rustc: Stabilize -C target-feature=+crt-static

This commit stabilizes the `crt-static` feature accepted by the compiler. Note
that this does not stabilize the `#[cfg]` attribute for `crt-static` as
that's going to be covered by #29717. This only stabilizes a few small pieces:

* The `crt-static` feature as accepted by the `-C target-feature` flag, and its
  connection with the platform-specific definition of `crt-static`.
* The semantics of `--print cfg` printing out activated `crt-static` feature, if
  available.

This should be enough to get the benefits of `crt-static` on stable Rust with
MSVC and with musl, but sidsteps the issue of stabilizing #29717 first.

Closes #37406

src/librustc_driver/lib.rs

@@ -635,11 +635,24 @@ impl RustcDefaultCalls {
                             node: ast::MetaItemKind::Word,
                             span: DUMMY_SP,
                         });
-                        if !allow_unstable_cfg && gated_cfg.is_some() {
-                            continue;
+
+                        // Note that crt-static is a specially recognized cfg
+                        // directive that's printed out here as part of
+                        // rust-lang/rust#37406, but in general the
+                        // `target_feature` cfg is gated under
+                        // rust-lang/rust#29717. For now this is just
+                        // specifically allowing the crt-static cfg and that's
+                        // it, this is intended to get into Cargo and then go
+                        // through to build scripts.
+                        let value = value.as_ref().map(|s| s.as_str());
+                        let value = value.as_ref().map(|s| s.as_ref());
+                        if name != "target_feature" || value != Some("crt-static") {
+                            if !allow_unstable_cfg && gated_cfg.is_some() {
+                                continue;
+                            }
                         }
 
-                        cfgs.push(if let &Some(ref value) = value {
+                        cfgs.push(if let Some(value) = value {
                             format!("{}=\"{}\"", name, value)
                         } else {
                             format!("{}", name)

src/librustc_driver/target_features.rs

@@ -12,7 +12,6 @@ use syntax::ast;
 use llvm::LLVMRustHasFeature;
 use rustc::session::Session;
 use rustc_trans::back::write::create_target_machine;
-use syntax::feature_gate::UnstableFeatures;
 use syntax::symbol::Symbol;
 use libc::c_char;
 
@@ -50,8 +49,6 @@ pub fn add_configuration(cfg: &mut ast::CrateConfig, sess: &Session) {
     }
 
     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");
 
@@ -65,14 +62,6 @@ pub fn add_configuration(cfg: &mut ast::CrateConfig, sess: &Session) {
         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.insert((tf, Some(Symbol::intern("crt-static"))));
     }