std::fs;
 use std::env::consts::EXE_SUFFIX;
@@ -16,7 +15,6 @@ use rustup_mock::clitools::{self, Config, Scenario,
                             expect_err,
                             set_current_dist_date,
                             this_host_triple};
-use regex::Regex;
 
 macro_rules! for_host { ($s: expr) => (&format!($s, this_host_triple())) }
 
@@ -621,15 +619,15 @@ fn show_toolchain_override_not_installed() {
     setup(&|config| {
         expect_ok(config, &["rustup", "override", "add", "nightly"]);
         expect_ok(config, &["rustup", "toolchain", "remove", "nightly"]);
-        // I'm not sure this should really be erroring when the toolchain
-        // is not installed; just capturing the behavior.
         let mut cmd = clitools::cmd(config, "rustup", &["show"]);
         clitools::env(config, &mut cmd);
         let out = cmd.output().unwrap();
         assert!(out.status.success());
         let stdout = String::from_utf8(out.stdout).unwrap();
+        let stderr = String::from_utf8(out.stderr).unwrap();
         assert!(!stdout.contains("not a directory"));
-        assert!(Regex::new(r"error: override toolchain 'nightly.*' is not installed, the directory override for '.*' specifies an uninstalled toolchain").unwrap().is_match(&stdout))
+        assert!(!stdout.contains("is not installed"));
+        assert!(stderr.contains("info: installing component 'rustc'"));
     });
 }
 
@@ -658,11 +656,11 @@ fn show_toolchain_env_not_installed() {
         clitools::env(config, &mut cmd);
         cmd.env("RUSTUP_TOOLCHAIN", "nightly");
         let out = cmd.output().unwrap();
-        // I'm not sure this should really be erroring when the toolchain
-        // is not installed; just capturing the behavior.
         assert!(out.status.success());
         let stdout = String::from_utf8(out.stdout).unwrap();
-        assert!(stdout.contains("override toolchain 'nightly' is not installed, the RUSTUP_TOOLCHAIN environment variable specifies an uninstalled toolchain"));
+        let stderr = String::from_utf8(out.stderr).unwrap();
+        assert!(!stdout.contains("is not installed"));
+        assert!(stderr.contains("info: installing component 'rustc'"));
     });
 }