Move the last few things from the forge

src/SUMMARY.md

@@ -17,6 +17,7 @@
         - [Adding new tests](./tests/adding.md)
         - [Using `compiletest` + commands to control test execution](./compiletest.md)
     - [Walkthrough: a typical contribution](./walkthrough.md)
+    - [Bug Fix Procedure](./bug-fix-procedure.md)
     - [Implementing new features](./implementing_new_features.md)
     - [Stability attributes](./stability.md)
     - [Stabilizing Features](./stabilization_guide.md)
@@ -27,6 +28,7 @@
     - [crates.io Dependencies](./crates-io.md)
     - [Emitting Errors and other Diagnostics](diagnostics.md)
         - [`LintStore`](./diagnostics/lintstore.md)
+        - [Diagnostic Codes](./diagnostics/diagnostic-codes.md)
     - [ICE-breaker teams](ice-breaker/about.md)
         - [LLVM ICE-breakers](ice-breaker/llvm.md)
 - [Part 2: How rustc works](./part-2-intro.md)
@@ -38,6 +40,7 @@
         - [Incremental compilation](./queries/incremental-compilation.md)
         - [Incremental compilation In Detail](./queries/incremental-compilation-in-detail.md)
         - [Debugging and Testing](./incrcomp-debugging.md)
+        - [Profiling Queries](./queries/profiling.md)
         - [Salsa](./salsa.md)
     - [Lexing and Parsing](./the-parser.md)
     - [`#[test]` Implementation](./test-implementation.md)

src/bug-fix-procedure.md

@@ -0,0 +1,330 @@
+# Rustc Bug Fix Procedure
+This page defines the best practices procedure for making bug fixes or soundness
+corrections in the compiler that can cause existing code to stop compiling. This
+text is based on
+[RFC 1589](https://github.com/rust-lang/rfcs/blob/master/text/1589-rustc-bug-fix-procedure.md).
+
+# Motivation
+
+[motivation]: #motivation
+
+From time to time, we encounter the need to make a bug fix, soundness
+correction, or other change in the compiler which will cause existing code to
+stop compiling. When this happens, it is important that we handle the change in
+a way that gives users of Rust a smooth transition. What we want to avoid is
+that existing programs suddenly stop compiling with opaque error messages: we
+would prefer to have a gradual period of warnings, with clear guidance as to
+what the problem is, how to fix it, and why the change was made. This RFC
+describes the procedure that we have been developing for handling breaking
+changes that aims to achieve that kind of smooth transition.
+
+One of the key points of this policy is that (a) warnings should be issued
+initially rather than hard errors if at all possible and (b) every change that
+causes existing code to stop compiling will have an associated tracking issue.
+This issue provides a point to collect feedback on the results of that change.
+Sometimes changes have unexpectedly large consequences or there may be a way to
+avoid the change that was not considered. In those cases, we may decide to
+change course and roll back the change, or find another solution (if warnings
+are being used, this is particularly easy to do).
+
+### What qualifies as a bug fix?
+
+Note that this RFC does not try to define when a breaking change is permitted.
+That is already covered under [RFC 1122][]. This document assumes that the
+change being made is in accordance with those policies. Here is a summary of the
+conditions from RFC 1122:
+
+- **Soundness changes:** Fixes to holes uncovered in the type system.
+- **Compiler bugs:** Places where the compiler is not implementing the specified
+  semantics found in an RFC or lang-team decision.
+- **Underspecified language semantics:** Clarifications to grey areas where the
+  compiler behaves inconsistently and no formal behavior had been previously
+  decided.
+
+Please see [the RFC][rfc 1122] for full details!
+
+# Detailed design
+
+[design]: #detailed-design
+
+The procedure for making a breaking change is as follows (each of these steps is
+described in more detail below):
+
+0. Do a **crater run** to assess the impact of the change.
+1. Make a **special tracking issue** dedicated to the change.
+1. Do not report an error right away. Instead, **issue forwards-compatibility
+   lint warnings**.
+   - Sometimes this is not straightforward. See the text below for suggestions
+     on different techniques we have employed in the past.
+   - For cases where warnings are infeasible:
+     - Report errors, but make every effort to give a targeted error message
+       that directs users to the tracking issue
+     - Submit PRs to all known affected crates that fix the issue
+       - or, at minimum, alert the owners of those crates to the problem and
+         direct them to the tracking issue
+1. Once the change has been in the wild for at least one cycle, we can
+   **stabilize the change**, converting those warnings into errors.
+
+Finally, for changes to libsyntax that will affect plugins, the general policy
+is to batch these changes. That is discussed below in more detail.
+
+### Tracking issue
+
+Every breaking change should be accompanied by a **dedicated tracking issue**
+for that change. The main text of this issue should describe the change being
+made, with a focus on what users must do to fix their code. The issue should be
+approachable and practical; it may make sense to direct users to an RFC or some
+other issue for the full details. The issue also serves as a place where users
+can comment with questions or other concerns.
