avoid embedding StabilityLevel::Unstable reason string into metadata multiple times

compiler/rustc_attr/src/builtin.rs

@@ -138,7 +138,7 @@ pub enum StabilityLevel {
     /// `#[unstable]`
     Unstable {
         /// Reason for the current stability level.
-        reason: Option<Symbol>,
+        reason: UnstableReason,
         /// Relevant `rust-lang/rust` issue.
         issue: Option<NonZeroU32>,
         is_soft: bool,
@@ -182,6 +182,32 @@ impl StabilityLevel {
     }
 }
 
+#[derive(Encodable, Decodable, PartialEq, Copy, Clone, Debug, Eq, Hash)]
+#[derive(HashStable_Generic)]
+pub enum UnstableReason {
+    None,
+    Default,
+    Some(Symbol),
+}
+
+impl UnstableReason {
+    fn from_opt_reason(reason: Option<Symbol>) -> Self {
+        // UnstableReason::Default constructed manually
+        match reason {
+            Some(r) => Self::Some(r),
+            None => Self::None,
+        }
+    }
+
+    pub fn to_opt_reason(&self) -> Option<Symbol> {
+        match self {
+            Self::None => None,
+            Self::Default => Some(sym::unstable_location_reason_default),
+            Self::Some(r) => Some(*r),
+        }
+    }
+}
+
 /// Collects stability info from all stability attributes in `attrs`.
 /// Returns `None` if no stability attributes are found.
 pub fn find_stability(
@@ -371,7 +397,12 @@ where
                                 );
                                 continue;
                             }
-                            let level = Unstable { reason, issue: issue_num, is_soft, implied_by };
+                            let level = Unstable {
+                                reason: UnstableReason::from_opt_reason(reason),
+                                issue: issue_num,
+                                is_soft,
+                                implied_by,
+                            };
                             if sym::unstable == meta_name {
                                 stab = Some((Stability { level, feature }, attr.span));
                             } else {

compiler/rustc_middle/src/middle/stability.rs

@@ -475,7 +475,13 @@ impl<'tcx> TyCtxt<'tcx> {
                 }
 
                 let suggestion = suggestion_for_allocator_api(self, def_id, span, feature);
-                EvalResult::Deny { feature, reason, issue, suggestion, is_soft }
+                EvalResult::Deny {
+                    feature,
+                    reason: reason.to_opt_reason(),
+                    issue,
+                    suggestion,
+                    is_soft,
+                }
             }
             Some(_) => {
                 // Stable APIs are always ok to call and deprecated APIs are

compiler/rustc_passes/src/stability.rs

@@ -2,7 +2,7 @@
 //! propagating default levels lexically from parent to children ast nodes.
 
 use attr::StabilityLevel;
-use rustc_attr::{self as attr, ConstStability, Stability, Unstable};
+use rustc_attr::{self as attr, ConstStability, Stability, Unstable, UnstableReason};
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap};
 use rustc_errors::{struct_span_err, Applicability};
 use rustc_hir as hir;
@@ -634,12 +634,9 @@ fn stability_index(tcx: TyCtxt<'_>, (): ()) -> Index {
         // while maintaining the invariant that all sysroot crates are unstable
         // by default and are unable to be used.
         if tcx.sess.opts.unstable_opts.force_unstable_if_unmarked {
-            let reason = "this crate is being loaded from the sysroot, an \
-                          unstable location; did you mean to load this crate \
-                          from crates.io via `Cargo.toml` instead?";
             let stability = Stability {
                 level: attr::StabilityLevel::Unstable {
-                    reason: Some(Symbol::intern(reason)),
+                    reason: UnstableReason::Default,
                     issue: NonZeroU32::new(27812),
                     is_soft: false,
                     implied_by: None,

compiler/rustc_resolve/src/macros.rs

@@ -812,7 +812,7 @@ impl<'a> Resolver<'a> {
                     stability::report_unstable(
                         self.session,
                         feature,
-                        reason,
+                        reason.to_opt_reason(),
                         issue,
                         None,
                         is_soft,

compiler/rustc_span/src/symbol.rs

@@ -1527,6 +1527,9 @@ symbols! {
         unsized_locals,
         unsized_tuple_coercion,
         unstable,
+        unstable_location_reason_default: "this crate is being loaded from the sysroot, an \
+                          unstable location; did you mean to load this crate \
+                          from crates.io via `Cargo.toml` instead?",
         untagged_unions,
         unused_imports,
         unused_qualifications,