rustc: Implement #[link(cfg(..))] and crt-static

This commit is an implementation of [RFC 1721] which adds a new target feature
to the compiler, `crt-static`, which can be used to select how the C runtime for
a target is linked. Most targets dynamically linke the C runtime by default with
the notable exception of some of the musl targets.

[RFC 1721]: https://github.com/rust-lang/rfcs/blob/master/text/1721-crt-static.md

This commit first adds the new target-feature, `crt-static`. If enabled, then
the `cfg(target_feature = "crt-static")` will be available. Targets like musl
will have this enabled by default. This feature can be controlled through the
standard target-feature interface, `-C target-feature=+crt-static` or
`-C target-feature=-crt-static`.

Next this adds an gated and unstable `#[link(cfg(..))]` feature to enable the
`crt-static` semantics we want with libc. The exact behavior of this attribute
is a little squishy, but it's intended to be a forever-unstable
implementation detail of the liblibc crate.

Specifically the `#[link(cfg(..))]` annotation means that the `#[link]`
directive is only active in a compilation unit if that `cfg` value is satisfied.
For example when compiling an rlib, these directives are just encoded and
ignored for dylibs, and all staticlibs are continued to be put into the rlib as
usual. When placing that rlib into a staticlib, executable, or dylib, however,
the `cfg` is evaluated *as if it were defined in the final artifact* and the
library is decided to be linked or not.

Essentially, what'll happen is:

* On MSVC with `-C target-feature=-crt-static`, the `msvcrt.lib` library will be
  linked to.
* On MSVC with `-C target-feature=+crt-static`, the `libcmt.lib` library will be
  linked to.
* On musl with `-C target-feature=-crt-static`, the object files in liblibc.rlib
  are removed and `-lc` is passed instead.
* On musl with `-C target-feature=+crt-static`, the object files in liblibc.rlib
  are used and `-lc` is not passed.

This commit does **not** include an update to the liblibc module to implement
these changes. I plan to do that just after the 1.14.0 beta release is cut to
ensure we get ample time to test this feature.

cc #37406

src/librustc/middle/cstore.rs

@@ -89,6 +89,13 @@ pub enum NativeLibraryKind {
     NativeUnknown,   // default way to specify a dynamic library
 }
 
+#[derive(Clone, Hash, RustcEncodable, RustcDecodable)]
+pub struct NativeLibrary {
+    pub kind: NativeLibraryKind,
+    pub name: String,
+    pub cfg: Option<P<ast::MetaItem>>,
+}
+
 /// The data we save and restore about an inlined item or method.  This is not
 /// part of the AST that we parse from a file, but it becomes part of the tree
 /// that we trans.
@@ -204,7 +211,7 @@ pub trait CrateStore<'tcx> {
     fn crate_hash(&self, cnum: CrateNum) -> Svh;
     fn crate_disambiguator(&self, cnum: CrateNum) -> InternedString;
     fn plugin_registrar_fn(&self, cnum: CrateNum) -> Option<DefId>;
-    fn native_libraries(&self, cnum: CrateNum) -> Vec<(NativeLibraryKind, String)>;
+    fn native_libraries(&self, cnum: CrateNum) -> Vec<NativeLibrary>;
     fn reachable_ids(&self, cnum: CrateNum) -> Vec<DefId>;
     fn is_no_builtins(&self, cnum: CrateNum) -> bool;
 
@@ -231,7 +238,7 @@ pub trait CrateStore<'tcx> {
     // This is basically a 1-based range of ints, which is a little
     // silly - I may fix that.
     fn crates(&self) -> Vec<CrateNum>;
-    fn used_libraries(&self) -> Vec<(String, NativeLibraryKind)>;
+    fn used_libraries(&self) -> Vec<NativeLibrary>;
     fn used_link_args(&self) -> Vec<String>;
 
