Adds documentation to cover spanview output

Documents the feature introduced in #76074.

src/compiler-debugging.md

@@ -260,6 +260,24 @@ $ dot -T pdf maybe_init_suffix.dot > maybe_init_suffix.pdf
 $ firefox maybe_init_suffix.pdf # Or your favorite pdf viewer
 ```
 
+## Viewing Spanview output (.html files)
+[viewing-spanview-output]: #viewing-spanview-output
+
+In addition to [graphviz output](#formatting-graphviz-output), MIR debugging
+flags include an option to generate a MIR representation called `Spanview` that
+uses HTML to highlight code regions in the original source code and display
+compiler metadata associated with each region.
+[`-Zdump-mir-spanview`](./mir/debugging.md), for example, highlights spans
+associated with each MIR `Statement`, `Terminator`, and/or `BasicBlock`.
+
+These `.html` files use CSS features to dynamically expand spans obscured by
+overlapping spans, and native tooltips (based on the HTML `title` attribute) to
+reveal the actual MIR elements, as text.
+
+To view these files, simply use a modern browser, or a CSS-capable HTML preview
+feature in a modern IDE. (The default HTML preview pane in *VS Code* is known to
+work, for instance.
+
 ## Narrowing (Bisecting) Regressions
 
 The [cargo-bisect-rustc][bisect] tool can be used as a quick and easy way to

src/mir/debugging.md

@@ -1,8 +1,16 @@
 # MIR Debugging
 
-The `-Zdump-mir` flag can be used to dump a text representation of the MIR. The
-`-Zdump-mir-graphviz` flag can be used to dump a `.dot` file that represents
-MIR as a control-flow graph.
+The `-Zdump-mir` flag can be used to dump a text representation of the MIR.
+The following optional flags, used in combination with `-Zdump-mir`, enable
+additional output formats, including:
+
+* `-Zdump-mir-graphviz` - dumps a `.dot` file that represents MIR as a
+control-flow graph
+* `-Zdump-mir-spanview` - dumps an `.html` file that highlights the source
+spans associated with MIR elements (including mouse-over actions to reveal
+elements obscured by overlaps, and tooltips to view the MIR statements).
+This flag takes an optional value: `statement` (the default), `terminator`, or
+`block`, to generate span highlights with different levels of granulatity.
 
 `-Zdump-mir=F` is a handy compiler options that will let you view the MIR for
 each function at each stage of compilation. `-Zdump-mir` takes a **filter** `F`