rust-releases 0.2.0

Index and access all available Rust releases
Documentation 
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# rust-releases


## Introduction

The Rust programming language uses deterministic versioning for toolchain releases. Stable versions use SemVer, 
while nightly, beta and historical builds can be accessed by using dated builds (YY-MM-DD).

Unfortunately, a clean index of releases is not available any more. I decided to research which resources where still available
and found the following solutions:
    
  1) Use the AWS index (e.g. `aws --no-sign-request s3 ls static-rust-lang-org/dist/ > dist.txt`)
      * Rate-limited (only obtaining the index took ~40 seconds)
      * [source](https://github.com/rust-lang/rust/issues/56971#issuecomment-527199391)
  2) Build from individual [release manifests](https://static.rust-lang.org/manifests.txt)
      * Requires parsing many documents
      * Approx. one week delay after a new release
      * Also has more specific toolchain information
      * Rate-limited
      * [source](https://github.com/rust-lang/rust/issues/56971#issuecomment-532783994)
  3) Parse Rust in-repo [RELEASES.md](https://raw.githubusercontent.com/rust-lang/rust/master/RELEASES.md)
      * Fast
      * Stable channel only

Each of these options requires additional parsing, which is where this crate comes in: the `rust-releases` crate
can obtain, parse and build an index from the above resources.

For each solution, the `Strategy` trait has been implemented, which provides a `build_index` method. This method
returns a `ReleaseIndex`, which can be used to iterate over Rust releases. In addition, some implement a `FetchResources`
trait which, through the `fetch_channel` method, can obtain the resources necessary to build the index of releases.

## Implemented options

<table>
<thead>
  <tr>
    <th>strategy name</th>
    <th>trait</th>
    <th>implemented</th>
    <th>notes</th>
  </tr>
</thead>
<tbody>
  <tr>
    <td rowspan="2">DistIndex</td>
    <td>Strategy</td>
    <td>✅</td>
    <td></td>
  </tr>
  <tr>
    <td>FetchResources</td>
    <td>❌</td>
    <td>slow (~1 minute)</td>
  </tr>
  <tr>
    <td rowspan="2">FromManifests</td>
    <td>Strategy</td>
    <td>✅</td>
    <td></td>
  </tr>
  <tr>
    <td>FetchResources</td>
    <td>✅ </td>
    <td>very slow</td>
  </tr>
  <tr>
    <td rowspan="2">ReleasesMd</td>
    <td>Strategy</td>
    <td>✅</td>
    <td></td>
  </tr>
  <tr>
    <td>FetchResources</td>
    <td>✅</td>
    <td>stable channel only</td>
  </tr>
</tbody>
</table>

## Technical options (eventually)

* Bring your own download tool (planned, will be a cfg option in the future)
* Optionally, use built in download tool

## Applications

[cargo-msrv](https://github.com/foresterre/cargo-msrv) is a tool which can be used to determine the minimal supported Rust version (MSRV).
In cargo-msrv I started by parsing the latest channel manifest, and then decreasing the minor semver version.

This is not great for many reasons:
* Except for the latest released version, we are left guessing the decreased version numbers
  actually exist
* Only stable versions are supported, not nightly, beta, or other channels
* Only 1.x.0 versions are supported

This is not ideal, thus `rust-releases` was born. Now cargo-msrv can iterate over Rust releases of which we know they exist and are available.