rustc: print elided regions as '_ instead of nothing, and use a separate check when optional.

src/librustc/infer/error_reporting/mod.rs

@@ -826,11 +826,15 @@ fn push_ty_ref<'tcx>(
             mutbl: hir::Mutability,
             s: &mut DiagnosticStyledString,
         ) {
-            let r = &r.to_string();
+            let mut r = r.to_string();
+            if r == "'_" {
+                r.clear();
+            } else {
+                r.push(' ');
+            }
             s.push_highlighted(format!(
-                "&{}{}{}",
+                "&{}{}",
                 r,
-                if r == "" { "" } else { " " },
                 if mutbl == hir::MutMutable { "mut " } else { "" }
             ));
             s.push_normal(ty.to_string());

src/librustc/traits/specialize/mod.rs

@@ -411,7 +411,7 @@ fn to_pretty_impl_header(tcx: TyCtxt<'_, '_, '_>, impl_def_id: DefId) -> Option<
         w.push('<');
         w.push_str(&substs.iter()
             .map(|k| k.to_string())
-            .filter(|k| !k.is_empty())
+            .filter(|k| k != "'_")
             .collect::<Vec<_>>().join(", "));
         w.push('>');
     }

src/librustc/ty/error.rs

@@ -190,7 +190,7 @@ pub fn sort_string(&self, tcx: TyCtxt<'a, 'gcx, 'lcx>) -> Cow<'static, str> {
                 let tymut_string = tymut.to_string();
                 if tymut_string == "_" ||         //unknown type name,
                    tymut_string.len() > 10 ||     //name longer than saying "reference",
-                   region.to_string() != ""       //... or a complex type
+                   region.to_string() != "'_"     //... or a complex type
                 {
                     format!("{}reference", match mutbl {
                         hir::Mutability::MutMutable => "mutable ",

src/librustc/ty/print/pretty.rs

@@ -222,14 +222,10 @@ fn always_print_region_in_paths(
         false
     }
 
-    // HACK(eddyb) Trying to print a lifetime might not print anything, which
-    // may need special handling in the caller (of `ty::RegionKind::print`).
-    // To avoid printing to a temporary string (which isn't even supported),
-    // the `print_region_outputs_anything` method can instead be used to
-    // determine this, ahead of time.
-    //
-    // NB: this must be kept in sync with the implementation of `print_region`.
-    fn print_region_outputs_anything(
+    /// Return `true` if the region should be printed in
+    /// optional positions, e.g. `&'a T` or `dyn Tr + 'b`.
+    /// This is typically the case for all non-`'_` regions.
+    fn region_should_not_be_omitted(
         self: &PrintCx<'_, '_, '_, Self>,
         region: ty::Region<'_>,
     ) -> bool;
@@ -497,7 +493,7 @@ pub fn pretty_path_generic_args(
             match substs[param.index as usize].unpack() {
                 UnpackedKind::Lifetime(r) => {
                     self.always_print_region_in_paths(r) ||
-                    self.print_region_outputs_anything(r)
+                    self.region_should_not_be_omitted(r)
                 }
                 _ => false,
             }
@@ -535,19 +531,6 @@ pub fn pretty_path_generic_args(
             for arg in arg0.into_iter().chain(args) {
                 maybe_comma(&mut cx)?;
 
-                if let UnpackedKind::Lifetime(region) = arg.unpack() {
-                    if !cx.print_region_outputs_anything(region) {
-                        // This happens when the value of the region
-                        // parameter is not easily serialized. This may be
-                        // because the user omitted it in the first place,
-                        // or because it refers to some block in the code,
-                        // etc. I'm not sure how best to serialize this.
-                        p!(write("'_"));
-
-                        continue;
-                    }
-                }
-
                 p!(print(arg));
             }
 
@@ -822,7 +805,7 @@ fn always_print_region_in_paths(
         *region != ty::ReErased
     }
 
