Address problems pointed out in the comments.

completion/available/cargo.completion.bash

@@ -3,13 +3,14 @@
 # cargo (Rust package manager) completion
 # This script is taken and modified from https://github.com/rust-lang/cargo/blob/master/src/etc/cargo.bashcomp.sh
 
-source $HOME/.cargo/env
+if ! _command_exists cargo; then
+    return 0
+fi
 
 # Required for bash versions < 4.1
 # Default bash version is 3.2 on latest macOS. See #6874
 shopt -s extglob
 
-command -v cargo >/dev/null 2>&1 &&
 _cargo() {
     local cur prev words cword
     _get_comp_words_by_ref cur prev words cword
@@ -99,7 +100,7 @@ command -v cargo >/dev/null 2>&1 &&
         if [[ "$cur" == -* ]]; then
             COMPREPLY=($(compgen -W "${opt___nocmd}" -- "$cur"))
         elif [[ "$cur" == +* ]]; then
-                COMPREPLY=($(compgen -W "$(_toolchains)" -- "$cur"))
+            COMPREPLY=($(compgen -W "$(__cargo_toolchains)" -- "$cur"))
         else
             COMPREPLY=($(compgen -W "$__cargo_commands" -- "$cur"))
         fi
@@ -118,19 +119,19 @@ command -v cargo >/dev/null 2>&1 &&
             _filedir toml
             ;;
         --bin)
-                COMPREPLY=($(compgen -W "$(_bin_names)" -- "$cur"))
+            COMPREPLY=($(compgen -W "$(__cargo_bin_names)" -- "$cur"))
             ;;
         --test)
-                COMPREPLY=($(compgen -W "$(_test_names)" -- "$cur"))
+            COMPREPLY=($(compgen -W "$(__cargo_test_names)" -- "$cur"))
             ;;
         --bench)
-                COMPREPLY=($(compgen -W "$(_benchmark_names)" -- "$cur"))
+            COMPREPLY=($(compgen -W "$(__cargo_benchmark_names)" -- "$cur"))
             ;;
         --example)
-                COMPREPLY=($(compgen -W "$(_get_examples)" -- "$cur"))
+            COMPREPLY=($(compgen -W "$(__cargo_get_examples)" -- "$cur"))
             ;;
         --target)
-                COMPREPLY=($(compgen -W "$(_get_targets)" -- "$cur"))
+            COMPREPLY=($(compgen -W "$(__cargo_get_targets)" -- "$cur"))
             ;;
         --target-dir)
             _filedir -d
@@ -153,19 +154,20 @@ command -v cargo >/dev/null 2>&1 &&
     # compopt does not work in bash version 3
 
     return 0
-    } &&
+}
+
 complete -F _cargo cargo
 
 __cargo_commands=$(cargo --list 2>/dev/null | awk 'NR>1 {print $1}')
 
-_locate_manifest() {
+__cargo_locate_manifest() {
     cargo locate-project --message-format plain 2>/dev/null
 }
 
 # Extracts the values of "name" from the array given in $1 and shows them as
 # command line options for completion
-_get_names_from_array() {
-    local manifest=$(_locate_manifest)
+__cargo_get_names_from_array() {
+    local manifest=$(__cargo_locate_manifest)
     if [[ -z $manifest ]]; then
         return 0
     fi
@@ -198,22 +200,22 @@ _get_names_from_array() {
 }
 
 #Gets the bin names from the manifest file
-_bin_names() {
-    _get_names_from_array "bin"
+__cargo_bin_names() {
+    __cargo_get_names_from_array "bin"
 }
 
 #Gets the test names from the manifest file
-_test_names() {
-    _get_names_from_array "test"
+__cargo_test_names() {
+    __cargo_get_names_from_array "test"
 }
 
 #Gets the bench names from the manifest file
-_benchmark_names() {
-    _get_names_from_array "bench"
+__cargo_benchmark_names() {
+    __cargo_get_names_from_array "bench"
 }
 
-_get_examples() {
-    local manifest=$(_locate_manifest)
+__cargo_get_examples() {
+    local manifest=$(__cargo_locate_manifest)
     [ -z "$manifest" ] && return 0
 
     local files=("${manifest%/*}"/examples/*.rs)
@@ -225,7 +227,7 @@ _get_examples() {
     fi
 }
 
-_get_targets() {
+__cargo_get_targets() {
     local result=()
     local targets=$(rustup target list)
     while read line; do
@@ -236,7 +238,7 @@ _get_targets() {
     echo "${result[@]}"
 }
 
-_toolchains() {
+__cargo_toolchains() {
     local result=()
     local toolchains=$(rustup toolchain list)
     local channels="nightly|beta|stable|[0-9]\.[0-9]{1,2}\.[0-9]"