+
+A template for these breaking-change tracking issues can be found below. An
+example of how such an issue should look can be [found
+here][breaking-change-issue].
+
+The issue should be tagged with (at least) `B-unstable` and `T-compiler`.
+
+### Tracking issue template
+
+This is a template to use for tracking issues:
+
+```
+This is the **summary issue** for the `YOUR_LINT_NAME_HERE`
+future-compatibility warning and other related errors. The goal of
+this page is describe why this change was made and how you can fix
+code that is affected by it. It also provides a place to ask questions
+or register a complaint if you feel the change should not be made. For
+more information on the policy around future-compatibility warnings,
+see our [breaking change policy guidelines][guidelines].
+
+[guidelines]: LINK_TO_THIS_RFC
+
+#### What is the warning for?
+
+*Describe the conditions that trigger the warning and how they can be
+fixed. Also explain why the change was made.**
+
+#### When will this warning become a hard error?
+
+At the beginning of each 6-week release cycle, the Rust compiler team
+will review the set of outstanding future compatibility warnings and
+nominate some of them for **Final Comment Period**. Toward the end of
+the cycle, we will review any comments and make a final determination
+whether to convert the warning into a hard error or remove it
+entirely.
+```
+
+### Issuing future compatibility warnings
+
+The best way to handle a breaking change is to begin by issuing
+future-compatibility warnings. These are a special category of lint warning.
+Adding a new future-compatibility warning can be done as follows.
+
+```rust
+// 1. Define the lint in `src/librustc/lint/builtin.rs`:
+declare_lint! {
+    pub YOUR_ERROR_HERE,
+    Warn,
+    "illegal use of foo bar baz"
+}
+
+// 2. Add to the list of HardwiredLints in the same file:
+impl LintPass for HardwiredLints {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(
+            ..,
+            YOUR_ERROR_HERE
+        )
+    }
+}
+
+// 3. Register the lint in `src/librustc_lint/lib.rs`:
+store.register_future_incompatible(sess, vec![
+    ...,
+    FutureIncompatibleInfo {
+        id: LintId::of(YOUR_ERROR_HERE),
+        reference: "issue #1234", // your tracking issue here!
+    },
+]);
+
+// 4. Report the lint:
+tcx.lint_node(
+    lint::builtin::YOUR_ERROR_HERE,
+    path_id,
+    binding.span,
+    format!("some helper message here"));
+```
+
+#### Helpful techniques
+
+It can often be challenging to filter out new warnings from older, pre-existing
+errors. One technique that has been used in the past is to run the older code
+unchanged and collect the errors it would have reported. You can then issue
+warnings for any errors you would give which do not appear in that original set.
+Another option is to abort compilation after the original code completes if
+errors are reported: then you know that your new code will only execute when
+there were no errors before.
+
+#### Crater and crates.io
+
+We should always do a crater run to assess impact. It is polite and considerate
+to at least notify the authors of affected crates the breaking change. If we can
+submit PRs to fix the problem, so much the better.
+
+#### Is it ever acceptable to go directly to issuing errors?
+
+Changes that are believed to have negligible impact can go directly to issuing
+an error. One rule of thumb would be to check against `crates.io`: if fewer than
+10 **total** affected projects are found (**not** root errors), we can move
+straight to an error. In such cases, we should still make the "breaking change"
+page as before, and we should ensure that the error directs users to this page.
+In other words, everything should be the same except that users are getting an
+error, and not a warning. Moreover, we should submit PRs to the affected
+projects (ideally before the PR implementing the change lands in rustc).
+
+If the impact is not believed to be negligible (e.g., more than 10 crates are
+affected), then warnings are required (unless the compiler team agrees to grant
+a special exemption in some particular case). If implementing warnings is not
+feasible, then we should make an aggressive strategy of migrating crates before
+we land the change so as to lower the number of affected crates. Here are some
+techniques for approaching this scenario:
+
+1. Issue warnings for subparts of the problem, and reserve the new errors for
+   the smallest set of cases you can.
+2. Try to give a very precise error message that suggests how to fix the problem
+   and directs users to the tracking issue.
+3. It may also make sense to layer the fix:
+   - First, add warnings where possible and let those land before proceeding to
+     issue errors.
+   - Work with authors of affected crates to ensure that corrected versions are
+     available _before_ the fix lands, so that downstream users can use them.
+
+### Stabilization
+
+After a change is made, we will **stabilize** the change using the same process
+that we use for unstable features:
+
+- After a new release is made, we will go through the outstanding tracking
+  issues corresponding to breaking changes and nominate some of them for **final
+  comment period** (FCP).
+- The FCP for such issues lasts for one cycle. In the final week or two of the
+  cycle, we will review comments and make a final determination:
+
+  - Convert to error: the change should be made into a hard error.
