Found cause - rust-lang/rust#47384?

README.md

@@ -0,0 +1,31 @@
+Functions from the library crate marked `#[no_mangle]` seem to be removed from executables if no references are made to the library,
+even when a linker arg like `-rdynamic` is used.
+This behaviour seems to be described in this bug report: https://github.com/rust-lang/rust/issues/47384.
+However, that bug report is supposedly fixed by https://github.com/rust-lang/rust/pull/95604.
+
+```
+# cat src/bin/without_extern_crate.rs
+fn main() {
+	println!("Hello, world!");
+}
+```
+
+```
+# RUSTFLAGS='-C link-arg=-rdynamic' cargo build && nm -g target/debug/without_extern_crate.rs | grep foo
+# nothing
+```
+
+```
+# cat src/bin/with_extern_crate.rs
+extern crate export_symbols;
+
+fn main() {
+	println!("Hello, world!");
+}
+```
+
+```
+# RUSTFLAGS='-C link-arg=-rdynamic' cargo build && nm -g target/debug/with_extern_crate.rs | grep foo
+000000000006dfa0 T foo1
+000000000006dfb0 T foo2
+```

src/bin/with_extern_crate.rs

@@ -0,0 +1,5 @@
+extern crate export_symbols;
+
+fn main() {
+	println!("Hello, world!");
+}

src/bin/without_extern_crate.rs

@@ -0,0 +1,3 @@
+fn main() {
+	println!("Hello, world!");
+}