Add expression parsing

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/syn/span.rs

@@ -1,4 +1,7 @@
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+use std::fmt::{Display, Formatter, self};
+
+#[derive(Debug, Clone, Copy, Eq)]
+#[cfg_attr(not(test), derive(PartialEq))]
 pub struct Pos {
     pub source: usize,
     pub line: usize,
@@ -7,6 +10,12 @@ pub struct Pos {
     pub len: usize,
 }
 
+impl Display for Pos {
+    fn fmt(&self, fmt: &mut Formatter) -> fmt::Result {
+        write!(fmt, "line {} at {}", self.line + 1, self.col + 1)
+    }
+}
+
 impl Default for Pos {
     fn default() -> Self {
         Pos {
@@ -19,6 +28,13 @@ impl Default for Pos {
     }
 }
 
+#[cfg(test)]
+impl PartialEq for Pos {
+    fn eq(&self, _other: &Pos) -> bool {
+        true
+    }
+}
+
 impl Pos {
     pub fn from_char(c: char, source: usize, line: usize, col: usize, byte: usize) -> Self {
         Pos {
@@ -77,6 +93,18 @@ impl Span {
     }
 }
 
+impl Display for Span {
+    fn fmt(&self, fmt: &mut Formatter) -> fmt::Result {
+        if self.start == self.end {
+            Display::fmt(&self.start, fmt)
+        } else if self.start.line == self.end.line {
+            write!(fmt, "line {} at {}-{}", self.start.line + 1, self.start.col + 1, self.end.col + 1)
+        } else {
+            write!(fmt, "lines {} to {}", self.start.line + 1, self.end.line + 1)
+        }
+    }
+}
+
 pub trait Spanned {
     fn span(&self) -> Span;
 
@@ -92,6 +120,23 @@ impl Spanned for Span {
     }
 }
 
+pub struct Sourced<'t, T: Spanned> {
+    text: &'t str,
+    inner: T,
+}
+
+impl<'t, T: Spanned> Sourced<'t, T> {
+    fn text(&self) -> &'t str {
+        self.text_at(self.text)
+    }
+}
+
+impl<T: Spanned> Spanned for Sourced<'_, T> {
+    fn span(&self) -> Span {
+        self.inner.span()
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;