     // utility functions
@@ -377,7 +384,7 @@ impl<'tcx> CrateStore<'tcx> for DummyCrateStore {
                            -> InternedString { bug!("crate_disambiguator") }
     fn plugin_registrar_fn(&self, cnum: CrateNum) -> Option<DefId>
         { bug!("plugin_registrar_fn") }
-    fn native_libraries(&self, cnum: CrateNum) -> Vec<(NativeLibraryKind, String)>
+    fn native_libraries(&self, cnum: CrateNum) -> Vec<NativeLibrary>
         { bug!("native_libraries") }
     fn reachable_ids(&self, cnum: CrateNum) -> Vec<DefId> { bug!("reachable_ids") }
     fn is_no_builtins(&self, cnum: CrateNum) -> bool { bug!("is_no_builtins") }
@@ -412,7 +419,9 @@ impl<'tcx> CrateStore<'tcx> for DummyCrateStore {
     // This is basically a 1-based range of ints, which is a little
     // silly - I may fix that.
     fn crates(&self) -> Vec<CrateNum> { vec![] }
-    fn used_libraries(&self) -> Vec<(String, NativeLibraryKind)> { vec![] }
+    fn used_libraries(&self) -> Vec<NativeLibrary> {
+        vec![]
+    }
     fn used_link_args(&self) -> Vec<String> { vec![] }
 
     // utility functions

src/librustc_back/target/linux_musl_base.rs

@@ -16,7 +16,6 @@ pub fn opts() -> TargetOptions {
     // Make sure that the linker/gcc really don't pull in anything, including
     // default objects, libs, etc.
     base.pre_link_args.push("-nostdlib".to_string());
-    base.pre_link_args.push("-static".to_string());
 
     // At least when this was tested, the linker would not add the
     // `GNU_EH_FRAME` program header to executables generated, which is required
@@ -67,5 +66,8 @@ pub fn opts() -> TargetOptions {
     base.has_rpath = false;
     base.position_independent_executables = false;
 
+    // These targets statically link libc by default
+    base.crt_static_default = true;
+
     base
 }

src/librustc_back/target/mod.rs

@@ -373,6 +373,9 @@ pub struct TargetOptions {
     /// A blacklist of ABIs unsupported by the current target. Note that generic
     /// ABIs are considered to be supported on all platforms and cannot be blacklisted.
     pub abi_blacklist: Vec<Abi>,
+
+    /// Whether or not the CRT is statically linked by default.
+    pub crt_static_default: bool,
 }
 
 impl Default for TargetOptions {
@@ -425,6 +428,7 @@ impl Default for TargetOptions {
             max_atomic_width: None,
             panic_strategy: PanicStrategy::Unwind,
             abi_blacklist: vec![],
+            crt_static_default: false,
         }
     }
 }
@@ -585,6 +589,7 @@ impl Target {
         key!(no_integrated_as, bool);
         key!(max_atomic_width, Option<u64>);
         try!(key!(panic_strategy, PanicStrategy));
+        key!(crt_static_default, bool);
 
         if let Some(array) = obj.find("abi-blacklist").and_then(Json::as_array) {
             for name in array.iter().filter_map(|abi| abi.as_string()) {
@@ -745,6 +750,7 @@ impl ToJson for Target {
         target_option_val!(no_integrated_as);
         target_option_val!(max_atomic_width);
         target_option_val!(panic_strategy);
+        target_option_val!(crt_static_default);
 
         if default.abi_blacklist != self.options.abi_blacklist {
             d.insert("abi-blacklist".to_string(), self.options.abi_blacklist.iter()

