Update book and templates with feature freeze (#14456)

This PR announces the feature freeze period talked about in
https://github.com/rust-lang/rust-clippy/issues/14364

1. Add the page to the book
2. Modify the in-Github templates.
3. The third commit (to be squashed into the first) rolls the date
mentioned 6 weeks (so, starting on May 9th and ending on August 20th).
This gives us a comfortable buffer to make choices for this period.

We have a pending discussion on the #14364. So having some more time to
make choices is very nice. I'm also preparing a Github action for
detecting new lints and posting a comment about the feature freeze
(being worked on in another branch)

Something I'd like comment on is the date formatting. I'm not sure if
"May 9th to the first of August" is the correct way of writing this
information in a book 😅.

changelog: Announce the feature freeze from May 9th to the first of
August

.github/ISSUE_TEMPLATE/new_lint.yml

@@ -1,5 +1,7 @@
 name: New lint suggestion
-description: Suggest a new Clippy lint.
+description: |
+  Suggest a new Clippy lint (currently not accepting new lints)
+  Check out the Clippy book for more information about the feature freeze.
 labels: ["A-lint"]
 body:
   - type: markdown

.github/PULL_REQUEST_TEMPLATE.md

@@ -32,6 +32,10 @@ order to get feedback.
 
 Delete this line and everything above before opening your PR.
 
+Note that we are currently not taking in new PRs that add new lints. We are in a
+feature freeze. Check out the book for more information. If you open a
+feature-adding pull request, its review will be delayed.
+
 ---
 
 *Please write a short comment explaining your change (or "none" for internal only changes)*

.github/workflows/feature_freeze.yml

@@ -0,0 +1,25 @@
+name: Feature freeze check
+
+on:
+  pull_request:
+    paths:
+      - 'clippy_lints/src/declared_lints.rs'
+
+jobs:
+  auto-comment:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Check PR Changes
+      id: pr-changes
+      run: echo "::set-output name=changes::${{ toJson(github.event.pull_request.changed_files) }}"
+
+    - name: Create Comment
+      if: steps.pr-changes.outputs.changes != '[]'
+      run: |
+        # Use GitHub API to create a comment on the PR
+        PR_NUMBER=${{ github.event.pull_request.number }}
+        COMMENT="**Seems that you are trying to add a new lint!**\nWe are currently in a [feature freeze](https://doc.rust-lang.org/nightly/clippy/development/feature_freeze.html), so we are delaying all lint-adding PRs to August 1st and focusing on bugfixes.\nThanks a lot for your contribution, and sorry for the inconvenience.\nWith ❤ from the Clippy team"
+        GITHUB_TOKEN=${{ secrets.GITHUB_TOKEN }}
+        COMMENT_URL="https://api.github.com/repos/${{ github.repository }}/issues/${PR_NUMBER}/comments"
+        curl -s -H "Authorization: token ${GITHUB_TOKEN}" -X POST $COMMENT_URL -d "{\"body\":\"$COMMENT\"}"

book/src/README.md

@@ -1,5 +1,9 @@
 # Clippy
 
+[### IMPORTANT NOTE FOR CONTRIBUTORS ================](development/feature_freeze.md)
+
+----
+
 [![License: MIT OR Apache-2.0](https://img.shields.io/crates/l/clippy.svg)](https://github.com/rust-lang/rust-clippy#license)
 
 A collection of lints to catch common mistakes and improve your

book/src/SUMMARY.md

@@ -13,6 +13,7 @@
     - [GitLab CI](continuous_integration/gitlab.md)
     - [Travis CI](continuous_integration/travis.md)
 - [Development](development/README.md)
+    - [IMPORTANT: FEATURE FREEZE](development/feature_freeze.md)
     - [Basics](development/basics.md)
     - [Adding Lints](development/adding_lints.md)
     - [Defining Lints](development/defining_lints.md)

book/src/development/adding_lints.md

@@ -1,5 +1,8 @@
 # Adding a new lint
 
+[### IMPORTANT NOTE FOR CONTRIBUTORS ================](feature_freeze.md)
+
+
 You are probably here because you want to add a new lint to Clippy. If this is
 the first time you're contributing to Clippy, this document guides you through
 creating an example lint from scratch.

book/src/development/feature_freeze.md

@@ -0,0 +1,55 @@
+# IMPORTANT: FEATURE FREEZE
+
+This is a temporary notice.
+
+From the 26th of June until the 18th of September we will perform a feature freeze. Only bugfix PRs will be reviewed
+except already open ones. Every feature-adding PR opened in between those dates will be moved into a
+milestone to be reviewed separately at another time.
+
+We do this because of the long backlog of bugs that need to be addressed
+in order to continue being the state-of-the-art linter that Clippy has become known for being.
+
+## For contributors
+
+If you are a contributor or are planning to become one, **please do not open a lint-adding PR**, we have lots of open
+bugs of all levels of difficulty that you can address instead!
+
+We currently have about 800 lints, each one posing a maintainability challenge that needs to account to every possible
+use case of the whole ecosystem. Bugs are natural in every software, but the Clippy team considers that Clippy needs a
+refinement period.
+
+If you open a PR at this time, we will not review it but push it into a milestone until the refinement period ends,
+adding additional load into our reviewing schedules.
+
+## I want to help, what can I do
+
+Thanks a lot to everyone who wants to help Clippy become better software in this feature freeze period!
+If you'd like to help, making a bugfix, making sure that it works, and opening a PR is a great step!
+
+To find things to fix, go to the [tracking issue][tracking_issue], find an issue that you like, go there and claim that
+issue with `@rustbot claim`.
+
+As a general metric and always taking into account your skill and knowledge level, you can use this guide:
+
+- 🟥 [ICEs][search_ice], these are compiler errors that causes Clippy to panic and crash. Usually involves high-level
+debugging, sometimes interacting directly with the upstream compiler. Difficult to fix but a great challenge that
+improves a lot developer workflows!
+
+- 🟧 [Suggestion causes bug][sugg_causes_bug], Clippy suggested code that changed logic in some silent way.
+Unacceptable, as this may have disastrous consequences. Easier to fix than ICEs
+
+- 🟨 [Suggestion causes error][sugg_causes_error], Clippy suggested code snippet that caused a compiler error
+when applied. We need to make sure that Clippy doesn't suggest using a variable twice at the same time or similar
+easy-to-happen occurrences.
+
+- 🟩 [False positives][false_positive], a lint should not have fired, the easiest of them all, as this is "just"
+identifying the root of a false positive and making an exception for those cases.
+
+Note that false negatives do not have priority unless the case is very clear, as they are a feature-request in a
+trench coat.
+
+[search_ice]: https://github.com/rust-lang/rust-clippy/issues?q=sort%3Aupdated-desc+state%3Aopen+label%3A%22I-ICE%22
+[sugg_causes_bug]: https://github.com/rust-lang/rust-clippy/issues?q=sort%3Aupdated-desc%20state%3Aopen%20label%3AI-suggestion-causes-bug
+[sugg_causes_error]: https://github.com/rust-lang/rust-clippy/issues?q=sort%3Aupdated-desc%20state%3Aopen%20label%3AI-suggestion-causes-error%20
+[false_positive]: https://github.com/rust-lang/rust-clippy/issues?q=sort%3Aupdated-desc%20state%3Aopen%20label%3AI-false-positive
+[tracking_issue]: https://github.com/rust-lang/rust-clippy/issues/15086