Add stability inheritance

This commit makes several changes to the stability index infrastructure:

* Stability levels are now inherited lexically, i.e., each item's
  stability level becomes the default for any nested items.

* The computed stability level for an item is stored as part of the
  metadata. When using an item from an external crate, this data is
  looked up and cached.

* The stability lint works from the computed stability level, rather
  than manual stability attribute annotations. However, the lint still
  checks only a limited set of item uses (e.g., it does not check every
  component of a path on import). This will be addressed in a later PR,
  as part of issue rust-lang#8962.

* The stability lint only applies to items originating from external
  crates, since the stability index is intended as a promise to
  downstream crates.

* The "experimental" lint is now _allow_ by default. This is because
  almost all existing crates have been marked "experimental", pending
  library stabilization. With inheritance in place, this would generate
  a massive explosion of warnings for every Rust program.

  The lint should be changed back to deny-by-default after library
  stabilization is complete.

* The "deprecated" lint still warns by default.

The net result: we can begin tracking stability index for the standard
libraries as we stabilize, without impacting most clients.

Closes rust-lang#13540.

src/doc/rust.md

@@ -2300,28 +2300,43 @@ One can indicate the stability of an API using the following attributes:
 These levels are directly inspired by
 [Node.js' "stability index"](http://nodejs.org/api/documentation.html).
 
-There are lints for disallowing items marked with certain levels:
-`deprecated`, `experimental` and `unstable`; the first two will warn
-by default. Items with not marked with a stability are considered to
-be unstable for the purposes of the lint. One can give an optional
+Stability levels are inherited, so an items's stability attribute is the
+default stability for everything nested underneath it.
+
+There are lints for disallowing items marked with certain levels: `deprecated`,
+`experimental` and `unstable`. For now, only `deprecated` warns by default, but
+this will change once the standard library has been stabilized.
+Stability levels are meant to be promises at the crate
+ level, so these lints only apply when referencing
+items from an _external_ crate, not to items defined within the
+current crate. Items with no stability level are considered
+to be unstable for the purposes of the lint. One can give an optional
 string that will be displayed when the lint flags the use of an item.
 
-~~~~ {.ignore}
-#![warn(unstable)]
+For example, if we define one crate called `stability_levels`:
 
+~~~~ {.ignore}
 #[deprecated="replaced by `best`"]
-fn bad() {
+pub fn bad() {
     // delete everything
 }
 
-fn better() {
+pub fn better() {
     // delete fewer things
 }
 
 #[stable]
-fn best() {
+pub fn best() {
     // delete nothing
 }
+~~~~
+
+then the lints will work as follows for a client crate:
+
+~~~~ {.ignore}
+#![warn(unstable)]
+extern crate stability_levels;
+use stability_levels::{bad, better, best};
 
 fn main() {
     bad(); // "warning: use of deprecated item: replaced by `best`"

src/librustc/driver/driver.rs

@@ -20,7 +20,7 @@ use metadata::common::LinkMeta;
 use metadata::creader;
 use middle::cfg;
 use middle::cfg::graphviz::LabelledCFG;
-use middle::{trans, freevars, kind, ty, typeck, lint, reachable};
+use middle::{trans, freevars, stability, kind, ty, typeck, lint, reachable};
 use middle::dependency_format;
 use middle;
 use plugin::load::Plugins;
@@ -312,8 +312,11 @@ pub fn phase_3_run_analysis_passes(sess: Session,
     time(time_passes, "loop checking", (), |_|
          middle::check_loop::check_crate(&sess, krate));
 
+    let stability_index = time(time_passes, "stability index", (), |_|
+                               stability::Index::build(krate));
+
     let ty_cx = ty::mk_ctxt(sess, def_map, named_region_map, ast_map,
-                            freevars, region_map, lang_items);
+                            freevars, region_map, lang_items, stability_index);
 
     // passes are timed inside typeck
     typeck::check_crate(&ty_cx, trait_map, krate);

src/librustc/lib.rs

@@ -80,6 +80,7 @@ pub mod middle {
     pub mod weak_lang_items;
     pub mod save;
     pub mod intrinsicck;
+    pub mod stability;
 }
 
 pub mod front {

src/librustc/metadata/common.rs

@@ -210,6 +210,9 @@ pub static tag_method_argument_name: uint = 0x8f;
 pub static tag_reachable_extern_fns: uint = 0x90;
 pub static tag_reachable_extern_fn_id: uint = 0x91;
 
+pub static tag_items_data_item_stability: uint = 0x92;
+
+
 #[deriving(Clone, Show)]
 pub struct LinkMeta {
     pub crateid: CrateId,

src/librustc/metadata/csearch.rs

@@ -25,6 +25,7 @@ use serialize::ebml::reader;
 use std::rc::Rc;
 use syntax::ast;
 use syntax::ast_map;
+use syntax::attr;
 use syntax::diagnostic::expect;
 use syntax::parse::token;
 
