为什么要在WASM库中查看Rust函数需要`#[no_mangle]`

我创建了一个新的空Rust库.我将crate-type设置为cdylib，以便生成.wasm个文件.这是我的lib.rs:

#[no_mangle]
pub extern fn fibonacci(n: usize) -> usize {
  if n < 2 {
     return n;
  }

  return fibonacci(n - 1) + fibonacci(n - 2);
}

如果我删除#[no_mangle]，那么Cargo会生成一个WASM二进制文件，它甚至不包含任何代码:

$ wasm-objdump -h -x target/wasm32-unknown-unknown/release/fibonacci.wasm

fibonacci.wasm: file format wasm 0x1

Sections:

    Table start=0x0000000a end=0x0000000f (size=0x00000005) count: 1
   Memory start=0x00000011 end=0x00000014 (size=0x00000003) count: 1
   Global start=0x00000016 end=0x0000002f (size=0x00000019) count: 3
   Export start=0x00000031 end=0x00000056 (size=0x00000025) count: 3
   Custom start=0x00000059 end=0x00000f5a (size=0x00000f01) ".debug_abbrev"
   Custom start=0x00000f5e end=0x0005c6df (size=0x0005b781) ".debug_info"
   Custom start=0x0005c6e3 end=0x00084461 (size=0x00027d7e) ".debug_ranges"
   Custom start=0x00084465 end=0x00118f6a (size=0x00094b05) ".debug_str"
   Custom start=0x00118f6e end=0x00152a48 (size=0x00039ada) ".debug_line"
   Custom start=0x00152a4a end=0x00152a63 (size=0x00000019) "name"
   Custom start=0x00152a65 end=0x00152ab2 (size=0x0000004d) "producers"
   Custom start=0x00152ab4 end=0x00152ae0 (size=0x0000002c) "target_features"

Section Details:

Table[1]:
 - table[0] type=funcref initial=1 max=1
Memory[1]:
 - memory[0] pages: initial=16
Global[3]:
 - global[0] i32 mutable=1 <__stack_pointer> - init i32=1048576
 - global[1] i32 mutable=0 <__data_end> - init i32=1048576
 - global[2] i32 mutable=0 <__heap_base> - init i32=1048576
Export[3]:
 - memory[0] -> "memory"
 - global[1] -> "__data_end"
 - global[2] -> "__heap_base"
Custom:
 - name: ".debug_abbrev"
Custom:
 - name: ".debug_info"
Custom:
 - name: ".debug_ranges"
Custom:
 - name: ".debug_str"
Custom:
 - name: ".debug_line"
Custom:
 - name: "name"
 - global[0] <__stack_pointer>
Custom:
 - name: "producers"
Custom:
 - name: "target_features"
  - [+] mutable-globals
  - [+] sign-ext

有没有什么方法可以在不使用#[no_mangle]的情况下强制将函数包含在二进制文件中？使用extern本身是行不通的.