Auto merge of #6401 - ebroto:pin_to_a_nightly, r=<try>

📌 Pin Clippy to a nightly 📌

changelog: none

Addresses partially #5561. As proposed there in [this comment](#5561 (comment)), this kicks off the process, to help us get acquainted with how the syncs should work, before working on improving the tooling.

Open questions:
* When performing a rustup, we will need to exclude the commits that were merged that same day, or else wait until that nightly is released. I did not update the documentation about this part, mainly because I'm not sure about how to do that.
* When should we perform the rustups now? My first idea is to do it at the same time we do the clippyups, to have a clear cadence and to avoid the two copies of the repo to diverge enough to make the process painful.
* Who does the rustups now? If we follow my previous idea and do both rustup and clippyup at the same time, it would be more work for `@flip1995` who currently does the clippyups. I would prefer to establish some kind of rotation to spead the work. Other ideas?
* I'm not sure if this affects the release process in any way.
* ???

`@rust-lang/clippy` thoughts?

r? `@flip1995`

.github/workflows/clippy.yml

@@ -52,13 +52,10 @@ jobs:
       uses: actions/cache@v2
       with:
         path: ~/.cargo
-        key: ${{ runner.os }}-x86_64-unknown-linux-gnu-${{ hashFiles('Cargo.lock') }}
+        key: ${{ runner.os }}-x86_64-unknown-linux-gnu-v2-${{ hashFiles('Cargo.lock') }}
         restore-keys: |
           ${{ runner.os }}-x86_64-unknown-linux-gnu
 
-    - name: Master Toolchain Setup
-      run: bash setup-toolchain.sh
-
     # Run
     - name: Set LD_LIBRARY_PATH (Linux)
       run: |

.github/workflows/clippy_bors.yml

@@ -101,15 +101,10 @@ jobs:
       uses: actions/cache@v2
       with:
         path: ~/.cargo
-        key: ${{ runner.os }}-${{ matrix.host }}-${{ hashFiles('Cargo.lock') }}
+        key: ${{ runner.os }}-${{ matrix.host }}-v2-${{ hashFiles('Cargo.lock') }}
         restore-keys: |
           ${{ runner.os }}-${{ matrix.host }}
 
-    - name: Master Toolchain Setup
-      run: bash setup-toolchain.sh
-      env:
-        HOST_TOOLCHAIN: ${{ matrix.host }}
-
     # Run
     - name: Set LD_LIBRARY_PATH (Linux)
       if: runner.os == 'Linux'
@@ -188,13 +183,10 @@ jobs:
       uses: actions/cache@v2
       with:
         path: ~/.cargo
-        key: ${{ runner.os }}-x86_64-unknown-linux-gnu-${{ hashFiles('Cargo.lock') }}
+        key: ${{ runner.os }}-x86_64-unknown-linux-gnu-v2-${{ hashFiles('Cargo.lock') }}
         restore-keys: |
           ${{ runner.os }}-x86_64-unknown-linux-gnu
 
-    - name: Master Toolchain Setup
-      run: bash setup-toolchain.sh
-
     # Run
     - name: Build Integration Test
       run: cargo test --test integration --features integration --no-run
@@ -269,13 +261,10 @@ jobs:
       uses: actions/cache@v2
       with:
         path: ~/.cargo
-        key: ${{ runner.os }}-x86_64-unknown-linux-gnu-${{ hashFiles('Cargo.lock') }}
+        key: ${{ runner.os }}-x86_64-unknown-linux-gnu-v2-${{ hashFiles('Cargo.lock') }}
         restore-keys: |
           ${{ runner.os }}-x86_64-unknown-linux-gnu
 
-    - name: Master Toolchain Setup
-      run: bash setup-toolchain.sh
-
     # Download
     - name: Download target dir
       uses: actions/download-artifact@v1

CONTRIBUTING.md

@@ -182,18 +182,26 @@ That's why the `else_if_without_else` example uses the `register_early_pass` fun
 [early_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html
 [late_lint_pass]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.LateLintPass.html
 
-## Fixing build failures caused by Rust
+## Syncing changes between Clippy and [`rust-lang/rust`]
 
-Clippy currently gets built with `rustc` of the `rust-lang/rust` `master`
-branch. Most of the times we have to adapt to the changes and only very rarely
-there's an actual bug in Rust.
+Clippy currently gets built with a pinned nightly version.
 
-If you decide to make Clippy work again with a Rust commit that breaks it, you
-have to sync the `rust-lang/rust-clippy` repository with the `subtree` copy of
-Clippy in the `rust-lang/rust` repository.
+In the `rust-lang/rust` repository, where rustc resides, there's a copy of Clippy
+that compiler hackers modify from time to time to adapt to changes in the unstable
+API of the compiler.
 
