Use right options in custom completions (#13698)

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# Description
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This issue was reported by kira in
[Discord](https://discord.com/channels/601130461678272522/1276981416307069019).
In https://github.com/nushell/nushell/pull/13311, I accidentally made it
so that custom completions are filtered according to the user's
configured completion options (`$env.config.completions`) rather than
the completion options provided as part of the custom completions. This
PR is a quick fix for that.

# User-Facing Changes
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It should once again be possible to override match algorithm, case
sensitivity, and substring matching (`positional`) in custom
completions.

# Tests + Formatting
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Added a couple tests.

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fdncred and I discussed this in Discord a bit and we thought it might be
better to not allow custom completions to override the user's config.
However, `positional` can't currently be set inside one's config, so you
can only do strict prefix matching, no substring matching. Another PR
could do one of the following:
- Document the fact that you can provide completion options inside
custom completions
- Remove the ability to provide completion options with custom
completions and add a `$env.config.completions.positional: true` option
- Remove the ability to provide completion options with custom
completions and add a new match algorithm `substring` (this is the one I
like most, since `positional` only applies to prefix matching anyway)

Separately from these options, we could also allow completers to specify
that they don't Nushell to do any filtering and sorting on the provided
custom completions.

crates/nu-cli/src/completions/custom_completions.rs

@@ -126,7 +126,7 @@ impl Completer for CustomCompletion {
         let options = custom_completion_options
             .as_ref()
             .unwrap_or(completion_options);
-        let suggestions = filter(&prefix, suggestions, completion_options);
+        let suggestions = filter(&prefix, suggestions, options);
         sort_suggestions(&String::from_utf8_lossy(&prefix), suggestions, options)
     }
 }

crates/nu-cli/tests/completions/mod.rs

@@ -61,6 +61,32 @@ fn extern_completer() -> NuCompleter {
     NuCompleter::new(Arc::new(engine), Arc::new(stack))
 }
 
+#[fixture]
+fn completer_strings_with_options() -> NuCompleter {
+    // Create a new engine
+    let (dir, _, mut engine, mut stack) = new_engine();
+    // Add record value as example
+    let record = r#"
+        # To test that the config setting has no effect on the custom completions
+        $env.config.completions.algorithm = "fuzzy"
+        def animals [] {
+            {
+                # Very rare and totally real animals
+                completions: ["Abcdef", "Foo Abcdef", "Acd Bar" ],
+                options: {
+                    completion_algorithm: "prefix",
+                    positional: false,
+                    case_sensitive: false,
+                }
+            }
+        }
+        def my-command [animal: string@animals] { print $animal }"#;
+    assert!(support::merge_input(record.as_bytes(), &mut engine, &mut stack, dir).is_ok());
+
+    // Instantiate a new completer
+    NuCompleter::new(Arc::new(engine), Arc::new(stack))
+}
+
 #[fixture]
 fn custom_completer() -> NuCompleter {
     // Create a new engine
@@ -169,6 +195,20 @@ fn variables_customcompletion_subcommands_with_customcompletion_2(
     match_suggestions(&expected, &suggestions);
 }
 
+#[rstest]
+fn customcompletions_substring_matching(mut completer_strings_with_options: NuCompleter) {
+    let suggestions = completer_strings_with_options.complete("my-command Abcd", 15);
+    let expected: Vec<String> = vec!["Abcdef".into(), "Foo Abcdef".into()];
+    match_suggestions(&expected, &suggestions);
+}
+
+#[rstest]
+fn customcompletions_case_insensitive(mut completer_strings_with_options: NuCompleter) {
+    let suggestions = completer_strings_with_options.complete("my-command foo", 14);
+    let expected: Vec<String> = vec!["Foo Abcdef".into()];
+    match_suggestions(&expected, &suggestions);
+}
+
 #[test]
 fn dotnu_completions() {
     // Create a new engine