src/librustc_driver/target_features.rs

@@ -12,6 +12,7 @@ use syntax::{ast, attr};
 use llvm::LLVMRustHasFeature;
 use rustc::session::Session;
 use rustc_trans::back::write::create_target_machine;
+use syntax::feature_gate::UnstableFeatures;
 use syntax::parse::token::InternedString;
 use syntax::parse::token::intern_and_get_ident as intern;
 use libc::c_char;
@@ -47,4 +48,32 @@ pub fn add_configuration(cfg: &mut ast::CrateConfig, sess: &Session) {
             cfg.push(attr::mk_name_value_item_str(tf.clone(), intern(&feat[..feat.len() - 1])))
         }
     }
+
+    let requested_features = sess.opts.cg.target_feature.split(',');
+    let unstable_options = sess.opts.debugging_opts.unstable_options;
+    let is_nightly = UnstableFeatures::from_environment().is_nightly_build();
+    let found_negative = requested_features.clone().any(|r| r == "-crt-static");
+    let found_positive = requested_features.clone().any(|r| r == "+crt-static");
+
+    // If the target we're compiling for requests a static crt by default,
+    // then see if the `-crt-static` feature was passed to disable that.
+    // Otherwise if we don't have a static crt by default then see if the
+    // `+crt-static` feature was passed.
+    let crt_static = if sess.target.target.options.crt_static_default {
+        !found_negative
+    } else {
+        found_positive
+    };
+
+    // If we switched from the default then that's only allowed on nightly, so
+    // gate that here.
+    if (found_positive || found_negative) && (!is_nightly || !unstable_options) {
+        sess.fatal("specifying the `crt-static` target feature is only allowed \
+                    on the nightly channel with `-Z unstable-options` passed \
+                    as well");
+    }
+
+    if crt_static {
+        cfg.push(attr::mk_name_value_item_str(tf.clone(), intern("crt-static")));
+    }
 }

src/librustc_metadata/creader.rs

@@ -23,6 +23,7 @@ use rustc::session::search_paths::PathKind;
 use rustc::middle;
 use rustc::middle::cstore::{CrateStore, validate_crate_name, ExternCrate};
 use rustc::util::nodemap::{FxHashMap, FxHashSet};
+use rustc::middle::cstore::NativeLibrary;
 use rustc::hir::map::Definitions;
 
 use std::cell::{RefCell, Cell};
@@ -35,6 +36,7 @@ use syntax::ast;
 use syntax::abi::Abi;
 use syntax::attr;
 use syntax::ext::base::SyntaxExtension;
+use syntax::feature_gate::{self, GateIssue};
 use syntax::parse::token::{InternedString, intern};
 use syntax_pos::{Span, DUMMY_SP};
 use log;
@@ -77,9 +79,8 @@ struct ExternCrateInfo {
 fn register_native_lib(sess: &Session,
                        cstore: &CStore,
                        span: Option<Span>,
-                       name: String,
-                       kind: cstore::NativeLibraryKind) {
-    if name.is_empty() {
+                       lib: NativeLibrary) {
+    if lib.name.is_empty() {
         match span {
             Some(span) => {
                 struct_span_err!(sess, span, E0454,
@@ -94,17 +95,21 @@ fn register_native_lib(sess: &Session,
         return
     }
     let is_osx = sess.target.target.options.is_like_osx;
-    if kind == cstore::NativeFramework && !is_osx {
+    if lib.kind == cstore::NativeFramework && !is_osx {
         let msg = "native frameworks are only available on OSX targets";
         match span {
-            Some(span) => {
-                span_err!(sess, span, E0455,
-                          "{}", msg)
-            }
+            Some(span) => span_err!(sess, span, E0455, "{}", msg),
             None => sess.err(msg),
         }
     }
-    cstore.add_used_library(name, kind);
+    if lib.cfg.is_some() && !sess.features.borrow().link_cfg {
+        feature_gate::emit_feature_err(&sess.parse_sess,
+                                       "link_cfg",
+                                       span.unwrap(),
+                                       GateIssue::Language,
+                                       "is feature gated");
+    }
+    cstore.add_used_library(lib);
 }
 