-For general information about `subtree`s in the Rust repository see [Rust's
-`CONTRIBUTING.md`][subtree].
+We need to sync these changes back to this repository periodically, and the changes
+made to this repository in the meantime also need to be synced to the `rust-lang/rust` repository. 
+
+To avoid flooding the `rust-lang/rust` PR queue, this two-way sync process is done
+in a bi-weekly basis if there's no urgent changes. This is done starting on the day of
+the Rust stable release and then every other week. That way we guarantee that we keep
+this repo up to date with the latest compiler API, and every feature in Clippy is available 
+for 2 weeks in nightly, before it can get to beta. For reference, the first sync
+following this cadence was performed the 2020-08-27.
+
+This process is described in detail in the following sections. For general information 
+about `subtree`s in the Rust repository see [Rust's `CONTRIBUTING.md`][subtree].
 
 ### Patching git-subtree to work with big repos
 
@@ -222,13 +230,14 @@ This shell has a hardcoded recursion limit set to 1000. In order to make this pr
 you need to force the script to run `bash` instead. You can do this by editing the first
 line of the `git-subtree` script and changing `sh` to `bash`.
 
-### Performing the sync
+### Performing the sync from [`rust-lang/rust`] to Clippy
 
 Here is a TL;DR version of the sync process (all of the following commands have
 to be run inside the `rust` directory):
 
-1. Clone the [`rust-lang/rust`] repository
-2. Sync the changes to the rust-copy of Clippy to your Clippy fork:
+1. Clone the [`rust-lang/rust`] repository or make sure it is up to date.
+2. Checkout the commit from the latest available nightly. You can get it using `rustup check`.
+3. Sync the changes to the rust-copy of Clippy to your Clippy fork:
     ```bash
     # Make sure to change `your-github-name` to your github name in the following command
     git subtree push -P src/tools/clippy git@github.com:your-github-name/rust-clippy sync-from-rust
@@ -246,17 +255,11 @@ to be run inside the `rust` directory):
     git checkout sync-from-rust
     git merge upstream/master
     ```
-3. Open a PR to `rust-lang/rust-clippy` and wait for it to get merged (to
+4. Open a PR to `rust-lang/rust-clippy` and wait for it to get merged (to
    accelerate the process ping the `@rust-lang/clippy` team in your PR and/or
    ~~annoy~~ ask them in the [Zulip] stream.)
-
-### Syncing back changes in Clippy to [`rust-lang/rust`]
 
-To avoid flooding the [`rust-lang/rust`] PR queue, changes in Clippy's repo are synced back
-in a bi-weekly basis if there's no urgent changes. This is done starting on the day of
-the Rust stable release and then every other week. That way we guarantee that
-every feature in Clippy is available for 2 weeks in nightly, before it can get to beta.
-For reference, the first sync following this cadence was performed the 2020-08-27.
+### Performing the sync from Clippy to [`rust-lang/rust`]
 
 All of the following commands have to be run inside the `rust` directory.
 

doc/basics.md

@@ -1,16 +1,14 @@
 # Basics for hacking on Clippy
 
 This document explains the basics for hacking on Clippy. Besides others, this
-includes how to set-up the development environment, how to build and how to test
-Clippy. For a more in depth description on the codebase take a look at [Adding
-Lints] or [Common Tools].
+includes how to build and test Clippy. For a more in depth description on
+the codebase take a look at [Adding Lints] or [Common Tools].
 
 [Adding Lints]: https://github.com/rust-lang/rust-clippy/blob/master/doc/adding_lints.md
 [Common Tools]: https://github.com/rust-lang/rust-clippy/blob/master/doc/common_tools_writing_lints.md
 
 - [Basics for hacking on Clippy](#basics-for-hacking-on-clippy)
   - [Get the code](#get-the-code)
-  - [Setup](#setup)
   - [Building and Testing](#building-and-testing)
   - [`cargo dev`](#cargo-dev)
   - [PR](#pr)
@@ -36,31 +34,11 @@ git checkout master
 git rebase upstream/master
 # push to the master branch of your fork
 git push
-```
-
-## Setup
-
-Next we need to setup the toolchain to compile Clippy. Since Clippy heavily
-relies on compiler internals it is build with the latest rustc master. To get
-this toolchain, you can just use the `setup-toolchain.sh` script or use
-`rustup-toolchain-install-master`:
-
-```bash
-bash setup-toolchain.sh
-# OR
-cargo install rustup-toolchain-install-master
-# For better IDE integration also add `-c rustfmt -c rust-src` (optional)
-rustup-toolchain-install-master -f -n master -c rustc-dev -c llvm-tools
-rustup override set master
-```
-
-_Note:_ Sometimes you may get compiler errors when building Clippy, even if you
-didn't change anything. Normally those will be fixed by a maintainer in a few hours. 
+``` 
 
 ## Building and Testing
 
-Once the `master` toolchain is installed, you can build and test Clippy like
-every other Rust project:
+You can build and test Clippy like every other Rust project:
 
 ```bash
 cargo build  # builds Clippy

rust-toolchain

@@ -1 +1,3 @@
-nightly
+[toolchain]
+channel = "nightly-2020-12-09"
+components = ["llvm-tools-preview", "rustc-dev", "rust-src", "rustfmt"]