[TVM Discuss] [Questions] How to test wasm32 target on Rust runtime binding

[Leon Wang via TVM Discuss]

Hi @tqchen @jroesch and others, I was working for embedding TVM Rust runtime 
for WASM target (see 
[here](https://github.com/leonwanghui/ms-backend-wasm/tree/master/wasm-backend-tvm)
 for the details), but came across some problems when building test-wasm32 
crate.

## Error elaboration
Firstly I need to clarify that I can build and run the 
[test-wasm32](https://github.com/apache/incubator-tvm/tree/master/rust/runtime/tests/test_wasm32)
 package successfully, but what I wanted is to build the `lib` rather than 
`bin` type of package, so I changed several lines in original `main.rs` and 
copy it to `lib.rs`:
```rust
extern "C" {
    static __tvm_module_ctx: i32;
}

#[no_mangle]
unsafe fn __get_tvm_module_ctx() -> i32 {
    // Refer a symbol in the libtest_wasm32.a to make sure that the link of the
    // library is not optimized out.
    __tvm_module_ctx
}

extern crate ndarray;
#[macro_use]
extern crate tvm_runtime;

use ndarray::Array;
use tvm_runtime::{DLTensor, Module as _, SystemLibModule};

#[no_mangle]
pub extern "C" fn run() {
    // try static
    let mut a = Array::from_vec(vec![1f32, 2., 3., 4.]);
    let mut b = Array::from_vec(vec![1f32, 0., 1., 0.]);
    let mut c = Array::from_vec(vec![0f32; 4]);
    let e = Array::from_vec(vec![2f32, 2., 4., 4.]);
    let mut a_dl: DLTensor = (&mut a).into();
    let mut b_dl: DLTensor = (&mut b).into();
    let mut c_dl: DLTensor = (&mut c).into();

    let syslib = SystemLibModule::default();
    let add = syslib
        .get_function("default_function")
        .expect("main function not found");
    call_packed!(add, &mut a_dl, &mut b_dl, &mut c_dl).unwrap();
    println!("{:?}", c_dl.data);
    assert!(c.all_close(&e, 1e-8f32));
}
```
But when I tried to compile the library, some errors occurred:
```
undefined symbol: __tvm_module_ctx
```

I was guessing something required to modified when changing the type from `bin` 
to `lib`, but I have no idea what to do about it because of missing the 
context, would be appreciated it if anyone could take a look.





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