message: README: Suggest using a previous nightly, revert other approach
diff: @@ -53,6 +53,16 @@ Install Miri via `rustup`:
  rustup component add miri
  ```
  +### error: component 'miri' is unavailable for download (nightly)
  +
  +The development of rustc's internals is quite fast paced. Downstream projects that rely on nightly internals, particularly clippy, can break fairly often because of this.
  +
  +When such breakages occur the nightly release will be missing Miri. This is a trade-off compared with the other option of just not publishing the night's release, but does avoid blocking the rust nightly releases for people that don't need clippy/Miri.
  +
  +To mitigate the issues we have:
  +* rustup will warn if the update is missing any components you currently have. This means you can no longer accidentally update to a no-Miri release. Once Miri is available again it'll update.
  +* However, if you need latest nightly Miri you can use <https://rust-lang.github.io/rustup-components-history/> to find and install a dated nightly release e.g. `rustup install nightly-2018-12-06`.
  +
  Now you can run your project in Miri:
  1. Run `cargo clean` to eliminate any cached dependencies. Miri needs your
  @@ -88,19 +98,9 @@ fn does_not_work_on_miri() {
  ### Common Problems
  -When using the above instructions, you may encounter a number of confusing
  +When using the above instructions, you may encounter a number of confusing compiler
  errors.
  -#### "component 'miri' for target '\<target\>' is unavailable for download for channel 'nightly'"
  -
  -The latest nightly may have broken Miri. If [this is the case][rust-toolstate], instead try to install Miri the old way with
  -
  -```sh
  -cargo +nightly install --force --git https://github.com/rust-lang/miri miri
  -```
  -
  -[rust-toolstate]:
  -
  #### "found possibly newer version of crate `std` which `<dependency>` depends on"
  Your build directory may contain artifacts from an earlier build that have/have
  