port tests/run-make/extern-fn-reachable to rmake

uses helper functions added in rust-lang#128147, must not be merged before that
PR.

src/tools/run-make-support/src/external_deps/rustc.rs

@@ -194,6 +194,11 @@ impl Rustc {
         self
     }
 
+    /// Make `rustc` prefere dynamic linking
+    pub fn prefer_dynamic(&mut self) -> &mut Self {
+        self.arg("-Cprefer-dynamic")
+    }
+
     /// Specify directory path used for profile generation
     pub fn profile_generate<P: AsRef<Path>>(&mut self, path: P) -> &mut Self {
         let mut arg = OsString::new();

src/tools/run-make-support/src/symbols.rs

@@ -42,3 +42,29 @@ pub fn any_symbol_contains(path: impl AsRef<Path>, substrings: &[&str]) -> bool
         false
     })
 }
+
+/// Check if an object file contains *all* of the given symbols.
+///
+/// The symbol names must match exactly.
+///
+/// Panics if `path` is not a valid object file readable by the current user.
+pub fn contains_exact_symbols(path: impl AsRef<Path>, symbol_names: &[&str]) -> bool {
+    let mut found = vec![false; symbol_names.len()];
+    with_symbol_iter(path, |syms| {
+        for sym in syms {
+            for (i, symbol_name) in symbol_names.iter().enumerate() {
+                found[i] |= sym.name_bytes().unwrap() == symbol_name.as_bytes();
+            }
+        }
+    });
+    let result = found.iter().all(|x| *x);
+    if !result {
+        eprintln!("does not contain symbol(s): ");
+        for i in 0..found.len() {
+            if !found[i] {
+                eprintln!("* {}", symbol_names[i]);
+            }
+        }
+    }
+    result
+}

src/tools/tidy/src/allowed_run_make_makefiles.txt

@@ -5,7 +5,6 @@ run-make/dep-info-doesnt-run-much/Makefile
 run-make/dep-info-spaces/Makefile
 run-make/dep-info/Makefile
 run-make/emit-to-stdout/Makefile
-run-make/extern-fn-reachable/Makefile
 run-make/incr-add-rust-src-component/Makefile
 run-make/issue-84395-lto-embed-bitcode/Makefile
 run-make/jobserver-error/Makefile

tests/run-make/extern-fn-reachable/rmake.rs

@@ -0,0 +1,7 @@
+//@ ignore-cross-compile
+use run_make_support::{rustc, symbols::contains_exact_symbols};
+
+fn main() {
+    rustc().input("dylib.rs").output("dylib.so").prefer_dynamic().run();
+    assert!(contains_exact_symbols("dylib.so", &["fun1", "fun2", "fun3", "fun4", "fun5"]));
+}