Introduce target feature crt_static_allows_dylibs

Most UNIX-like platforms do not allow shared libraries to statically
link their own libc, as libc expects to have consistent process-global
state. On those platforms, when we do not have a shared libc available,
we must not attempt to link dylibs or cdylibs. On Windows, however, it
is expected to statically link the CRT into dynamic libraries.

This feature is only relevant for targets that support both fully-static
and fully-dynamic linkage, such as musl on Linux.

Author: smaeul

src/librustc_back/target/mod.rs

@@ -416,6 +416,8 @@ pub struct TargetOptions {
     /// ABIs are considered to be supported on all platforms and cannot be blacklisted.
     pub abi_blacklist: Vec<Abi>,
 
+    /// Whether or not linking dylibs to a static CRT is allowed.
+    pub crt_static_allows_dylibs: bool,
     /// Whether or not the CRT is statically linked by default.
     pub crt_static_default: bool,
     /// Whether or not crt-static is respected by the compiler (or is a no-op).
@@ -480,6 +482,7 @@ impl Default for TargetOptions {
             max_atomic_width: None,
             panic_strategy: PanicStrategy::Unwind,
             abi_blacklist: vec![],
+            crt_static_allows_dylibs: false,
             crt_static_default: false,
             crt_static_respected: false,
             stack_probes: false,
@@ -717,6 +720,7 @@ impl Target {
         key!(max_atomic_width, Option<u64>);
         key!(min_atomic_width, Option<u64>);
         try!(key!(panic_strategy, PanicStrategy));
+        key!(crt_static_allows_dylibs, bool);
         key!(crt_static_default, bool);
         key!(crt_static_respected, bool);
         key!(stack_probes, bool);
@@ -906,6 +910,7 @@ impl ToJson for Target {
         target_option_val!(min_atomic_width);
         target_option_val!(max_atomic_width);
         target_option_val!(panic_strategy);
+        target_option_val!(crt_static_allows_dylibs);
         target_option_val!(crt_static_default);
         target_option_val!(crt_static_respected);
         target_option_val!(stack_probes);

src/librustc_back/target/windows_msvc_base.rs

@@ -63,6 +63,7 @@ pub fn opts() -> TargetOptions {
         is_like_windows: true,
         is_like_msvc: true,
         pre_link_args: args,
+        crt_static_allows_dylibs: true,
         crt_static_respected: true,
 
         .. Default::default()

src/librustc_trans_utils/link.rs

@@ -123,8 +123,11 @@ pub fn invalid_output_for_target(sess: &Session,
     match (sess.target.target.options.dynamic_linking,
            sess.target.target.options.executables, crate_type) {
         (false, _, config::CrateTypeCdylib) |
-        (false, _, config::CrateTypeProcMacro) |
-        (false, _, config::CrateTypeDylib) => true,
+        (false, _, config::CrateTypeDylib) |
+        (false, _, config::CrateTypeProcMacro) => true,
+        (true, _, config::CrateTypeCdylib) |
+        (true, _, config::CrateTypeDylib) => sess.crt_static() &&
+            !sess.target.target.options.crt_static_allows_dylibs,
         (_, false, config::CrateTypeExecutable) => true,
         _ => false
     }