rustup-init.sh ksh and zsh compatibility fixes

rustup-init.sh

@@ -1,5 +1,6 @@
 #!/bin/sh
 # shellcheck shell=dash
+# shellcheck disable=SC2039  # local is non-POSIX
 
 # This is just a little script that can be downloaded from the internet to
 # install rustup. It just does platform detection, downloads the installer
@@ -8,14 +9,20 @@
 # It runs on Unix shells like {a,ba,da,k,z}sh. It uses the common `local`
 # extension. Note: Most shells limit `local` to 1 var per line, contra bash.
 
-if [ "$KSH_VERSION" = 'Version JM 93t+ 2010-03-05' ]; then
-    # The version of ksh93 that ships with many illumos systems does not
-    # support the "local" extension.  Print a message rather than fail in
-    # subtle ways later on:
-    echo 'rustup does not work with this ksh93 version; please try bash!' >&2
-    exit 1
-fi
+# Some versions of ksh have no `local` keyword. Alias it to `typeset`, but
+# beware this makes variables global with f()-style function syntax in ksh93.
+# mksh has this alias by default.
+has_local() {
+    # shellcheck disable=SC2034  # deliberately unused
+    local _has_local
+}
 
+has_local 2>/dev/null || alias local=typeset
+
+# zsh does not split words by default, Required for curl retry arguments below.
+if [ -n "$ZSH_VERSION" ]; then
+    setopt shwordsplit
+fi
 
 set -u