diff --git a/tamer/src/diagnose.rs b/tamer/src/diagnose.rs
index 424a8f97..f425acdd 100644
--- a/tamer/src/diagnose.rs
+++ b/tamer/src/diagnose.rs
@@ -27,7 +27,7 @@ mod report;
 mod resolver;
 
 pub use report::{Reporter, VisualReporter};
-pub use resolver::*;
+pub use resolver::FsSpanResolver;
 
 use core::fmt;
 use std::{borrow::Cow, error::Error, fmt::Display};
diff --git a/tamer/src/diagnose/report.rs b/tamer/src/diagnose/report.rs
index e249820c..55580f0a 100644
--- a/tamer/src/diagnose/report.rs
+++ b/tamer/src/diagnose/report.rs
@@ -20,8 +20,10 @@
 //! Rendering of diagnostic information.
 
 use super::{
-    AnnotatedSpan, Diagnostic, Label, Level, ResolvedSpan, SpanResolver,
-    SpanResolverError,
+    resolver::{
+        ResolvedSpan, ResolvedSpanData, SpanResolver, SpanResolverError,
+    },
+    AnnotatedSpan, Diagnostic, Label, Level,
 };
 use crate::span::{Context, Span, UNKNOWN_SPAN};
 use std::fmt::{self, Display, Write};
@@ -216,10 +218,7 @@ impl<'s> Display for SpanHeader<'s> {
 ///   offsets should be rendered in place of lines and columns.
 #[derive(Debug)]
 enum HeaderLineNum<'s> {
-    /// Failed to resolve the [`Span`] into a [`ResolvedSpan`].
     Unresolved(Span),
-
-    /// The [`Span`] was resolved into one or more [`SourceLine`]s.
     Resolved(&'s ResolvedSpan),
 }
 
diff --git a/tamer/src/diagnose/report/test/integration.rs b/tamer/src/diagnose/report/test/integration.rs
index d1992738..8ab0581a 100644
--- a/tamer/src/diagnose/report/test/integration.rs
+++ b/tamer/src/diagnose/report/test/integration.rs
@@ -52,11 +52,9 @@
 //!   and separate reporters will be provided for machine-readable
 //!   formats if that is what is needed.
 
-use crate::{
-    diagnose::{Annotate, BufSpanResolver},
-    span::Context,
-};
+use crate::span::Context;
 
+use super::super::super::{resolver::BufSpanResolver, Annotate};
 use super::*;
 use std::{
     collections::HashMap,
diff --git a/tamer/src/diagnose/resolver.rs b/tamer/src/diagnose/resolver.rs
index 2e30fd39..5e3b93e4 100644
--- a/tamer/src/diagnose/resolver.rs
+++ b/tamer/src/diagnose/resolver.rs
@@ -29,8 +29,6 @@ use std::{
     io::{self, BufRead, BufReader, Seek},
     mem::take,
     num::NonZeroU32,
-    ops::Deref,
-    slice::SliceIndex,
     str::Utf8Error,
 };
 use unicode_width::UnicodeWidthChar;
@@ -134,34 +132,58 @@ pub struct ResolvedSpan {
     lines: NonEmptyVec<SourceLine>,
 }
 
-impl ResolvedSpan {
+/// Data interpreted from a [`ResolvedSpan`] or equivalent.
+pub trait ResolvedSpanData {
     /// Line number representing the offset of the [`Span`].
-    pub fn line_num(&self) -> NonZeroU32 {
-        self.lines.first().num
-    }
+    ///
+    /// This is intended to answer the question of "what line?",
+    ///   to which one would typically reply with the line that a particular
+    ///   issue starts on.
+    ///
+    /// More concretely,
+    ///   this is the line number expected to appear in a report heading
+    ///   alongside the context,
+    ///     or in an error summary
+    ///     (e.g. "path/to/file:1:2").
+    fn line_num(&self) -> NonZeroU32;
 
-    /// Column number(s) relative to the beginning of the line.
+    /// Column number(s) relative to the beginning of the first line
+    ///   representing the offset of the [`Span`].
     ///
     /// 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
-    }
+    fn col_num(&self) -> Option<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 {
+    fn first_line_span(&self) -> Span;
+
+    /// [`Context`] of the [`Span`] used for resolution.
+    fn ctx(&self) -> Context;
+
+    /// The original [`Span`] before resolution.
+    fn unresolved_span(&self) -> Span;
+}
+
+impl ResolvedSpanData for ResolvedSpan {
+    fn line_num(&self) -> NonZeroU32 {
+        self.lines.first().num
+    }
+
+    fn col_num(&self) -> Option<Column> {
+        self.lines.first().column
+    }
+
+    fn first_line_span(&self) -> Span {
         self.lines.first().span
     }
 
-    /// [`Context`] of the [`Span`] used for resolution.
-    pub fn ctx(&self) -> Context {
+    fn ctx(&self) -> Context {
         self.span.ctx()
     }
 
-    /// The original [`Span`] before resolution.
-    pub fn unresolved_span(&self) -> Span {
+    fn unresolved_span(&self) -> Span {
         self.span
     }
 }