Auto merge of #40113 - smaeul:native-musl, r=alexcrichton

Support dynamically-linked and/or native musl targets

These changes allow native compilation on musl-based distributions and the use of dynamic libraries on linux-musl targets. This is intended to remove limitations based on past assumptions about musl targets, while maintaining existing behavior by default.

A minor related bugfix is included.

Author: bors

config.toml.example

@@ -292,6 +292,12 @@
 # build native code.
 #android-ndk = "/path/to/ndk"
 
+# Force static or dynamic linkage of the standard library for this target. If
+# this target is a host for rustc, this will also affect the linkage of the
+# compiler itself. This is useful for building rustc on targets that normally
+# only use static libraries. If unset, the target's default linkage is used.
+#crt-static = false
+
 # The root location of the MUSL installation directory. The library directory
 # will also need to contain libunwind.a for an unwinding implementation. Note
 # that this option only makes sense for MUSL targets that produce statically

src/bootstrap/bin/rustc.rs

@@ -237,9 +237,13 @@ fn main() {
             }
         }
 
-        if target.contains("pc-windows-msvc") {
-            cmd.arg("-Z").arg("unstable-options");
-            cmd.arg("-C").arg("target-feature=+crt-static");
+        if let Ok(s) = env::var("RUSTC_CRT_STATIC") {
+            if s == "true" {
+                cmd.arg("-C").arg("target-feature=+crt-static");
+            }
+            if s == "false" {
+                cmd.arg("-C").arg("target-feature=-crt-static");
+            }
         }
 
         // Force all crates compiled by this compiler to (a) be unstable and (b)

src/bootstrap/builder.rs

@@ -503,6 +503,10 @@ impl<'a> Builder<'a> {
             cargo.env("RUSTC_METADATA_SUFFIX", "rustc");
         }
 
+        if let Some(x) = self.crt_static(target) {
+            cargo.env("RUSTC_CRT_STATIC", x.to_string());
+        }
+
         // Enable usage of unstable features
         cargo.env("RUSTC_BOOTSTRAP", "1");
         self.add_rust_test_threads(&mut cargo);

src/bootstrap/compile.rs

@@ -77,6 +77,14 @@ impl Step for Std {
                 target,
             });
             println!("Uplifting stage1 std ({} -> {})", from.host, target);
+
+            // Even if we're not building std this stage, the new sysroot must
+            // still contain the musl startup objects.
+            if target.contains("musl") && !target.contains("mips") {
+                let libdir = builder.sysroot_libdir(compiler, target);
+                copy_musl_third_party_objects(build, target, &libdir);
+            }
+
             builder.ensure(StdLink {
                 compiler: from,
                 target_compiler: compiler,
@@ -89,6 +97,11 @@ impl Step for Std {
         println!("Building stage{} std artifacts ({} -> {})", compiler.stage,
                 &compiler.host, target);
 
+        if target.contains("musl") && !target.contains("mips") {
+            let libdir = builder.sysroot_libdir(compiler, target);
+            copy_musl_third_party_objects(build, target, &libdir);
+        }
+
         let out_dir = build.cargo_out(compiler, Mode::Libstd, target);
         build.clear_if_dirty(&out_dir, &builder.rustc(compiler));
         let mut cargo = builder.cargo(compiler, Mode::Libstd, target, "build");
@@ -105,6 +118,20 @@ impl Step for Std {
     }
 }
 
+/// Copies the crt(1,i,n).o startup objects
+///
+/// Since musl supports fully static linking, we can cross link for it even
+/// with a glibc-targeting toolchain, given we have the appropriate startup
+/// files. As those shipped with glibc won't work, copy the ones provided by
+/// musl so we have them on linux-gnu hosts.
+fn copy_musl_third_party_objects(build: &Build,
+                                 target: Interned<String>,
+                                 into: &Path) {
+    for &obj in &["crt1.o", "crti.o", "crtn.o"] {
+        copy(&build.musl_root(target).unwrap().join("lib").join(obj), &into.join(obj));
+    }
+}
+
 /// Configure cargo to compile the standard library, adding appropriate env vars
 /// and such.
 pub fn std_cargo(build: &Build,
@@ -189,10 +216,6 @@ impl Step for StdLink {
         let libdir = builder.sysroot_libdir(target_compiler, target);
         add_to_sysroot(&libdir, &libstd_stamp(build, compiler, target));
 
-        if target.contains("musl") && !target.contains("mips") {
-            copy_musl_third_party_objects(build, target, &libdir);
-        }
-
         if build.config.sanitizers && compiler.stage != 0 && target == "x86_64-apple-darwin" {
             // The sanitizers are only built in stage1 or above, so the dylibs will
             // be missing in stage0 and causes panic. See the `std()` function above
@@ -208,15 +231,6 @@ impl Step for StdLink {
     }
 }
 