@@ -328,3 +329,10 @@ pub fn is_typedef(cstore: &cstore::CStore, did: ast::DefId) -> bool {
     let cdata = cstore.get_crate_data(did.krate);
     decoder::is_typedef(&*cdata, did.node)
 }
+
+pub fn get_stability(cstore: &cstore::CStore,
+                     def: ast::DefId)
+                     -> Option<attr::Stability> {
+    let cdata = cstore.get_crate_data(def.krate);
+    decoder::get_stability(&*cdata, def.node)
+}

src/librustc/metadata/decoder.rs

@@ -439,6 +439,14 @@ pub fn get_type(cdata: Cmd, id: ast::NodeId, tcx: &ty::ctxt)
     }
 }
 
+pub fn get_stability(cdata: Cmd, id: ast::NodeId) -> Option<attr::Stability> {
+    let item = lookup_item(id, cdata.data());
+    reader::maybe_get_doc(item, tag_items_data_item_stability).map(|doc| {
+        let mut decoder = reader::Decoder::new(doc);
+        Decodable::decode(&mut decoder).unwrap()
+    })
+}
+
 pub fn get_impl_trait(cdata: Cmd,
                       id: ast::NodeId,
                       tcx: &ty::ctxt) -> Option<Rc<ty::TraitRef>>

src/librustc/metadata/encoder.rs

@@ -327,6 +327,10 @@ fn encode_enum_variant_info(ecx: &EncodeContext,
         encode_parent_item(ebml_w, local_def(id));
         encode_visibility(ebml_w, variant.node.vis);
         encode_attributes(ebml_w, variant.node.attrs.as_slice());
+
+        let stab = ecx.tcx.stability.borrow().lookup_local(variant.node.id);
+        encode_stability(ebml_w, stab);
+
         match variant.node.kind {
             ast::TupleVariantKind(ref args)
                     if args.len() > 0 && generics.ty_params.len() == 0 => {
@@ -588,6 +592,7 @@ fn encode_info_for_mod(ecx: &EncodeContext,
 
     encode_path(ebml_w, path.clone());
     encode_visibility(ebml_w, vis);
+    encode_stability(ebml_w, ecx.tcx.stability.borrow().lookup_local(id));
 
     // Encode the reexports of this module, if this module is public.
     if vis == Public {
@@ -717,6 +722,8 @@ fn encode_info_for_struct_ctor(ecx: &EncodeContext,
         encode_symbol(ecx, ebml_w, ctor_id);
     }
 
+    encode_stability(ebml_w, ecx.tcx.stability.borrow().lookup_local(ctor_id));
+
     // indicate that this is a tuple struct ctor, because downstream users will normally want
     // the tuple struct definition, but without this there is no way for them to tell that
     // they actually have a ctor rather than a normal function
@@ -761,6 +768,9 @@ fn encode_info_for_method(ecx: &EncodeContext,
     encode_method_ty_fields(ecx, ebml_w, m);
     encode_parent_item(ebml_w, local_def(parent_id));
 
+    let stab = ecx.tcx.stability.borrow().lookup_local(m.def_id.node);
+    encode_stability(ebml_w, stab);
+
     // The type for methods gets encoded twice, which is unfortunate.
     let tpt = lookup_item_type(ecx.tcx, m.def_id);
     encode_bounds_and_type(ebml_w, ecx, &tpt);
@@ -880,6 +890,14 @@ fn encode_sized(ebml_w: &mut Encoder, sized: Sized) {
     ebml_w.end_tag();
 }
 
+fn encode_stability(ebml_w: &mut Encoder, stab_opt: Option<attr::Stability>) {
+    stab_opt.map(|stab| {
+        ebml_w.start_tag(tag_items_data_item_stability);
+        stab.encode(ebml_w).unwrap();
+        ebml_w.end_tag();
+    });
+}
+
 fn encode_info_for_item(ecx: &EncodeContext,
                         ebml_w: &mut Encoder,
                         item: &Item,
@@ -900,6 +918,8 @@ fn encode_info_for_item(ecx: &EncodeContext,
            ecx.tcx.sess.codemap().span_to_str(item.span));
 
     let def_id = local_def(item.id);
+    let stab = tcx.stability.borrow().lookup_local(item.id);
+
     match item.node {
       ItemStatic(_, m, _) => {
         add_to_index(item, ebml_w, index);
@@ -921,6 +941,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
             encode_inlined_item(ecx, ebml_w, IIItemRef(item));
         }
         encode_visibility(ebml_w, vis);
+        encode_stability(ebml_w, stab);
         ebml_w.end_tag();
       }
       ItemFn(ref decl, fn_style, _, ref generics, _) => {
@@ -939,6 +960,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
             encode_symbol(ecx, ebml_w, item.id);
         }
         encode_visibility(ebml_w, vis);
+        encode_stability(ebml_w, stab);
         encode_method_argument_names(ebml_w, &**decl);
         ebml_w.end_tag();
       }
@@ -968,6 +990,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
             ebml_w.end_tag();
         }
         encode_visibility(ebml_w, vis);
+        encode_stability(ebml_w, stab);
         ebml_w.end_tag();
       }
       ItemTy(..) => {
@@ -979,6 +1002,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
         encode_name(ebml_w, item.ident.name);
         encode_path(ebml_w, path);
         encode_visibility(ebml_w, vis);
+        encode_stability(ebml_w, stab);
         ebml_w.end_tag();
       }
       ItemEnum(ref enum_definition, ref generics) => {
@@ -1001,6 +1025,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
         encode_inherent_implementations(ecx, ebml_w, def_id);
 
         encode_visibility(ebml_w, vis);
+        encode_stability(ebml_w, stab);
         ebml_w.end_tag();
 
         encode_enum_variant_info(ecx,
@@ -1035,6 +1060,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
         encode_name(ebml_w, item.ident.name);
         encode_attributes(ebml_w, item.attrs.as_slice());
         encode_path(ebml_w, path.clone());
+        encode_stability(ebml_w, stab);
         encode_visibility(ebml_w, vis);
 
