Add '| undefined' to optional types in generated .d.ts

[rhysd]

Hi,

#[wasm_bindgen]
struct X {
    pub foo: Option<i32>,
}

#[wasm_bindgen]
impl X {
    pub fn f(x: Option<i32>) -> Option<u32> {
        x.map(|i| i as u32)
    }
}

These option values generate TS type definition in .d.ts file as follows

export class X {
free(): void;
foo: number;

static  f(arg0: number): number | undefined;

}

However, by this type definition, foo and arg0 cannot accept undefined though the original values are Option when using strictNullChecks compiler flag of tsc. Passing undefined to the arg0 causes TypeScript compilation error. And the compiler thinks accessing to foo always returns non-undefined value so compiler cannot check the case where foo is undefined.

Expected type definition is:

export class X {
free(): void;
foo: number | undefined;

static  f(arg0: number | undefined): number | undefined;

}

number | undefined means a value which is number or undefined. It should make tsc happy.

This patch modifies TS type definition file generation to generate the types T | undefind for Option<T> values.

[alexcrichton]

Thanks!