Linear builders

A builder appends in place into one growing buffer. To keep that invisible, each builder value is used exactly once: every operation consumes its builder and push returns the builder to use next.

Operation
StrBuf()	an empty string builder
ListBuf()	an empty ListBuf(T); T comes from the expected type
b.push(s)	StrBuf: append a Str
b.push_char(c)	StrBuf: append the code point c
b.push(x)	ListBuf(T): append an element
b.finish()	the finished Str or List(T)

push is amortized O(1) and finish hands over the buffer without copying.

StrBuf digits(Int i, Int n, StrBuf acc) {
    if (i >= n) { return acc; }
    return digits(i + 1, n, acc.push(f"{i}"));
}

Int main() {
    ListBuf(Str) l0 = ListBuf();
    List(Str) words = l0.push("a").push("b").finish();
    println(f"{digits(0, 10, StrBuf()).finish()} {words}");
    return 0;
}

Output

0123456789 ["a", "b"]

Rules

Code	Violation
E0401	a builder is not consumed on some path (including a return that drops it)
E0402	a builder is used twice, or after it was consumed
E0403	a builder bound outside a loop is used inside it, or captured by a lambda
E0404	a builder type appears inside another type (a field, a list element, an option, a closure type)

Branches must consume the same builders, except a branch that leaves by return, break or continue. The right operand of && / || must not consume a builder bound outside it. Passing a builder along (return b;, or as an argument) counts as its one use.