         /* Encode def_ids for each field and method
@@ -1095,6 +1121,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
             encode_impl_vtables(ebml_w, ecx, &impl_vtables);
         }
         encode_path(ebml_w, path.clone());
+        encode_stability(ebml_w, stab);
         ebml_w.end_tag();
 
         // Iterate down the methods, emitting them. We rely on the
@@ -1138,6 +1165,7 @@ fn encode_info_for_item(ecx: &EncodeContext,
         // should no longer need this ugly little hack either.
         encode_sized(ebml_w, sized);
         encode_visibility(ebml_w, vis);
+        encode_stability(ebml_w, stab);
         for &method_def_id in ty::trait_method_def_ids(tcx, def_id).iter() {
             ebml_w.start_tag(tag_item_trait_method);
             encode_def_id(ebml_w, method_def_id);
@@ -1176,9 +1204,11 @@ fn encode_info_for_item(ecx: &EncodeContext,
             ebml_w.start_tag(tag_items_data_item);
 
             encode_method_ty_fields(ecx, ebml_w, &*method_ty);
-
             encode_parent_item(ebml_w, def_id);
 
+            let stab = tcx.stability.borrow().lookup_local(method_def_id.node);
+            encode_stability(ebml_w, stab);
+
             let elem = ast_map::PathName(method_ty.ident.name);
             encode_path(ebml_w, path.clone().chain(Some(elem).move_iter()));
 

src/librustc/middle/lint.rs

@@ -372,7 +372,8 @@ static lint_table: &'static [(&'static str, LintSpec)] = &[
      LintSpec {
         lint: Experimental,
         desc: "detects use of #[experimental] items",
-        default: Warn
+        // FIXME #6875: Change to Warn after std library stabilization is complete
+        default: Allow
     }),
 
     ("unstable",
@@ -1661,6 +1662,8 @@ fn check_missing_doc_variant(cx: &Context, v: &ast::Variant) {
 /// Checks for use of items with #[deprecated], #[experimental] and
 /// #[unstable] (or none of them) attributes.
 fn check_stability(cx: &Context, e: &ast::Expr) {
+    let tcx = cx.tcx;
+
     let id = match e.node {
         ast::ExprPath(..) | ast::ExprStruct(..) => {
             match cx.tcx.def_map.borrow().find(&e.id) {
@@ -1670,16 +1673,16 @@ fn check_stability(cx: &Context, e: &ast::Expr) {
         }
         ast::ExprMethodCall(..) => {
             let method_call = typeck::MethodCall::expr(e.id);
-            match cx.tcx.method_map.borrow().find(&method_call) {
+            match tcx.method_map.borrow().find(&method_call) {
                 Some(method) => {
                     match method.origin {
                         typeck::MethodStatic(def_id) => {
                             // If this implements a trait method, get def_id
                             // of the method inside trait definition.
                             // Otherwise, use the current def_id (which refers
                             // to the method inside impl).
-                            ty::trait_method_of_method(
-                                cx.tcx, def_id).unwrap_or(def_id)
+                            ty::trait_method_of_method(cx.tcx, def_id)
+                               .unwrap_or(def_id)
                         }
                         typeck::MethodParam(typeck::MethodParam {
                             trait_id: trait_id,
@@ -1699,32 +1702,11 @@ fn check_stability(cx: &Context, e: &ast::Expr) {
         _ => return
     };
 
-    let stability = if ast_util::is_local(id) {
-        // this crate
-        let s = cx.tcx.map.with_attrs(id.node, |attrs| {
-            attrs.map(|a| attr::find_stability(a.as_slice()))
-        });
-        match s {
-            Some(s) => s,
+    // stability attributes are promises made across crates; do not
+    // check anything for crate-local usage.
+    if ast_util::is_local(id) { return }
 
-            // no possibility of having attributes
-            // (e.g. it's a local variable), so just
-            // ignore it.
-            None => return
-        }
-    } else {
-        // cross-crate
-
-        let mut s = None;
-        // run through all the attributes and take the first
-        // stability one.
-        csearch::get_item_attrs(&cx.tcx.sess.cstore, id, |attrs| {
-            if s.is_none() {
-                s = attr::find_stability(attrs.as_slice())
-            }
-        });
-        s
-    };
+    let stability = tcx.stability.borrow_mut().lookup(&tcx.sess.cstore, id);
 
     let (lint, label) = match stability {
         // no stability attributes == Unstable