lint for shadowing live variables

[nikomatsakis]

At least as currently implemented, Dada distinguishes declaring a new variable (p = expr) from re-assigning the value of an existing variable (p := expr). This can easily be confusing. Consider this program:

fn main() {
    n = 0
    while n < 3 {
        n = n + 1
    }
}

This program will never terminate because the n = n + 1 declares a new variable.

We should either get rid of the idea of := or have a lint that detects when you shadow a live variable (variable may be accessed again later on). Therefore the above code would warn. This code would not:

async fn main() {
    n = 0
    print(n).await
    n = 1
    print(n).await
}

and neither would this

async fn main() {
    n = 0
    print(n).await
    if n > 0 {
        n = 5
        print(n).await
    }
    n := 1 # ignores current value of `n`
    print(n).await
}

but this would:

async fn main() {
    n = 0
    print(n).await
    if n > 0 {
        n = 5
        print(n).await
    }
    n += 1 # uses current value of `n`
    print(n).await
}

I would do this analysis on the validated tree.