I have little idea of the infrastructure (I had a look at middle/liveness.rs, which didn't help me much), but how difficult would it be to add an used_mutably_count to the analysis that counts the maximum number of times a variable could possibly be assigned or used in a context that requires mut (accounting for loops)?

e.g.

let mut a;
a = 1;
// used_mutably_count(a) == Known(1)

let mut b = 2;
b = 1;
// used_mutably_count(b) == Known(2)

let mut c = ~[];
for vec::each(args) |arg| {
    c.push(copy *args); 
}
// used_mutably_count(c) == Unknown

Then if used_mutably_count(var) == Known(1), rustc can warn that var should be immutable (or not, if var is already immutable).

It could be computed with rules like

umc(var, if cond { blk1 } else { blk2 }) = max(umc(var, blk1), umc(var, blk2))
umc(var, { stmt1; stmt2; ... }) = umc(var, stmt1) + umc(var, stmt2) + ...
umc(var, while expr { body }) = if umc(var, body) > Known(0) || umc(var, expr) > Known(0) { Unknown } else { Known(0) }
umc(var, let var = ...;) = Known(1)
umc(var, let var;) = Known(0)

where umc(var, ast) counts the maximum possibly assignments & mutable-uses of var in ast, the operations are such that the result is Unknown if any of the inputs are Unknown and Unknown > Known(k) for all k.

(I guess the count could even be compressed to UsedNever, UsedOnce and UsedMany.)

Uh oh!

Uh oh!

Warn when a local is "mut" but clearly not mutated #3083

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