Support multiple stability attributes on items

Cargo.lock

@@ -3338,6 +3338,7 @@ dependencies = [
  "rustc_serialize",
  "rustc_session",
  "rustc_span",
+ "smallvec",
 ]
 
 [[package]]
@@ -3511,6 +3512,7 @@ dependencies = [
  "rustc_target",
  "rustc_trait_selection",
  "rustc_type_ir",
+ "smallvec",
  "tracing",
 ]
 
@@ -4228,6 +4230,7 @@ dependencies = [
  "rustc_span",
  "rustc_target",
  "rustc_trait_selection",
+ "smallvec",
  "tracing",
 ]
 

compiler/rustc_attr/Cargo.toml

@@ -17,4 +17,5 @@ rustc_macros = { path = "../rustc_macros" }
 rustc_serialize = { path = "../rustc_serialize" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
+smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
 # tidy-alphabetical-end

compiler/rustc_attr/messages.ftl

@@ -83,7 +83,10 @@ attr_multiple_item =
     multiple '{$item}' items
 
 attr_multiple_stability_levels =
-    multiple stability levels
+    multiple stability levels for feature `{$feature}`
+
+attr_multiple_unstable_reasons =
+    multiple reasons provided for unstability
 
 attr_non_ident_feature =
     'feature' is not an identifier
@@ -97,6 +100,9 @@ attr_rustc_const_stable_indirect_pairing =
 attr_rustc_promotable_pairing =
     `rustc_promotable` attribute must be paired with either a `rustc_const_unstable` or a `rustc_const_stable` attribute
 
+attr_soft_inconsistent =
+    `soft` must be present on either none or all of an item's `unstable` attributes
+
 attr_soft_no_args =
     `soft` should not have any arguments