From 91bf01a9775ac1d2e2633f8702481d023d42fb24 Mon Sep 17 00:00:00 2001
From: Chet Nichols III <chetn@nvidia.com>
Date: Sat, 22 Nov 2025 00:06:45 -0800
Subject: [PATCH] feat for issue #809: impl std::error::Error for
 cursive::theme::Error

https://github.com/gyscos/cursive/issues/809 had been opened inquiring about implementing `std::error::Error` for `cursive::theme::Error` so they could do something like this:

```
fn load_theme(siv: &mut CursiveRunnable, path: PathBuf) -> anyhow::Result<()> {
    siv.load_theme_file(path)?;
    Ok(())
}
```

Instead of having to do this:

```
fn load_theme(siv: &mut CursiveRunnable, path: PathBuf) -> anyhow::Result<()> {
    match siv.load_theme_file(path) {
        Ok(_) => Ok(()),
        Err(e) => match e {
            cursive::theme::Error::Io(e) => Err(e.into()),
            cursive::theme::Error::Parse(e) => Err(e.into()),
        }
    }
}
```

It seemed like low hanging fruit, so I figured I'd offer up a PR. Included some tests for the different `Error::Io`, `Error::Parse` and `?` cases.

Thank you for considering this change!
---
 cursive-core/src/theme.rs | 85 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 85 insertions(+)

diff --git a/cursive-core/src/theme.rs b/cursive-core/src/theme.rs
index a26127b9..46468756 100644
--- a/cursive-core/src/theme.rs
+++ b/cursive-core/src/theme.rs
@@ -181,6 +181,30 @@ impl From<toml::de::Error> for Error {
     }
 }
 
+impl std::fmt::Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match self {
+            Error::Io(err) => write!(f, "An error occured when reading the theme file: {}", err),
+            #[cfg(feature = "toml")]
+            Error::Parse(err) => write!(
+                f,
+                "An error occured while parsing the theme TOML content: {}",
+                err
+            ),
+        }
+    }
+}
+
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            Error::Io(err) => Some(err),
+            #[cfg(feature = "toml")]
+            Error::Parse(err) => Some(err),
+        }
+    }
+}
+
 /// Loads a theme from file.
 ///
 /// Must have the `toml` feature enabled.
@@ -215,3 +239,64 @@ pub fn load_toml(content: &str) -> Result<Theme, Error> {
 pub fn load_default() -> Theme {
     Theme::default()
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::error::Error as _;
+
+    #[test]
+    // Make sure Error::Io correctly implements std::error::Error.
+    fn test_io_error_implements_std_error() {
+        let io_error = io::Error::new(io::ErrorKind::NotFound, "file not found");
+        let theme_error = Error::Io(io_error);
+
+        // Should be able to use as std::error::Error.
+        let _error_trait: &dyn std::error::Error = &theme_error;
+
+        // Should have a Display implementation.
+        let display_str = format!("{}", theme_error);
+        assert!(display_str.contains("when reading the theme file"));
+
+        // And a source.
+        assert!(theme_error.source().is_some());
+    }
+
+    #[cfg(feature = "toml")]
+    #[test]
+    // Make sure Error::Parse correctly implements std::error::Error.
+    fn test_parse_error_implements_std_error() {
+        let parse_result = load_toml("invalid toml {{{");
+        assert!(parse_result.is_err());
+
+        let theme_error = parse_result.unwrap_err();
+
+        // Should be able to use as std::error::Error.
+        let _error_trait: &dyn std::error::Error = &theme_error;
+
+        // Should have a Display implementation.
+        let display_str = format!("{}", theme_error);
+        assert!(display_str.contains("while parsing the theme"));
+
+        // And a source.
+        assert!(theme_error.source().is_some());
+    }
+
+    #[cfg(feature = "toml")]
+    #[test]
+    // Make sure ? works for returning Error results.
+    fn test_error_works_with_question_mark_operator() {
+        fn load_theme_simple(content: &str) -> Result<Theme, Box<dyn std::error::Error>> {
+            let theme = load_toml(content)?;
+            Ok(theme)
+        }
+
+        // Test with "invalid" TOML.
+        let result = load_theme_simple("invalid toml {{{");
+        assert!(result.is_err());
+
+        // Test with valid TOML.
+        let result = load_theme_simple("shadow = false");
+        assert!(result.is_ok());
+    }
+}