Fancy multipart forms API

[sagebind]

Add an easy-to-use API for creating a multipart/form-data request body and sending it. This API should allow for referencing one or more file streams as form parts. Allowing for files is tricky as described in #7.

There are a couple existing crates we could make use of:

• https://crates.io/crates/multipart
• https://crates.io/crates/multipart-async
• https://crates.io/crates/mime_multipart
• https://crates.io/crates/formdata

At first glance, none of these appear to support a fully async writing of forms though, except multipart-async which looks unstable, which is unfortunate.

[ayrat555]

will this issue be ever resolved? 😄

I wanted to use isahc for my telegram client. but this issue was a blocker :[
https://github.com/ayrat555/frankenstein/blob/master/examples/api_trait_implementation.rs#L113

[sagebind]

Technically you don't need this in order to send multipart forms since a multipart form is just bytes. You can craft the request body yourself or use one of the various multipart form libraries on Crates.io to generate such a request body.

One of the primary use-cases for multipart forms is to upload files, but since Isahc's core logic is async, using blocking file I/O is a non-starter. The help wanted label is totally accurate here, since this is a big feature that will require a lot of design and implementation time to do correctly. I'd like to add this at some point though.