Print syntax contexts and marks when printing hygiene information

src/librustc_driver/pretty.rs

@@ -326,6 +326,7 @@ impl<'hir> pprust::PpAnn for IdentifiedAnnotation<'hir> {
     }
     fn post(&self, s: &mut pprust::State<'_>, node: pprust::AnnNode<'_>) {
         match node {
+            pprust::AnnNode::Crate(_) |
             pprust::AnnNode::Ident(_) |
             pprust::AnnNode::Name(_) => {},
 
@@ -431,14 +432,18 @@ impl<'a> pprust::PpAnn for HygieneAnnotation<'a> {
         match node {
             pprust::AnnNode::Ident(&ast::Ident { name, span }) => {
                 s.s.space();
-                // FIXME #16420: this doesn't display the connections
-                // between syntax contexts
                 s.synth_comment(format!("{}{:?}", name.as_u32(), span.ctxt()))
             }
             pprust::AnnNode::Name(&name) => {
                 s.s.space();
                 s.synth_comment(name.as_u32().to_string())
             }
+            pprust::AnnNode::Crate(_) => {
+                s.s.hardbreak();
+                let verbose = self.sess.verbose();
+                s.synth_comment(syntax_pos::hygiene::debug_hygiene_data(verbose));
+                s.s.hardbreak_if_not_bol();
+            }
             _ => {}
         }
     }

src/libsyntax/print/pprust.rs

@@ -35,6 +35,7 @@ pub enum AnnNode<'a> {
     SubItem(ast::NodeId),
     Expr(&'a ast::Expr),
     Pat(&'a ast::Pat),
+    Crate(&'a ast::Crate),
 }
 
 pub trait PpAnn {
@@ -140,6 +141,7 @@ pub fn print_crate<'a>(cm: &'a SourceMap,
 
     s.print_mod(&krate.module, &krate.attrs);
     s.print_remaining_comments();
+    s.ann.post(&mut s, AnnNode::Crate(krate));
     s.s.eof()
 }
 

src/libsyntax_pos/hygiene.rs

@@ -343,6 +343,38 @@ pub fn update_dollar_crate_names(mut get_name: impl FnMut(SyntaxContext) -> Symb
     }))
 }
 
+pub fn debug_hygiene_data(verbose: bool) -> String {
+    HygieneData::with(|data| {
+        if verbose {
+            format!("{:#?}", data)
+        } else {
+            let mut s = String::from("");
+            s.push_str("Expansions:");
+            data.expn_data.iter().enumerate().for_each(|(id, expn_info)| {
+                let expn_info = expn_info.as_ref().expect("no expansion data for an expansion ID");
+                s.push_str(&format!(
+                    "\n{}: parent: {:?}, call_site_ctxt: {:?}, kind: {:?}",
+                    id,
+                    expn_info.parent,
+                    expn_info.call_site.ctxt(),
+                    expn_info.kind,
+                ));
+            });
+            s.push_str("\n\nSyntaxContexts:");
+            data.syntax_context_data.iter().enumerate().for_each(|(id, ctxt)| {
+                s.push_str(&format!(
+                    "\n#{}: parent: {:?}, outer_mark: ({:?}, {:?})",
+                    id,
+                    ctxt.parent,
+                    ctxt.outer_expn,
+                    ctxt.outer_transparency,
+                ));
+            });
+            s
+        }
+    })
+}
+
 impl SyntaxContext {
     #[inline]
     pub const fn root() -> Self {

src/test/ui/hygiene/unpretty-debug.stdout

@@ -13,3 +13,13 @@ macro_rules! foo /* 0#0 */ { ($ x : ident) => { y + $ x } }
 fn bar /* 0#0 */() { let x /* 0#0 */ = 1; y /* 0#1 */ + x /* 0#0 */ }
 
 fn y /* 0#0 */() { }
+
+/*
+Expansions:
+0: parent: ExpnId(0), call_site_ctxt: #0, kind: Root
+1: parent: ExpnId(0), call_site_ctxt: #0, kind: Macro(Bang, foo)
+
+SyntaxContexts:
+#0: parent: #0, outer_mark: (ExpnId(0), Opaque)
+#1: parent: #0, outer_mark: (ExpnId(1), SemiTransparent)
+*/