If save-ing with non-existing parent dir, return directory_not_found (#15961)

crates/nu-protocol/src/engine/engine_state.rs

@@ -768,6 +768,30 @@ impl EngineState {
         &[0u8; 0]
     }
 
+    /// If the span's content starts with the given prefix, return two subspans
+    /// corresponding to this prefix, and the rest of the content.
+    pub fn span_match_prefix(&self, span: Span, prefix: &[u8]) -> Option<(Span, Span)> {
+        let contents = self.get_span_contents(span);
+
+        if contents.starts_with(prefix) {
+            span.split_at(prefix.len())
+        } else {
+            None
+        }
+    }
+
+    /// If the span's content ends with the given postfix, return two subspans
+    /// corresponding to the rest of the content, and this postfix.
+    pub fn span_match_postfix(&self, span: Span, prefix: &[u8]) -> Option<(Span, Span)> {
+        let contents = self.get_span_contents(span);
+
+        if contents.ends_with(prefix) {
+            span.split_at(span.len() - prefix.len())
+        } else {
+            None
+        }
+    }
+
     /// Get the global config from the engine state.
     ///
     /// Use [`Stack::get_config()`] instead whenever the `Stack` is available, as it takes into

crates/nu-protocol/src/span.rs

@@ -132,6 +132,45 @@ impl Span {
         Self::new(self.start - offset, self.end - offset)
     }
 
+    /// Return length of the slice.
+    pub fn len(&self) -> usize {
+        self.end - self.start
+    }
+
+    /// Indicate if slice has length 0.
+    pub fn is_empty(&self) -> bool {
+        self.start == self.end
+    }
+
+    /// Return another span fully inside the [`Span`].
+    ///
+    /// `start` and `end` are relative to `self.start`, and must lie within the `Span`.
+    /// In other words, both `start` and `end` must be `<= self.len()`.
+    pub fn subspan(&self, offset_start: usize, offset_end: usize) -> Option<Self> {
+        let len = self.len();
+
+        if offset_start > len || offset_end > len || offset_start > offset_end {
+            None
+        } else {
+            Some(Self::new(
+                self.start + offset_start,
+                self.start + offset_end,
+            ))
+        }
+    }
+
+    /// Return two spans that split the ['Span'] at the given position.
+    pub fn split_at(&self, offset: usize) -> Option<(Self, Self)> {
+        if offset < self.len() {
+            Some((
+                Self::new(self.start, self.start + offset),
+                Self::new(self.start + offset, self.end),
+            ))
+        } else {
+            None
+        }
+    }
+
     pub fn contains(&self, pos: usize) -> bool {
         self.start <= pos && pos < self.end
     }