 // Extra info about a crate loaded for plugins or exported macros.
@@ -635,9 +640,9 @@ impl<'a> CrateLoader<'a> {
 
     fn register_statically_included_foreign_items(&mut self) {
         let libs = self.cstore.get_used_libraries();
-        for (lib, list) in self.foreign_item_map.iter() {
-            let is_static = libs.borrow().iter().any(|&(ref name, kind)| {
-                lib == name && kind == cstore::NativeStatic
+        for (foreign_lib, list) in self.foreign_item_map.iter() {
+            let is_static = libs.borrow().iter().any(|lib| {
+                *foreign_lib == lib.name && lib.kind == cstore::NativeStatic
             });
             if is_static {
                 for id in list {
@@ -898,7 +903,18 @@ impl<'a> CrateLoader<'a> {
                     InternedString::new("foo")
                 }
             };
-            register_native_lib(self.sess, self.cstore, Some(m.span), n.to_string(), kind);
+            let cfg = items.iter().find(|k| {
+                k.check_name("cfg")
+            }).and_then(|a| a.meta_item_list());
+            let cfg = cfg.map(|list| {
+                list[0].meta_item().unwrap().clone()
+            });
+            let lib = NativeLibrary {
+                name: n.to_string(),
+                kind: kind,
+                cfg: cfg,
+            };
+            register_native_lib(self.sess, self.cstore, Some(m.span), lib);
         }
 
         // Finally, process the #[linked_from = "..."] attribute
@@ -924,7 +940,12 @@ impl<'a> middle::cstore::CrateLoader for CrateLoader<'a> {
         }
 
         for &(ref name, kind) in &self.sess.opts.libs {
-            register_native_lib(self.sess, self.cstore, None, name.clone(), kind);
+            let lib = NativeLibrary {
+                name: name.clone(),
+                kind: kind,
+                cfg: None,
+            };
+            register_native_lib(self.sess, self.cstore, None, lib);
         }
         self.register_statically_included_foreign_items();
     }

src/librustc_metadata/cstore.rs

@@ -31,7 +31,7 @@ use syntax::{ast, attr};
 use syntax::ext::base::SyntaxExtension;
 use syntax_pos;
 
-pub use rustc::middle::cstore::{NativeLibraryKind, LinkagePreference};
+pub use rustc::middle::cstore::{NativeLibrary, LinkagePreference};
 pub use rustc::middle::cstore::{NativeStatic, NativeFramework, NativeUnknown};
 pub use rustc::middle::cstore::{CrateSource, LinkMeta};
 
@@ -97,7 +97,7 @@ pub struct CStore {
     metas: RefCell<FxHashMap<CrateNum, Rc<CrateMetadata>>>,
     /// Map from NodeId's of local extern crate statements to crate numbers
     extern_mod_crate_map: RefCell<NodeMap<CrateNum>>,
-    used_libraries: RefCell<Vec<(String, NativeLibraryKind)>>,
+    used_libraries: RefCell<Vec<NativeLibrary>>,
     used_link_args: RefCell<Vec<String>>,
     statically_included_foreign_items: RefCell<NodeSet>,
     pub inlined_item_cache: RefCell<DefIdMap<Option<CachedInlinedItem>>>,
@@ -212,12 +212,12 @@ impl CStore {
         libs
     }
 
-    pub fn add_used_library(&self, lib: String, kind: NativeLibraryKind) {
-        assert!(!lib.is_empty());
-        self.used_libraries.borrow_mut().push((lib, kind));
+    pub fn add_used_library(&self, lib: NativeLibrary) {
+        assert!(!lib.name.is_empty());
+        self.used_libraries.borrow_mut().push(lib);
     }
 
-    pub fn get_used_libraries<'a>(&'a self) -> &'a RefCell<Vec<(String, NativeLibraryKind)>> {
+    pub fn get_used_libraries(&self) -> &RefCell<Vec<NativeLibrary>> {
         &self.used_libraries
     }
 

src/librustc_metadata/cstore_impl.rs

@@ -14,7 +14,7 @@ use locator;
 use schema;
 
 use rustc::middle::cstore::{InlinedItem, CrateStore, CrateSource, DepKind, ExternCrate};
-use rustc::middle::cstore::{NativeLibraryKind, LinkMeta, LinkagePreference, LoadedMacro};
+use rustc::middle::cstore::{NativeLibrary, LinkMeta, LinkagePreference, LoadedMacro};
 use rustc::hir::def::{self, Def};
 use rustc::middle::lang_items;
 use rustc::session::Session;
@@ -295,7 +295,7 @@ impl<'tcx> CrateStore<'tcx> for cstore::CStore {
         })
     }
 
