feat(Source): impl Source for str, &str (make &'static str usable for testing) (#40)

src/source_impls.rs

@@ -5,12 +5,11 @@ use std::{borrow::Cow, sync::Arc};
 
 use crate::{MietteError, MietteSpanContents, Source, SourceSpan, SpanContents};
 
-impl Source for String {
+fn start_line_column(string: &str, span: &SourceSpan) -> Result<(usize, usize), MietteError> {
-    fn read_span(&self, span: &SourceSpan) -> Result<Box<dyn SpanContents + '_>, MietteError> {
     let mut offset = 0usize;
     let mut start_line = 0usize;
     let mut start_column = 0usize;
-        let mut iter = self.chars().peekable();
+    let mut iter = string.chars().peekable();
     while let Some(char) = iter.next() {
         if offset < span.offset() {
             match char {
@@ -32,16 +31,47 @@ impl Source for String {
         }
 
         if offset >= span.offset() + span.len() - 1 {
+            return Ok((start_line, start_column));
+        }
+
+        offset += char.len_utf8();
+    }
+    Err(MietteError::OutOfBounds)
+}
+
+// The basic impl here is on str (not &str), because otherwise String's impl cannot reuse it
+// without creating a temporary &str inside its read_span implementation, and then returning data
+// that refers to that temporary.
+impl Source for str {
+    fn read_span<'a>(
+        &'a self,
+        span: &SourceSpan,
+    ) -> Result<Box<dyn SpanContents + 'a>, MietteError> {
+        let (start_line, start_column) = start_line_column(self, span)?;
         return Ok(Box::new(MietteSpanContents::new(
             &self.as_bytes()[span.offset()..span.offset() + span.len()],
             start_line,
             start_column,
         )));
     }
-
-            offset += char.len_utf8();
 }
-        Err(MietteError::OutOfBounds)
+
+/// Makes `src: &'static str` or `struct S<'a> { src: &'a str }` usable.
+impl<'s> Source for &'s str {
+    fn read_span<'a>(
+        &'a self,
+        span: &SourceSpan,
+    ) -> Result<Box<dyn SpanContents + 'a>, MietteError> {
+        <str as Source>::read_span(self, span)
+    }
+}
+
+impl Source for String {
+    fn read_span<'a>(
+        &'a self,
+        span: &SourceSpan,
+    ) -> Result<Box<dyn SpanContents + 'a>, MietteError> {
+        <str as Source>::read_span(self, span)
     }
 }