add SourceFile::span_for_substr

src/source.rs

@@ -141,6 +141,34 @@ impl SourceFile {
         }
         Some(SourceSpan::from(start..end))
     }
+
+    /// Creates a span for an item using a substring of `contents`
+    ///
+    /// Note that substr must be a literal substring, as in it must be
+    /// a pointer into the same string! If it's not we'll return None.
+    pub fn span_for_substr(&self, substr: &str) -> Option<SourceSpan> {
+        // Get the bounds of the full string
+        let base_addr = self.inner.contents.as_ptr() as usize;
+        let base_len = self.inner.contents.len();
+
+        // Get the bounds of the substring
+        let substr_addr = substr.as_ptr() as usize;
+        let substr_len = substr.len();
+
+        // The index of the substring is just the number of bytes it is from the start
+        // (This will bail out if the """substring""" has an address *before* the full string)
+        let start = substr_addr.checked_sub(base_addr)?;
+        // The end index (exclusive) is just the start index + sublen
+        // (This will bail out if this overflows)
+        let end = start.checked_add(substr_len)?;
+        // Finally, make sure the substr endpoint isn't past the end of the full string
+        if end > base_len {
+            return None;
+        }
+
+        // At this point it's definitely a substring, nice!
+        Some(SourceSpan::from(start..end))
+    }
 }
 
 impl SourceCode for SourceFile {

tests/source.rs

@@ -0,0 +1,31 @@
+use miette::SourceCode;
+
+#[test]
+fn substr_span() {
+    // Make the file
+    let contents = String::from("hello !there!");
+    let source = axoasset::SourceFile::new("file.md", contents).unwrap();
+
+    // Do some random parsing operation
+    let mut parse = source.contents().split('!');
+    let _ = parse.next();
+    let there = parse.next().unwrap();
+
+    // Get the span
+    let there_span = source.span_for_substr(there).unwrap();
+
+    // Assert the span is correct
+    let span_bytes = source.read_span(&there_span, 0, 0).unwrap().data();
+    assert_eq!(std::str::from_utf8(span_bytes).unwrap(), "there");
+}
+
+#[test]
+fn substr_span_invalid() {
+    // Make the file
+    let contents = String::from("hello !there!");
+    let source = axoasset::SourceFile::new("file.md", contents).unwrap();
+
+    // Get the span for a non-substring (string literal isn't pointing into the String)
+    let there_span = source.span_for_substr("there");
+    assert_eq!(there_span, None);
+}