Auto merge of #12606 - Angelin01:tab-completion-rustup-fallback-rustc, r=weihanglo

Tab completion for --target uses rustup but fallsback to rustc

### What does this PR try to resolve?

Fixes #12585

Currently, not only is the tab completion for `--target` inconsistent between bash and zsh, it depends on rustup and rustc respectively.
As discussed in the issue at hand, we'll use `rustup` if it is available and fallback to `rustc` if it is not, even if it is unfriendly.

### How should we test and review this PR?

Source the respective completion functions and test it out with `cargo build --target [TAB]`.

**I would appreciate if someone that regularly uses zsh would test this.** I did basic testing, but since I don't use zsh commonly I am unsure if everything is as it should be.

### Additional information

I switched to using `rustup target list --installed` instead of grabbing lines that contain "default" or "installed". I believe that any "default" target should be installed too, right?

Author: bors

src/etc/_cargo

@@ -408,9 +408,15 @@ _cargo_cmds() {
 }
 
 _cargo_target_triple() {
-    local -a targets
-	targets=( ${(f)"$(rustc --print target-list)"} )
-	_describe 'target triple' targets
+    local -a result
+
+    if (( $+commands[rustup] )); then
+        result=( ${(f)"$(rustup target list --installed)"} )
+    else
+        result=( ${(f)"$(rustc --print target-list)"} )
+    fi
+
+    _describe 'target triple' result
 }
 
 #FIXME: Disabled until fixed

src/etc/cargo.bashcomp.sh

@@ -250,15 +250,11 @@ _get_examples(){
 }
 
 _get_targets(){
-	local result=()
-	local targets=$(rustup target list)
-	while read line
-	do
-		if [[ "$line" =~ default|installed ]]; then
-			result+=("${line%% *}")
-		fi
-	done <<< "$targets"
-	echo "${result[@]}"
+	if command -v rustup >/dev/null 2>/dev/null; then
+	  rustup target list --installed
+	else
+	  rustc --print target-list
+	fi
 }
 
 _toolchains(){