feat(cli/help): discuss rustup toolchain install

src/cli/help.rs

@@ -49,16 +49,16 @@ pub(crate) fn update_help() -> String {
     updates rustup itself.
 
     If given a toolchain argument then `update` updates that
-    toolchain, the same as `rustup toolchain install`."
+    toolchain, the same as `rustup toolchain install`.
+
+{TOOLCHAIN_INSTALL_HINT}"
     )
 }
 
 pub(crate) fn install_help() -> String {
     format!(
         r"{HEADER}Discussion:{HEADER:#}
-    Installs a specific rust toolchain.
-
-    The 'install' command is an alias for 'rustup update <toolchain>'."
+    The 'install' command is an alias for 'rustup toolchain install'."
     )
 }
 
@@ -118,6 +118,13 @@ pub(crate) fn toolchain_help() -> String {
     )
 }
 
+pub(crate) fn toolchain_install_help() -> String {
+    format!(
+        r"{HEADER}Discussion:{HEADER:#}
+{TOOLCHAIN_INSTALL_HINT}"
+    )
+}
+
 pub(crate) fn toolchain_link_help() -> String {
     format!(
         r"{HEADER}Discussion:{HEADER:#}
@@ -362,3 +369,13 @@ pub(crate) fn topic_arg_help() -> &'static str {
 }
 
 const SUBHEADER: Style = Style::new().bold();
+
+const TOOLCHAIN_INSTALL_HINT: &str =
+    "    Some environment variables allow you to customize certain parameters
+    in toolchain installation, including:
+
+    - `RUSTUP_CONCURRENT_DOWNLOADS`: the number of concurrent downloads.
+    - `RUSTUP_DOWNLOAD_TIMEOUT`: the download timeout in seconds.
+
+    See <https://rust-lang.github.io/rustup/devel/environment-variables.html>
+    for more info.";

src/cli/rustup_mode.rs

@@ -32,7 +32,8 @@ use crate::{
             maybe_resolvable_toolchain_arg_help, official_toolchain_arg_help, override_help,
             override_unset_help, resolvable_local_toolchain_arg_help,
             resolvable_toolchain_arg_help, run_help, rustup_help, show_active_toolchain_help,
-            show_help, toolchain_help, toolchain_link_help, topic_arg_help, update_help,
+            show_help, toolchain_help, toolchain_install_help, toolchain_link_help, topic_arg_help,
+            update_help,
         },
         self_update::{self, SelfUpdateMode, check_rustup_update},
         topical_doc,
@@ -331,7 +332,7 @@ enum ToolchainSubcmd {
     },
 
     /// Install or update the given toolchains, or by default the active toolchain
-    #[command(aliases = ["update", "add"] )]
+    #[command(aliases = ["update", "add"], after_help = toolchain_install_help())]
     Install {
         #[command(flatten)]
         opts: UpdateOpts,