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

src/source_impls.rs

@@ -5,43 +5,73 @@ use std::{borrow::Cow, sync::Arc};
 
 use crate::{MietteError, MietteSpanContents, Source, SourceSpan, SpanContents};
 
-impl Source for String {
-    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();
-        while let Some(char) = iter.next() {
-            if offset < span.offset() {
-                match char {
-                    '\r' => {
-                        if iter.next_if_eq(&'\n').is_some() {
-                            offset += 1;
-                        }
-                        start_line += 1;
-                        start_column = 0;
-                    }
-                    '\n' => {
-                        start_line += 1;
-                        start_column = 0;
-                    }
-                    _ => {
-                        start_column += 1;
+fn start_line_column(string: &str, span: &SourceSpan) -> Result<(usize, usize), MietteError> {
+    let mut offset = 0usize;
+    let mut start_line = 0usize;
+    let mut start_column = 0usize;
+    let mut iter = string.chars().peekable();
+    while let Some(char) = iter.next() {
+        if offset < span.offset() {
+            match char {
+                '\r' => {
+                    if iter.next_if_eq(&'\n').is_some() {
+                        offset += 1;
                     }
+                    start_line += 1;
+                    start_column = 0;
+                }
+                '\n' => {
+                    start_line += 1;
+                    start_column = 0;
+                }
+                _ => {
+                    start_column += 1;
                 }
             }
-
-            if offset >= span.offset() + span.len() - 1 {
-                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)
+
+        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,
+        )));
+    }
+}
+
+/// 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)
     }
 }