tamer: diagnose::report: Vary mark character depending on level

Looking more and more Rust-like.  Shameless copy.

TBH I forget what character it uses for help, but it's easy enough to
change.

Also, to be clear: this is modeled after Rust, but it's not a requirement of
mine that it look exactly like it.  I just like the general style; I'll
surely deviate over time, as appropriate (or as I feel like it).

DEV-12151

tamer/src/diagnose/report.rs

@@ -558,8 +558,11 @@ impl<'d> Display for LineMark<'d> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self.col {
             Some(col) => {
-                let underline = "^"
-                    .repeat((col.end().get() - col.start().get()) as usize + 1);
+                let underline =
+                    self.level.mark_char().to_string().repeat(
+                        (col.end().get() - col.start().get()) as usize + 1,
+                    );
+
                 let lpad = col.start().get() as usize - 1;
 
                 write!(f, "   | {:lpad$}{underline}\n", "")?;
@@ -577,6 +580,22 @@ impl<'d> Display for LineMark<'d> {
     }
 }
 
+trait MarkChar {
+    fn mark_char(&self) -> char;
+}
+
+impl MarkChar for Level {
+    /// Character used to underline the columns applicable to a given span
+    ///   underneath a source line.
+    fn mark_char(&self) -> char {
+        match self {
+            Level::InternalError => '!',
+            Level::Error => '^',
+            Level::Note | Level::Help => '-',
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;

tamer/src/diagnose/report/test/integration.rs

@@ -357,7 +357,7 @@ internal error: multiple spans with labels of different severity level
   --> foo/bar:4:9
    |
    | foo/bar line 4
-   |         ^^^^^^
+   |         !!!!!!
    = internal error: an internal error
    = error: an error
    = note: a note
@@ -506,3 +506,44 @@ error: offset at newline
 "
     );
 }
+
+#[test]
+fn error_level_mark_styling() {
+    let ctx = Context::from("foo/bar");
+
+    assert_report!(
+        "multiple level mark styles",
+        vec![
+            ctx.span(0, 7).internal_error("A"),
+            ctx.span(15, 7).error("B"),
+            ctx.span(30, 7).note("C"),
+            ctx.span(45, 7).help("D"),
+        ],
+        "\
+internal error: multiple level mark styles
+  --> foo/bar:1:1
+   |
+   | foo/bar line 1
+   | !!!!!!!
+   = internal error: A
+
+  --> foo/bar:2:1
+   |
+   | foo/bar line 2
+   | ^^^^^^^
+   = error: B
+
+  --> foo/bar:3:1
+   |
+   | foo/bar line 3
+   | -------
+   = note: C
+
+  --> foo/bar:4:1
+   |
+   | foo/bar line 4
+   | -------
+   = help: D
+"
+    );
+}