compiler: support multiple unstable attributes on items

Author: dianne

Cargo.lock

@@ -3322,6 +3322,7 @@ dependencies = [
  "rustc_serialize",
  "rustc_session",
  "rustc_span",
+ "smallvec",
 ]
 
 [[package]]
@@ -4204,6 +4205,7 @@ 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