Swap additional context and label in I/O errors (#14954)

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# Description
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Tweaks the error style for I/O errors introduced #14927. Moves the
additional context to below the text that says "I/O error", and always
shows the error kind in the label.

Additional context|Before PR|After PR
:-:|:-:|:-:

yes|![image](https://github.com/user-attachments/assets/df4f2e28-fdf5-4693-b60c-255d019af25f)
|
![image](https://github.com/user-attachments/assets/5915e9d0-78d4-49a6-b495-502d0c6444fa)
no|
![image](https://github.com/user-attachments/assets/e4ecaada-ec8c-4940-b08a-bbfaa45083d5)
|
![image](https://github.com/user-attachments/assets/467163d8-ab39-47f0-a74f-e2effe2fe6af)



# User-Facing Changes
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helps us keep track of breaking changes. -->

N/A, as this is a follow-up to #14927 which has not been included in a
release

# Tests + Formatting
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N/A

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N/A

---------

Co-authored-by: Piepmatz <git+github@cptpiepmatz.de>

crates/nu-command/src/filesystem/open.rs

@@ -118,13 +118,16 @@ impl Command for Open {
                     #[cfg(unix)]
                     let err = {
                         let mut err = err;
-                        err.additional_context = Some(match path.metadata() {
-                            Ok(md) => format!(
-                                "The permissions of {:o} does not allow access for this user",
-                                md.permissions().mode() & 0o0777
-                            ),
-                            Err(e) => e.to_string(),
-                        });
+                        err.additional_context = Some(
+                            match path.metadata() {
+                                Ok(md) => format!(
+                                    "The permissions of {:o} does not allow access for this user",
+                                    md.permissions().mode() & 0o0777
+                                ),
+                                Err(e) => e.to_string(),
+                            }
+                            .into(),
+                        );
                         err
                     };
 

crates/nu-protocol/src/errors/shell_error/io.rs

@@ -115,7 +115,7 @@ pub struct IoError {
     ///
     /// Only set this field if it adds meaningful context.
     /// If [`ErrorKind`] already contains all the necessary information, leave this as [`None`].
-    pub additional_context: Option<String>,
+    pub additional_context: Option<AdditionalContext>,
 
     /// The precise location in the Rust code where the error originated.
     ///
@@ -139,6 +139,16 @@ pub enum ErrorKind {
     IsADirectory,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq, Error, Diagnostic)]
+#[error("{0}")]
+pub struct AdditionalContext(String);
+
+impl From<String> for AdditionalContext {
+    fn from(value: String) -> Self {
+        AdditionalContext(value)
+    }
+}
+
 impl IoError {
     /// Creates a new [`IoError`] with the given kind, span, and optional path.
     ///
@@ -209,7 +219,7 @@ impl IoError {
             kind: kind.into(),
             span,
             path,
-            additional_context: Some(additional_context.to_string()),
+            additional_context: Some(additional_context.to_string().into()),
             location: None,
         }
     }
@@ -249,7 +259,7 @@ impl IoError {
             kind: kind.into(),
             span: Span::unknown(),
             path: None,
-            additional_context: Some(additional_context.to_string()),
+            additional_context: Some(additional_context.to_string().into()),
             location: Some(location.to_string()),
         }
     }
@@ -283,7 +293,7 @@ impl IoError {
             kind: kind.into(),
             span: Span::unknown(),
             path: path.into(),
-            additional_context: Some(additional_context.to_string()),
+            additional_context: Some(additional_context.to_string().into()),
             location: Some(location.to_string()),
         }
     }
@@ -370,16 +380,14 @@ impl Diagnostic for IoError {
             (true, Some(location)) => SourceSpan::new(0.into(), location.len()),
         };
 
-        let label = match self.additional_context.as_ref() {
-            Some(ctx) => format!("{ctx}\n{}", self.kind),
-            None => self.kind.to_string(),
-        };
-        let label = LabeledSpan::new_with_span(Some(label), span);
+        let label = LabeledSpan::new_with_span(Some(self.kind.to_string()), span);
         Some(Box::new(std::iter::once(label)))
     }
 
     fn diagnostic_source(&self) -> Option<&dyn Diagnostic> {
-        Some(&self.kind as &dyn Diagnostic)
+        self.additional_context
+            .as_ref()
+            .map(|ctx| ctx as &dyn Diagnostic)
     }
 
     fn source_code(&self) -> Option<&dyn miette::SourceCode> {