complete/fish: Take RIPGREP_CONFIG_PATH into account

The fish completions now also pay attention to the configuration file
to determine whether to suggest negation options and not just to the
current command line.

This doesn't cover all edge cases. For example the config file is
cached, and so changes may not take effect until the next shell
session. But the cases it doesn't cover are hopefully very rare.

crates/core/flags/complete/fish.rs

@@ -6,11 +6,15 @@ use crate::flags::{defs::FLAGS, CompletionType};
 
 const TEMPLATE: &'static str = "complete -c rg !SHORT! -l !LONG! -d '!DOC!'";
 const TEMPLATE_NEGATED: &'static str =
-    "complete -c rg -l !NEGATED! -n '__fish_contains_opt !SHORT! !LONG!' -d '!DOC!'\n";
+    "complete -c rg -l !NEGATED! -n '__rg_contains_opt !LONG! !SHORT!' -d '!DOC!'\n";
 
 /// Generate completions for Fish.
+///
+/// Reference: https://fishshell.com/docs/current/completions.html
 pub(crate) fn generate() -> String {
     let mut out = String::new();
+    out.push_str(include_str!("prelude.fish"));
+    out.push('\n');
     for flag in FLAGS.iter() {
         let short = match flag.name_short() {
             None => "".to_string(),
@@ -55,6 +59,10 @@ pub(crate) fn generate() -> String {
         out.push_str(&completion);
 
         if let Some(negated) = flag.name_negated() {
+            let short = match flag.name_short() {
+                None => "".to_string(),
+                Some(byte) => char::from(byte).to_string(),
+            };
             out.push_str(
                 &TEMPLATE_NEGATED
                     .replace("!NEGATED!", &negated)

crates/core/flags/complete/prelude.fish

@@ -0,0 +1,31 @@
+# Usage: __rg_contains_opt LONG [SHORT]
+function __rg_contains_opt --description 'Specialized __fish_contains_opt'
+    # Cache the config file because this function is called many times per
+    # completion attempt.
+    # The cache will persist for the entire shell session (even if the
+    # variable or the file contents change).
+    if not set -q __rg_config
+        set -g __rg_config
+        if set -qx RIPGREP_CONFIG_PATH
+            set __rg_config (
+                cat -- $RIPGREP_CONFIG_PATH 2>/dev/null \
+                | string trim \
+                | string match -rv '^$|^#'
+            )
+        end
+    end
+
+    set -l commandline (commandline -cpo) (commandline -ct) $__rg_config
+
+    if contains -- "--$argv[1]" $commandline
+        return 0
+    end
+
+    if set -q argv[2]
+        if string match -qr -- "^-[^-]*$argv[2]" $commandline
+            return 0
+        end
+    end
+
+    return 1
+end