When we were previously removing allocators, we were only able to work
with containers whose allocators appeared as their last template
parameter.
Now we can replace allocators in the middle of a parameter list.
This fixes tests for folly::sorted_vector_set.
std::basic_string takes three template parameters:
1. CharT
2. Traits
3. Allocator
The Traits parameter was causing issues, as it requires a type which
exposes certain things, e.g. `Traits::value_type`.
We have a few options to resolve this:
1. Remove this parameter, as we do for allocators
Cons: removing a template parameter doesn't work if other
parameters appear after it
2. Stub this parameter, as we do for hashers/comparators
Cons: we need to hardcode an implementation that satisfies the
`Traits::value_type` requirements
3. Leave the parameter as-is
Cons: will not work if a non-standard Traits is used
By using the real implementation of this Traits parameter
(normally `std::char_traits<CharT>`), we get one that we know will
work as long as it is defined in a stdlib header.
Option 3 is what we use in this patch. Instead of adding more
configuration options to the container TOML file format (e.g.
`params_to_keep = [1]`), we add `std::char_traits` as a dummy
container type. Now, whenever `std::char_traits` appears, it will be
left as-is, i.e. not removed, replaced or reverse-engineered.
This is the same approach previously used for Thrift's isset_bitset.
This change is fully backwards compatible with the old ContainerInfo
parser, as it is just adding new fields.
The old fields will continue to be used by legacy OICodeGen until that
is removed.
Also add a README to document the format.
