Verify that rust version is recent enough to install rustlings.

I would have liked to write some tests for the vercomp function I
introduce, but there doesn't seem to be any CI setup yet?

install.sh

@@ -30,6 +30,58 @@ else
     echo "SUCCESS: Cargo is installed"
 fi
 
+# Function that compares two versions strings v1 and v2 given in arguments (e.g 1.31 and 1.33.0).
+# Returns 1 if v1 > v2, 0 if v1 == v2, 2 if v1 < v2.
+function vercomp() {
+    if [[ $1 == $2 ]]
+    then
+        return 0
+    fi
+    v1=( ${1//./ } )
+    v2=( ${2//./ } )
+    len1=${#v1[@]}
+    len2=${#v2[@]}
+    max_len=$len1
+    if [[ $max_len -lt $len2 ]]
+    then
+        max_len=$len2
+    fi
+    for i in `seq 0 $max_len`
+    do
+        # Fill empty fields with zeros in v1
+        if [ -z "${v1[$i]}" ]
+        then
+            v1[$i]=0
+        fi
+        if [ -z "${v2[$i]}" ]
+        then
+            v2[$i]=0
+        fi
+        # And in v2
+        if [ ${v1[$i]} -gt ${v2[$i]} ]
+        then
+            return 1
+        fi
+        if [ ${v1[$i]} -lt ${v2[$i]} ]
+        then
+            return 2
+        fi
+    done
+    return 0
+}
+
+RustVersion=$(rustc --version | cut -d " " -f 2)
+MinRustVersion=1.31
+vercomp $RustVersion $MinRustVersion
+if [ $? -eq 2 ]
+then
+    echo "WARNING: Rust version is too old: $RustVersion - needs at least $MinRustVersion"
+    echo "Please update Rust with 'rustup update'"
+    exit 1
+else
+    echo "SUCCESS: Rust is up to date"
+fi
+
 Path=${1:-rustlings/}
 echo "Cloning Rustlings at $Path..."
 git clone -q https://github.com/rust-lang/rustlings $Path