hygiene: Fix wording of desugaring descriptions

Use variant names rather than descriptions for identifying desugarings in `#[rustc_on_unimplemented]`.
Both are highly unstable, but variant name is at least a single identifier.

Author: petrochenkov

src/doc/unstable-book/src/language-features/on-unimplemented.md

@@ -98,7 +98,8 @@ application of these fields based on a variety of attributes when using
    `crate_local`) or matching against a particular method. Currently used
    for `try`.
  - `from_desugaring`: usable both as boolean (whether the flag is present)
-   or matching against a particular desugaring.
+   or matching against a particular desugaring. The desugaring is identified
+   with its variant name in the `DesugaringKind` enum.
 
 For example, the `Iterator` trait can be annotated in the following way:
 

src/libcore/ops/try.rs

@@ -8,12 +8,12 @@
 #[rustc_on_unimplemented(
    on(all(
        any(from_method="from_error", from_method="from_ok"),
-       from_desugaring="?"),
+       from_desugaring="QuestionMark"),
       message="the `?` operator can only be used in a \
                function that returns `Result` or `Option` \
                (or another type that implements `{Try}`)",
       label="cannot use the `?` operator in a function that returns `{Self}`"),
-   on(all(from_method="into_result", from_desugaring="?"),
+   on(all(from_method="into_result", from_desugaring="QuestionMark"),
       message="the `?` operator can only be applied to values \
                that implement `{Try}`",
       label="the `?` operator cannot be applied to type `{Self}`")

src/librustc/traits/error_reporting.rs

@@ -372,7 +372,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
         if let Some(k) = obligation.cause.span.desugaring_kind() {
             flags.push((sym::from_desugaring, None));
-            flags.push((sym::from_desugaring, Some(k.descr().to_string())));
+            flags.push((sym::from_desugaring, Some(format!("{:?}", k))));
         }
         let generics = self.tcx.generics_of(def_id);
         let self_ty = trait_ref.self_ty();

src/libsyntax_pos/hygiene.rs

@@ -734,15 +734,16 @@ pub enum DesugaringKind {
 }
 
 impl DesugaringKind {
-    pub fn descr(self) -> &'static str {
+    /// The description wording should combine well with "desugaring of {}".
+    fn descr(self) -> &'static str {
         match self {
-            DesugaringKind::CondTemporary => "if and while condition",
-            DesugaringKind::Async => "async",
-            DesugaringKind::Await => "await",
-            DesugaringKind::QuestionMark => "?",
-            DesugaringKind::TryBlock => "try block",
-            DesugaringKind::ExistentialType => "existential type",
-            DesugaringKind::ForLoop => "for loop",
+            DesugaringKind::CondTemporary => "`if` or `while` condition",
+            DesugaringKind::Async => "`async` block or function",
+            DesugaringKind::Await => "`await` expression",
+            DesugaringKind::QuestionMark => "operator `?`",
+            DesugaringKind::TryBlock => "`try` block",
+            DesugaringKind::ExistentialType => "`existential type`",
+            DesugaringKind::ForLoop => "`for` loop",
         }
     }
 }

src/libsyntax_pos/lib.rs

@@ -442,7 +442,7 @@ impl Span {
             // Don't print recursive invocations.
             if !info.call_site.source_equal(&prev_span) {
                 let (pre, post) = match info.kind {
-                    ExpnKind::Desugaring(..) => ("desugaring of `", "`"),
+                    ExpnKind::Desugaring(..) => ("desugaring of ", ""),
                     ExpnKind::Macro(macro_kind, _) => match macro_kind {
                         MacroKind::Bang => ("", "!"),
                         MacroKind::Attr => ("#[", "]"),