-/// Copies the crt(1,i,n).o startup objects
-///
-/// Only required for musl targets that statically link to libc
-fn copy_musl_third_party_objects(build: &Build, target: Interned<String>, into: &Path) {
-    for &obj in &["crt1.o", "crti.o", "crtn.o"] {
-        copy(&build.musl_root(target).unwrap().join("lib").join(obj), &into.join(obj));
-    }
-}
-
 fn copy_apple_sanitizer_dylibs(native_dir: &Path, platform: &str, into: &Path) {
     for &sanitizer in &["asan", "tsan"] {
         let filename = format!("libclang_rt.{}_{}_dynamic.dylib", sanitizer, platform);

src/bootstrap/config.rs

@@ -143,6 +143,7 @@ pub struct Target {
     pub cc: Option<PathBuf>,
     pub cxx: Option<PathBuf>,
     pub ndk: Option<PathBuf>,
+    pub crt_static: Option<bool>,
     pub musl_root: Option<PathBuf>,
     pub qemu_rootfs: Option<PathBuf>,
 }
@@ -275,6 +276,7 @@ struct TomlTarget {
     cc: Option<String>,
     cxx: Option<String>,
     android_ndk: Option<String>,
+    crt_static: Option<bool>,
     musl_root: Option<String>,
     qemu_rootfs: Option<String>,
 }
@@ -446,6 +448,7 @@ impl Config {
                 }
                 target.cxx = cfg.cxx.clone().map(PathBuf::from);
                 target.cc = cfg.cc.clone().map(PathBuf::from);
+                target.crt_static = cfg.crt_static.clone();
                 target.musl_root = cfg.musl_root.clone().map(PathBuf::from);
                 target.qemu_rootfs = cfg.qemu_rootfs.clone().map(PathBuf::from);
 

src/bootstrap/lib.rs

@@ -656,6 +656,16 @@ impl Build {
         base
     }
 
+    /// Returns if this target should statically link the C runtime, if specified
+    fn crt_static(&self, target: Interned<String>) -> Option<bool> {
+        if target.contains("pc-windows-msvc") {
+            Some(true)
+        } else {
+            self.config.target_config.get(&target)
+                .and_then(|t| t.crt_static)
+        }
+    }
+
     /// Returns the "musl root" for this `target`, if defined
     fn musl_root(&self, target: Interned<String>) -> Option<&Path> {
         self.config.target_config.get(&target)

src/bootstrap/sanity.rs

@@ -151,8 +151,15 @@ pub fn check(build: &mut Build) {
             panic!("the iOS target is only supported on macOS");
         }
 
-        // Make sure musl-root is valid if specified
+        // Make sure musl-root is valid
         if target.contains("musl") && !target.contains("mips") {
+            // If this is a native target (host is also musl) and no musl-root is given,
+            // fall back to the system toolchain in /usr before giving up
+            if build.musl_root(*target).is_none() && build.config.build == *target {
+                let target = build.config.target_config.entry(target.clone())
+                                 .or_insert(Default::default());
+                target.musl_root = Some("/usr".into());
+            }
             match build.musl_root(*target) {
                 Some(root) => {
                     if fs::metadata(root.join("lib/libc.a")).is_err() {

src/librustc/session/mod.rs

@@ -429,6 +429,31 @@ impl Session {
             .unwrap_or(self.opts.debug_assertions)
     }
 
+    pub fn crt_static(&self) -> bool {
+        // If the target does not opt in to crt-static support, use its default.
+        if self.target.target.options.crt_static_respected {
+            self.crt_static_feature()
+        } else {
+            self.target.target.options.crt_static_default
+        }
+    }
+
+    pub fn crt_static_feature(&self) -> bool {
+        let requested_features = self.opts.cg.target_feature.split(',');
+        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.
+        if self.target.target.options.crt_static_default {
+            !found_negative
+        } else {
+            found_positive
+        }
+    }
+
     pub fn must_not_eliminate_frame_pointers(&self) -> bool {
         self.opts.debuginfo != DebugInfoLevel::NoDebugInfo ||
         !self.target.target.options.eliminate_frame_pointer

src/librustc_back/target/linux_musl_base.rs

@@ -60,15 +60,10 @@ pub fn opts() -> TargetOptions {
     base.pre_link_objects_exe.push("crti.o".to_string());
     base.post_link_objects.push("crtn.o".to_string());
 
-    // MUSL support doesn't currently include dynamic linking, so there's no
-    // need for dylibs or rpath business. Additionally `-pie` is incompatible
-    // with `-static`, so we can't pass `-pie`.
-    base.dynamic_linking = false;
-    base.has_rpath = false;
-    base.position_independent_executables = false;
-
     // These targets statically link libc by default
     base.crt_static_default = true;
+    // These targets allow the user to choose between static and dynamic linking.
+    base.crt_static_respected = true;
 
     base
 }

src/librustc_back/target/mod.rs

@@ -416,8 +416,12 @@ 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).
+    pub crt_static_respected: bool,
 
     /// Whether or not stack probes (__rust_probestack) are enabled
     pub stack_probes: bool,
@@ -478,7 +482,9 @@ 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,
         }
     }
@@ -714,7 +720,9 @@ 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);
 
         if let Some(array) = obj.find("abi-blacklist").and_then(Json::as_array) {
@@ -902,7 +910,9 @@ 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);
 
         if default.abi_blacklist != self.options.abi_blacklist {