Updated get_toolchain, it now retrieves active toolchain.

[ateraz13]

As per this issue, this pull requests updates the function which retrieves the active toolchain name as the previous approach is now outdated.

[mcklatz]

Hi! What status of this PR, could you merge it?

[mcklatz]

One issue with the current approach, plus a suggestion for a more
robust fix.

The current diff can double-output on older rustup

function get_toolchain {
    rustup show | sed -nr 's/(.*) \(default\)/\1/p' | head -n 1
    rustup show active-toolchain | awk '{print $1}'
}

If we keep the rustup-based approach, the old line should be replaced, I guess, like:

function get_toolchain {
    rustup show active-toolchain | awk '{print $1}'
}

But I guess better way try to resolve via rustc --print sysroot and skip parsing entirely

 LUA_FILTER="$HOME/.local/bin/rustic-doc-filter.lua"
-function get_toolchain {
-    rustup show | sed -nr 's/(.*) \(default\)/\1/p' | head -n 1
-}

 if [ "$1" = "" ] || [ "$1" = "--help"  ]; then
     MY_NAME="$(basename "$0")"
@@
 if [ "$DEST_DIR" = "" ]; then
     LIBRARY="$1"
-    TARGET="$(get_toolchain)"
-
-    ## Users can change the location of the rustup directory
-    RUSTUP_HOME="${RUSTUP_HOME:-$HOME/.rustup}"
-    ## Set
-    DOC_PATH="$RUSTUP_HOME/toolchains/$TARGET/share/doc/rust/html/$LIBRARY"
+
+    ## Resolve the active toolchain's documentation directory directly via
+    ## rustc.  This avoids parsing `rustup show` (whose output format changed:
+    ## the default toolchain is now tagged "(active, default)" instead of
+    ## "(default)") and also works for rustup-less, distro-packaged Rust.
+    SYSROOT="$(rustc --print sysroot)"
+    if [ -z "$SYSROOT" ]; then
+        echo "rustic-doc: could not determine rustc sysroot; is rustc on PATH?" >&2
+        exit 1
+    fi
+    DOC_PATH="$SYSROOT/share/doc/rust/html/$LIBRARY"
     DEST_DIR="${XDG_DATA_HOME:-$HOME/.local/share}/emacs/rustic-doc/$LIBRARY"

     echo "Generating org files in: $DEST_DIR"
 fi

I could to open this as an alternative commit if you'd prefer.