A local declared without a value (long x, int[3] k, P p) reads whatever its stack slot held #3114

A local declared with a type and no value holds whatever its stack slot held before. Reading it is undefined behaviour in the generated C, and nothing reports it:

struct P {
    a: long
    w: long[4]
}

fn g() -> long {
    long x
    int[3] k
    P p
    return x + k[1] + p.a + p.w[2]
}

main() {
    println("${g()}")
}

This compiles with no diagnostic. The generated C is int64_t x; int k[3]; P p;, so g returns garbage. The same holds for an array a function means to fill in a loop when the loop misses an element (long[16] v in a block function).

Everywhere else the language gives a value a defined start:

• make([]T, n) returns zeroed elements;
• an array literal shorter than its array zeroes the rest;
• a function that falls off its end returns its type's zero value.

The reference never says what a declared local holds, so the program reads a stack slot.

Fix:

• A declaration without a value starts at its type's zero value: 0, false, "" (the null string), null, an all-zero array, and a struct with every field zero. Generated C: = 0 and = {0}. The C compiler drops the store when the program writes the value before reading it.
• The reference says so in the Variables section.
• Tests cover a scalar, an array, a struct with an array field, an actor handler's local and a closure's local.
• The cost is measured on code that declares large local arrays in hot functions (the cryptography modules after std and contrib call other modules' raw layer (97 calls in 49 files) instead of their public API #3107, the cross-language benchmarks).

Found on #3101 (#3107, moving crypto buffers onto local arrays).