Switch

const std = @import("std");
const mem = std.mem;

pub fn toRna(allocator: mem.Allocator, dna: []const u8) mem.Allocator.Error![]const u8 {
    var rna_slice = try allocator.alloc(u8, dna.len);
    for (dna, 0..) |dna_nucleotide, i| {
        switch (dna_nucleotide) {
            'A' => rna_slice[i] = 'U',
            'C' => rna_slice[i] = 'G',
            'G' => rna_slice[i] = 'C',
            'T' => rna_slice[i] = 'A',
            else => unreachable,
        }
    }
    return rna_slice;
}

A switch maps each of the four DNA bases to its RNA complement.

The else => unreachable prong states that no other byte can occur. unreachable is an assertion: in safe build modes reaching it panics, and in ReleaseFast/ReleaseSmall it is undefined behavior the optimizer may assume never happens. The exercise's inputs only ever contain A, C, G, and T, so the prong is never taken.