compiler: support multiple stability attributes on items

This changes the text for E0544. Unfortunately, the example doesn't make
much sense anymore.

The way this merges stability levels together is made to work for
stability-checking and rustdoc; as far as I can tell, only
`rustc_passes::lib_features` needs them separate, and it extracts them
itself.

This includes the clarified/fixed const-unstable semantics that were
previously in a different commit.

Author: dianne

Cargo.lock

@@ -3334,6 +3334,7 @@ dependencies = [
  "rustc_serialize",
  "rustc_session",
  "rustc_span",
+ "smallvec",
 ]
 
 [[package]]
@@ -3506,6 +3507,7 @@ dependencies = [
  "rustc_target",
  "rustc_trait_selection",
  "rustc_type_ir",
+ "smallvec",
  "tracing",
 ]
 
@@ -4218,6 +4220,7 @@ 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 +83,7 @@ attr_multiple_item =
     multiple '{$item}' items
 
 attr_multiple_stability_levels =
-    multiple stability levels
+    multiple stability levels for feature `{$feature}`
 
 attr_non_ident_feature =
     'feature' is not an identifier