-Z identify_regions toggles rendering of (previously hidden) unnamed regions.

Unlike `-Z verbose`, it is succinct.

It uniquely identifies regions when displaying them, and distinguishes
code extents from user-specified lifetimes in the output by leveraging
a syntactic restriction: you cannot write a lifetime that starts with
a numeric character.

For example, it prints '<num>ce for the more verbose
`ReScope(CodeExtent(<num>))`.

src/librustc/mir/mod.rs

@@ -1176,12 +1176,22 @@ impl<'tcx> Debug for Rvalue<'tcx> {
             UnaryOp(ref op, ref a) => write!(fmt, "{:?}({:?})", op, a),
             Discriminant(ref lval) => write!(fmt, "discriminant({:?})", lval),
             NullaryOp(ref op, ref t) => write!(fmt, "{:?}({:?})", op, t),
-            Ref(_, borrow_kind, ref lv) => {
+            Ref(region, borrow_kind, ref lv) => {
                 let kind_str = match borrow_kind {
                     BorrowKind::Shared => "",
                     BorrowKind::Mut | BorrowKind::Unique => "mut ",
                 };
-                write!(fmt, "&{}{:?}", kind_str, lv)
+
+                // When identifying regions, add trailing space if
+                // necessary.
+                let region = if ppaux::identify_regions() {
+                    let mut region = format!("{}", region);
+                    if region.len() > 0 { region.push(' '); }
+                    region
+                } else {
+                    "".to_owned()
+                };
+                write!(fmt, "&{}{}{:?}", region, kind_str, lv)
             }
 
             Aggregate(ref kind, ref lvs) => {

src/librustc/session/config.rs

@@ -895,6 +895,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
         "in general, enable more debug printouts"),
     span_free_formats: bool = (false, parse_bool, [UNTRACKED],
         "when debug-printing compiler state, do not include spans"), // o/w tests have closure@path
+    identify_regions: bool = (false, parse_bool, [UNTRACKED],
+        "make unnamed regions display as '# (where # is some non-ident unique id)"),
     time_passes: bool = (false, parse_bool, [UNTRACKED],
         "measure time of each rustc pass"),
     count_llvm_insns: bool = (false, parse_bool,

src/librustc/util/ppaux.rs

@@ -8,8 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use hir::BodyId;
 use hir::def_id::DefId;
 use hir::map::definitions::DefPathData;
+use middle::region::{CodeExtent, BlockRemainder};
 use ty::subst::{self, Subst};
 use ty::{BrAnon, BrEnv, BrFresh, BrNamed};
 use ty::{TyBool, TyChar, TyAdt};
@@ -32,6 +34,10 @@ pub fn verbose() -> bool {
     ty::tls::with(|tcx| tcx.sess.verbose())
 }
 
+pub fn identify_regions() -> bool {
+    ty::tls::with(|tcx| tcx.sess.opts.debugging_opts.identify_regions)
+}
+
 fn fn_sig(f: &mut fmt::Formatter,
           inputs: &[Ty],
           variadic: bool,
@@ -519,6 +525,23 @@ impl fmt::Display for ty::RegionKind {
             ty::ReSkolemized(_, br) => {
                 write!(f, "{}", br)
             }
+            ty::ReScope(code_extent) if identify_regions() => {
+                match code_extent {
+                    CodeExtent::Misc(node_id) =>
+                        write!(f, "'{}mce", node_id.as_u32()),
+                    CodeExtent::CallSiteScope(BodyId { node_id }) =>
+                        write!(f, "'{}cce", node_id.as_u32()),
+                    CodeExtent::ParameterScope(BodyId { node_id }) =>
+                        write!(f, "'{}pce", node_id.as_u32()),
+                    CodeExtent::DestructionScope(node_id) =>
+                        write!(f, "'{}dce", node_id.as_u32()),
+                    CodeExtent::Remainder(BlockRemainder { block, first_statement_index }) =>
+                        write!(f, "'{}_{}rce", block, first_statement_index),
+                }
+            }
+            ty::ReVar(region_vid) if identify_regions() => {
+                write!(f, "'{}rv", region_vid.index)
+            }
             ty::ReScope(_) |
             ty::ReVar(_) |
             ty::ReErased => Ok(()),