-    fn native_libraries(&self, cnum: CrateNum) -> Vec<(NativeLibraryKind, String)>
+    fn native_libraries(&self, cnum: CrateNum) -> Vec<NativeLibrary>
     {
         self.get_crate_data(cnum).get_native_libraries()
     }
@@ -524,7 +524,7 @@ impl<'tcx> CrateStore<'tcx> for cstore::CStore {
         result
     }
 
-    fn used_libraries(&self) -> Vec<(String, NativeLibraryKind)>
+    fn used_libraries(&self) -> Vec<NativeLibrary>
     {
         self.get_used_libraries().borrow().clone()
     }

src/librustc_metadata/decoder.rs

@@ -11,7 +11,7 @@
 // Decoding metadata from a single crate's metadata
 
 use astencode::decode_inlined_item;
-use cstore::{self, CrateMetadata, MetadataBlob, NativeLibraryKind};
+use cstore::{self, CrateMetadata, MetadataBlob, NativeLibrary};
 use index::Index;
 use schema::*;
 
@@ -980,7 +980,7 @@ impl<'a, 'tcx> CrateMetadata {
     }
 
 
-    pub fn get_native_libraries(&self) -> Vec<(NativeLibraryKind, String)> {
+    pub fn get_native_libraries(&self) -> Vec<NativeLibrary> {
         self.root.native_libraries.decode(self).collect()
     }
 

src/librustc_metadata/encoder.rs

@@ -13,7 +13,7 @@ use index::Index;
 use schema::*;
 
 use rustc::middle::cstore::{InlinedItemRef, LinkMeta};
-use rustc::middle::cstore::{LinkagePreference, NativeLibraryKind};
+use rustc::middle::cstore::{LinkagePreference, NativeLibrary};
 use rustc::hir::def;
 use rustc::hir::def_id::{CrateNum, CRATE_DEF_INDEX, DefIndex, DefId};
 use rustc::middle::dependency_format::Linkage;
@@ -1134,14 +1134,9 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
          self.lazy_seq_ref(&tcx.lang_items.missing))
     }
 
-    fn encode_native_libraries(&mut self) -> LazySeq<(NativeLibraryKind, String)> {
+    fn encode_native_libraries(&mut self) -> LazySeq<NativeLibrary> {
         let used_libraries = self.tcx.sess.cstore.used_libraries();
-        self.lazy_seq(used_libraries.into_iter().filter_map(|(lib, kind)| {
-            match kind {
-                cstore::NativeStatic => None, // these libraries are not propagated
-                cstore::NativeFramework | cstore::NativeUnknown => Some((kind, lib)),
-            }
-        }))
+        self.lazy_seq(used_libraries)
     }
 
     fn encode_codemap(&mut self) -> LazySeq<syntax_pos::FileMap> {

src/librustc_metadata/schema.rs

@@ -14,7 +14,7 @@ use index;
 use rustc::hir;
 use rustc::hir::def::{self, CtorKind};
 use rustc::hir::def_id::{DefIndex, DefId};
-use rustc::middle::cstore::{DepKind, LinkagePreference, NativeLibraryKind};
+use rustc::middle::cstore::{DepKind, LinkagePreference, NativeLibrary};
 use rustc::middle::lang_items;
 use rustc::mir;
 use rustc::ty::{self, Ty};
@@ -175,7 +175,7 @@ pub struct CrateRoot {
     pub dylib_dependency_formats: LazySeq<Option<LinkagePreference>>,
     pub lang_items: LazySeq<(DefIndex, usize)>,
     pub lang_items_missing: LazySeq<lang_items::LangItem>,
-    pub native_libraries: LazySeq<(NativeLibraryKind, String)>,
+    pub native_libraries: LazySeq<NativeLibrary>,
     pub codemap: LazySeq<syntax_pos::FileMap>,
     pub impls: LazySeq<TraitImpls>,
     pub reachable_ids: LazySeq<DefIndex>,