Added help for rustup toolchain link as documented in issue #954.

src/rustup-cli/help.rs

@@ -30,7 +30,7 @@ the same as `rustup toolchain install`.
 'toolchain' specifies a toolchain name, such as 'stable', 'nightly',
 or '1.8.0'. For more information see `rustup help toolchain`.";
 
-pub static TOOLCHAIN_INSTALL_HELP: &'static str =
+pub static INSTALL_HELP: &'static str =
 r"
 Installs a specific rust toolchain.
 
@@ -77,8 +77,29 @@ inferred, so the above could be written:
 
     $ rustup default stable-msvc
 
-Toolchain names that don't name a channel instead can be used to name
-custom toolchains with the `rustup toolchain link` command.";
+    rustup can also manage symlinked local toolchain builds, which are
+    often used to for developing Rust itself. For more information see
+    `rustup toolchain help link`.";
+
+pub static TOOLCHAIN_LINK_HELP: &'static str =
+r"DISCUSSION:
+    'toolchain' is the custom name to be assigned to the new toolchain.
+    Any name is permitted as long as it does not fully match an initial
+    substring of a standard release channel. For example, you can use
+    the names 'latest' or '2017-04-01' but you cannot use 'stable' or
+    'beta-i686' or 'nightly-x86_64-unknown-linux-gnu'.
+
+    'path' specifies the directory where the binaries and libraries for
+    the custom toolchain can be found. For example, when used for
+    development of Rust itself, toolchains can be linked directly out of
+    the build directory. After building, you can test out different
+    compiler versions as follows:
+
+        $ rustup toolchain link latest-stage1 build/x86_64-unknown-linux-gnu/stage1
+        $ rustup override set latest-stage1
+
+    If you now compile a crate in the current directory, the custom
+    toolchain 'latest-stage1' will be used.";
 
 pub static OVERRIDE_HELP: &'static str =
 r"

src/rustup-cli/rustup_mode.rs

@@ -7,7 +7,7 @@ use rustup_dist::manifest::Component;
 use rustup_dist::dist::{TargetTriple, PartialToolchainDesc, PartialTargetTriple};
 use rustup_utils::utils;
 use self_update;
-use std::path::Path;
+use std::path::{Path, PathBuf};
 use std::process::Command;
 use std::iter;
 use term2;
@@ -128,7 +128,7 @@ pub fn cli() -> App<'static, 'static> {
             .after_help(SHOW_HELP))
         .subcommand(SubCommand::with_name("install")
             .about("Update Rust toolchains")
-            .after_help(TOOLCHAIN_INSTALL_HELP)
+            .after_help(INSTALL_HELP)
             .setting(AppSettings::Hidden) // synonym for 'toolchain install'
             .arg(Arg::with_name("toolchain")
                 .required(true)))
@@ -167,6 +167,7 @@ pub fn cli() -> App<'static, 'static> {
                      .multiple(true)))
             .subcommand(SubCommand::with_name("link")
                 .about("Create a custom toolchain by symlinking to a directory")
+                .after_help(TOOLCHAIN_LINK_HELP)
                 .arg(Arg::with_name("toolchain")
                     .required(true))
                 .arg(Arg::with_name("path")
@@ -687,7 +688,15 @@ fn toolchain_link(cfg: &Cfg, m: &ArgMatches) -> Result<()> {
     let ref toolchain = m.value_of("toolchain").expect("");
     let ref path = m.value_of("path").expect("");
     let toolchain = try!(cfg.get_toolchain(toolchain, true));
-
+    let mut pathbuf = PathBuf::from(path);
+
+    pathbuf.push("lib");
+    try!(utils::assert_is_directory(&pathbuf));
+    pathbuf.pop();
+    pathbuf.push("bin");
+    try!(utils::assert_is_directory(&pathbuf));
+    pathbuf.push("rustc");
+    try!(utils::assert_is_file(&pathbuf));
     Ok(try!(toolchain.install_from_dir(Path::new(path), true)))
 }