feat(datafusion-cli): enhance CLI helper with default hint (#20310)

# Pull Request Description

## Which issue does this PR close?

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- Closes #.

## Rationale for this change

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When the CLI prompt is empty, users have no indication of how to get
help or exit. Showing a subtle gray hint (e.g. `\? for help, \q to
quit`) on an empty line improves discoverability without changing any
existing behavior.

## What changes are included in this PR?

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PR.
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- Implement `Hinter::hint` on `CliHelper` so that when the current line
is empty, a default hint is shown (e.g. `\? for help, \q to quit`) in
gray.
- Add a `DEFAULT_HINT_SUGGESTION` constant and use it for the hint text.
- Expose the highlighter’s `Color` type and its methods as `pub(crate)`
and add `Color::gray()` for styling the hint.

## Are these changes tested?

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No new tests were added; the change is a small UI hint.

Manual testing: start the CLI and confirm the hint appears on an empty
line and disappears when typing. Happy to add tests if the project
expects them for this behavior.

## Are there any user-facing changes?

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updated before approving the PR.
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Yes. When the input line is empty, users now see a gray hint: `\? for
help, \q to quit`. No API or CLI flag changes. Documentation could
optionally mention this hint in the CLI docs if the project documents
REPL UX.

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change` label.
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Author: dariocurr

datafusion-cli/src/helper.rs

@@ -20,7 +20,7 @@
 
 use std::borrow::Cow;
 
-use crate::highlighter::{NoSyntaxHighlighter, SyntaxHighlighter};
+use crate::highlighter::{Color, NoSyntaxHighlighter, SyntaxHighlighter};
 
 use datafusion::sql::parser::{DFParser, Statement};
 use datafusion::sql::sqlparser::dialect::dialect_from_str;
@@ -33,6 +33,9 @@ use rustyline::hint::Hinter;
 use rustyline::validate::{ValidationContext, ValidationResult, Validator};
 use rustyline::{Context, Helper, Result};
 
+/// Default suggestion shown when the input line is empty.
+const DEFAULT_HINT_SUGGESTION: &str = " \\? for help, \\q to quit";
+
 pub struct CliHelper {
     completer: FilenameCompleter,
     dialect: Dialect,
@@ -114,6 +117,15 @@ impl Highlighter for CliHelper {
 
 impl Hinter for CliHelper {
     type Hint = String;
+
+    fn hint(&self, line: &str, _pos: usize, _ctx: &Context<'_>) -> Option<String> {
+        if line.trim().is_empty() {
+            let suggestion = Color::gray(DEFAULT_HINT_SUGGESTION);
+            Some(suggestion)
+        } else {
+            None
+        }
+    }
 }
 
 /// returns true if the current position is after the open quote for

datafusion-cli/src/highlighter.rs

@@ -80,16 +80,20 @@ impl Highlighter for SyntaxHighlighter {
 }
 
 /// Convenient utility to return strings with [ANSI color](https://gist.github.com/JBlond/2fea43a3049b38287e5e9cefc87b2124).
-struct Color {}
+pub(crate) struct Color {}
 
 impl Color {
-    fn green(s: impl Display) -> String {
+    pub(crate) fn green(s: impl Display) -> String {
         format!("\x1b[92m{s}\x1b[0m")
     }
 
-    fn red(s: impl Display) -> String {
+    pub(crate) fn red(s: impl Display) -> String {
         format!("\x1b[91m{s}\x1b[0m")
     }
+
+    pub(crate) fn gray(s: impl Display) -> String {
+        format!("\x1b[90m{s}\x1b[0m")
+    }
 }
 
 #[cfg(test)]