+  - Revert: we should remove the warning and continue to allow the older code to
+    compile.
+  - Defer: can't decide yet, wait longer, or try other strategies.
+
+Ideally, breaking changes should have landed on the **stable branch** of the
+compiler before they are finalized.
+
+<a name="guide">
+
+### Removing a lint
+
+Once we have decided to make a "future warning" into a hard error, we need a PR
+that removes the custom lint. As an example, here are the steps required to
+remove the `overlapping_inherent_impls` compatibility lint. First, convert the
+name of the lint to uppercase (`OVERLAPPING_INHERENT_IMPLS`) ripgrep through the
+source for that string. We will basically by converting each place where this
+lint name is mentioned (in the compiler, we use the upper-case name, and a macro
+automatically generates the lower-case string; so searching for
+`overlapping_inherent_impls` would not find much).
+
+#### Remove the lint.
+
+The first reference you will likely find is the lint definition [in
+`librustc/lint/builtin.rs` that resembles this][defsource]:
+
+[defsource]: https://github.com/rust-lang/rust/blob/085d71c3efe453863739c1fb68fd9bd1beff214f/src/librustc/lint/builtin.rs#L171-L175
+
+```rust
+declare_lint! {
+    pub OVERLAPPING_INHERENT_IMPLS,
+    Deny, // this may also say Warning
+    "two overlapping inherent impls define an item with the same name were erroneously allowed"
+}
+```
+
+This `declare_lint!` macro creates the relevant data structures. Remove it. You
+will also find that there is a mention of `OVERLAPPING_INHERENT_IMPLS` later in
+the file as [part of a `lint_array!`][lintarraysource]; remove it too,
+
+[lintarraysource]: https://github.com/rust-lang/rust/blob/085d71c3efe453863739c1fb68fd9bd1beff214f/src/librustc/lint/builtin.rs#L252-L290
+
+Next, you see see [a reference to `OVERLAPPING_INHERENT_IMPLS` in
+`librustc_lint/lib.rs`][futuresource]. This defining the lint as a "future
+compatibility lint":
+
+```rust
+FutureIncompatibleInfo {
+    id: LintId::of(OVERLAPPING_INHERENT_IMPLS),
+    reference: "issue #36889 <https://github.com/rust-lang/rust/issues/36889>",
+},
+```
+
+Remove this too.
+
+#### Add the lint to the list of removed lists.
+
+In `src/librustc_lint/lib.rs` there is a list of "renamed and removed lints".
+You can add this lint to the list:
+
+```rust
+store.register_removed("overlapping_inherent_impls", "converted into hard error, see #36889");
+```
+
+where `#36889` is the tracking issue for your lint.
+
+#### Update the places that issue the lint
+
+Finally, the last class of references you will see are the places that actually
+**trigger** the lint itself (i.e., what causes the warnings to appear). These
+you do not want to delete. Instead, you want to convert them into errors. In
+this case, the [`add_lint` call][addlintsource] looks like this:
+
+```rust
+self.tcx.sess.add_lint(lint::builtin::OVERLAPPING_INHERENT_IMPLS,
+                       node_id,
+                       self.tcx.span_of_impl(item1).unwrap(),
+                       msg);
+```
+
+We want to convert this into an error. In some cases, there may be an existing
+error for this scenario. In others, we will need to allocate a fresh diagnostic
+code.
+[Instructions for allocating a fresh diagnostic code can be found here.](rustc-diagnostic-code.html)
+You may want to mention in the extended description that the compiler behavior
+changed on this point, and include a reference to the tracking issue for the
+change.
+
+Let's say that we've adopted `E0592` as our code. Then we can change the
+`add_lint()` call above to something like:
+
+```rust
+struct_span_err!(self.tcx.sess, self.tcx.span_of_impl(item1).unwrap(), msg)
+    .emit();
+```
+
+#### Update tests
+
+Finally, run the test suite. These should be some tests that used to reference
+the `overlapping_inherent_impls` lint, those will need to be updated. In
+general, if the test used to have `#[deny(overlapping_inherent_impls)]`, that
+can just be removed.
+
+```
+./x.py test
+```
+
+#### All done!
+
+Open a PR. =)
+
+[addlintsource]: https://github.com/rust-lang/rust/blob/085d71c3efe453863739c1fb68fd9bd1beff214f/src/librustc_typeck/coherence/inherent.rs#L300-L303
+[futuresource]: https://github.com/rust-lang/rust/blob/085d71c3efe453863739c1fb68fd9bd1beff214f/src/librustc_lint/lib.rs#L202-L205
+
+<!-- -Links--------------------------------------------------------------------- -->
+
+[rfc 1122]: https://github.com/rust-lang/rfcs/blob/master/text/1122-language-semver.md
+[breaking-change-issue]: https://gist.github.com/nikomatsakis/631ec8b4af9a18b5d062d9d9b7d3d967
+