Add test for musl dynamically linking

tests/run-make/musl-default-linking/rmake.rs

@@ -0,0 +1,62 @@
+use run_make_support::{rustc, serde_json};
+
+// Please do NOT add more targets to this list!
+// Per https://github.com/rust-lang/compiler-team/issues/422,
+// we should be trying to move these targets to dynamically link
+// musl libc by default.
+static LEGACY_STATIC_LINKING_TARGETS: &[&'static str] = &[
+    "aarch64-unknown-linux-musl",
+    "arm-unknown-linux-musleabi",
+    "arm-unknown-linux-musleabihf",
+    "armv5te-unknown-linux-musleabi",
+    "armv7-unknown-linux-musleabi",
+    "armv7-unknown-linux-musleabihf",
+    "i586-unknown-linux-musl",
+    "i686-unknown-linux-musl",
+    "mips64-unknown-linux-musl",
+    "mips64-unknown-linux-muslabi64",
+    "mips64el-unknown-linux-muslabi64",
+    "powerpc-unknown-linux-musl",
+    "powerpc-unknown-linux-muslspe",
+    "powerpc64-unknown-linux-musl",
+    "powerpc64le-unknown-linux-musl",
+    "riscv32gc-unknown-linux-musl",
+    "s390x-unknown-linux-musl",
+    "thumbv7neon-unknown-linux-musleabihf",
+    "x86_64-unknown-linux-musl",
+];
+
+fn main() {
+    let targets = rustc().print("target-list").run().stdout_utf8();
+
+    for target in targets.lines() {
+        let abi = target.split('-').last().unwrap();
+
+        if !abi.starts_with("musl") {
+            continue;
+        }
+
+        let target_spec_json = rustc()
+            .print("target-spec-json")
+            .target(target)
+            .arg("-Zunstable-options")
+            .run()
+            .stdout_utf8();
+
+        let target_spec: serde_json::Value =
+            serde_json::from_str(&target_spec_json).expect("failed to parse target-spec-json");
+        let default = &target_spec["crt-static-default"];
+
+        // If the value is `null`, then the default to dynamically link from
+        // musl_base was not overriden.
+        if default.is_null() {
+            continue;
+        }
+
+        if default.as_bool().expect("wasn't a boolean")
+            && !LEGACY_STATIC_LINKING_TARGETS.contains(&target)
+        {
+            panic!("{target} statically links musl libc when it should dynamically link it");
+        }
+    }
+}