tamer: diagnost::resolver::ResolvedSpan: Clear methods for all data

This (a) makes it clear the intent of these methods and (b) will allow
introducing a trait for mocking it.

DEV-12151

tamer/src/diagnose/report.rs

@@ -134,7 +134,7 @@ impl MaybeResolvedSpan {
     fn header(&self) -> SpanHeader {
         match self {
             Self::Resolved(rspan) => {
-                SpanHeader(rspan.span.ctx(), HeaderLineNum::Resolved(&rspan))
+                SpanHeader(rspan.ctx(), HeaderLineNum::Resolved(&rspan))
             }
 
             Self::Unresolved(span, _) => {
@@ -260,7 +260,7 @@ struct HeaderColNum<'s>(&'s ResolvedSpan);
 impl<'s> Display for HeaderColNum<'s> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let Self(rspan) = self;
-        let span = rspan.span;
+        let span = rspan.unresolved_span();
 
         match rspan.col_num() {
             Some(col) => write!(f, ":{}", col),

tamer/src/diagnose/resolver.rs

@@ -122,7 +122,7 @@ impl<T> AsRef<Vec<T>> for NonEmptyVec<T> {
 #[derive(Debug, PartialEq, Eq)]
 pub struct ResolvedSpan {
     /// The original [`Span`] whose resolution was requested.
-    pub span: Span,
+    span: Span,
 
     /// The lines of source code that correspond to this [`Span`],
     ///   if known.
@@ -135,17 +135,35 @@ pub struct ResolvedSpan {
 }
 
 impl ResolvedSpan {
+    /// Line number representing the offset of the [`Span`].
     pub fn line_num(&self) -> NonZeroU32 {
         self.lines.first().num
     }
 
+    /// Column number(s) relative to the beginning of the line.
+    ///
+    /// The column may not be able to be resolved if the line contains
+    ///   invalid UTF-8 data.
     pub fn col_num(&self) -> Option<Column> {
         self.lines.first().column
     }
 
+    /// A [`Span`] representing the first line.
+    ///
+    /// Note that many spans have only one line associated with them.
     pub fn first_line_span(&self) -> Span {
         self.lines.first().span
     }
+
+    /// [`Context`] of the [`Span`] used for resolution.
+    pub fn ctx(&self) -> Context {
+        self.span.ctx()
+    }
+
+    /// The original [`Span`] before resolution.
+    pub fn unresolved_span(&self) -> Span {
+        self.span
+    }
 }
 
 /// Source column offsets.