-    fn print_region_outputs_anything(
+    fn region_should_not_be_omitted(
         self: &PrintCx<'_, '_, '_, Self>,
         region: ty::Region<'_>,
     ) -> bool {
@@ -902,8 +885,9 @@ pub fn pretty_print_region(
         // `explain_region()` or `note_and_explain_region()`.
         match *region {
             ty::ReEarlyBound(ref data) => {
-                if data.name != "'_" {
+                if data.name != "" {
                     p!(write("{}", data.name));
+                    return self.ok();
                 }
             }
             ty::ReLateBound(_, br) |
@@ -919,6 +903,7 @@ pub fn pretty_print_region(
                 if let Some((region, counter)) = highlight.highlight_bound_region {
                     if br == region {
                         p!(write("'{}", counter));
+                        return self.ok();
                     }
                 }
             }
@@ -938,20 +923,33 @@ pub fn pretty_print_region(
                         first_statement_index.index()
                     )),
                 }
+                return self.ok();
             }
             ty::ReVar(region_vid) if identify_regions => {
                 p!(write("{:?}", region_vid));
+                return self.ok();
             }
             ty::ReVar(_) => {}
             ty::ReScope(_) |
             ty::ReErased => {}
-            ty::ReStatic => p!(write("'static")),
-            ty::ReEmpty => p!(write("'<empty>")),
+            ty::ReStatic => {
+                p!(write("'static"));
+                return self.ok();
+            }
+            ty::ReEmpty => {
+                p!(write("'<empty>"));
+                return self.ok();
+            }
 
             // The user should never encounter these in unsubstituted form.
-            ty::ReClosureBound(vid) => p!(write("{:?}", vid)),
+            ty::ReClosureBound(vid) => {
+                p!(write("{:?}", vid));
+                return self.ok();
+            }
         }
 
+        p!(write("'_"));
+
         self.ok()
     }
 }
@@ -978,7 +976,7 @@ pub fn pretty_print_type(
             }
             ty::Ref(r, ty, mutbl) => {
                 p!(write("&"));
-                if self.print_region_outputs_anything(r) {
+                if self.region_should_not_be_omitted(r) {
                     p!(print(r), write(" "));
                 }
                 p!(print(ty::TypeAndMut { ty, mutbl }))
@@ -1027,7 +1025,7 @@ pub fn pretty_print_type(
                 nest!(|cx| cx.print_def_path(def.did, Some(substs), iter::empty()));
             }
             ty::Dynamic(data, r) => {
-                let print_r = self.print_region_outputs_anything(r);
+                let print_r = self.region_should_not_be_omitted(r);
                 if print_r {
                     p!(write("("));
                 }

src/librustc_codegen_utils/symbol_names.rs

@@ -512,7 +512,7 @@ fn path_generic_args<'gcx, 'tcx>(
 }
 
 impl PrettyPrinter for SymbolPath {
-    fn print_region_outputs_anything(
+    fn region_should_not_be_omitted(
         self: &PrintCx<'_, '_, '_, Self>,
         _region: ty::Region<'_>,
     ) -> bool {

src/test/ui/issues/issue-20831-debruijn.stderr

@@ -11,7 +11,7 @@ LL | |     }
    | |_____^ lifetime mismatch
    |
    = note: expected type `'a`
-              found type ``
+              found type `'_`
 note: the anonymous lifetime #2 defined on the method body at 28:5...
   --> $DIR/issue-20831-debruijn.rs:28:5
    |
@@ -42,7 +42,7 @@ LL | |     }
    | |_____^ lifetime mismatch
    |
    = note: expected type `'a`
-              found type ``
+              found type `'_`
 note: the lifetime 'a as defined on the impl at 26:6...
   --> $DIR/issue-20831-debruijn.rs:26:6
    |

src/test/ui/regions/regions-addr-of-upvar-self.stderr

@@ -4,7 +4,7 @@ error[E0495]: cannot infer an appropriate lifetime for borrow expression due to
 LL |             let p: &'static mut usize = &mut self.food;
    |                                         ^^^^^^^^^^^^^^
    |
-note: first, the lifetime cannot outlive the lifetime  as defined on the body at 9:18...
+note: first, the lifetime cannot outlive the lifetime '_ as defined on the body at 9:18...
   --> $DIR/regions-addr-of-upvar-self.rs:9:18
    |
 LL |         let _f = || {

src/test/ui/regions/regions-return-ref-to-upvar-issue-17403.stderr

@@ -4,7 +4,7 @@ error[E0495]: cannot infer an appropriate lifetime for borrow expression due to
 LL |         let mut f = || &mut x;
    |                        ^^^^^^
    |
-note: first, the lifetime cannot outlive the lifetime  as defined on the body at 7:21...
+note: first, the lifetime cannot outlive the lifetime '_ as defined on the body at 7:21...
   --> $DIR/regions-return-ref-to-upvar-issue-17403.rs:7:21
    |
 LL |         let mut f = || &mut x;