Type check warning failed to locate the code who raising the warning

[erizmr]

Describe the bug
Type check warning failed to locate the code who raising the warning.

To Reproduce

import taichi as ti

ti.init(debug=True)

x = ti.field(dtype=ti.i32, shape=())
y = ti.field(dtype=ti.f32, shape=())
@ti.kernel
def bug_gen():
    x[None] += ti.cast(1.3, ti.f64)
    y[None] += ti.cast(2.5, ti.f64)
bug_gen()

Log/Screenshots

[strongoier]

Warnings are not something thrown out, so our current try-catch mechanism for error reporting cannot work. Maybe we still have to pass source code information inside expressions & statements... wdyt @lin-hitonami

[lin-hitonami]

Yes, but it seems that we are passing the traceback info to the Expr class in C++? https://github.com/taichi-dev/taichi/blob/master/python/taichi/lang/expr.py#L37

[strongoier]

Yes the missing info can be fixed. However, even if it is fixed, the result info will be the traceback mostly inside the ASTTransformer instead of anything locating source code.