Better error messages

Error messages with locations were good, and then they sucked, and now
they're good again.

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

src/syn/span.rs

@@ -1,6 +1,6 @@
 #![allow(dead_code)]
 
-use std::fmt::{self, Debug};
+use std::fmt::{self, Debug, Display};
 use std::rc::Rc;
 
 #[derive(Debug, Default, Clone, PartialEq, Eq)]
@@ -34,6 +34,30 @@ impl PartialEq for Span {
     }
 }
 
+impl Display for Span {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        if self.start == self.end {
+            write!(
+                fmt,
+                "line {} col {} in {}",
+                self.end.line, self.end.col, self.source
+            )
+        } else if self.end.source > self.start.source && self.end.line == self.start.line {
+            write!(
+                fmt,
+                "line {} cols {}-{} in {}",
+                self.end.line, self.start.col, self.end.col, self.source
+            )
+        } else {
+            write!(
+                fmt,
+                "lines {}-{} in {}",
+                self.start.line, self.end.line, self.source
+            )
+        }
+    }
+}
+
 #[derive(Clone, PartialEq, Eq)]
 pub struct Spanned<T> {
     span: Span,

src/vm/error.rs

@@ -18,7 +18,7 @@ pub enum RuntimeError {
     #[error("expected {0}")]
     WrongValue(String),
 
-    #[error("at XXX")]
+    #[error("at {0}")]
     Span(Span, Box<RuntimeError>),
 }