1.52.1 - Blog post explaining the fingerprint issue.

posts/2021-05-07-caught-red-handed.md

@@ -6,9 +6,11 @@ team: the compiler team <https://www.rust-lang.org/governance/teams/compiler>
 release: true
 ---
 
-The Rust team has prepared a new release, 1.52.1, working around a bug
-introduced in 1.52.0. We recommend all Rust users, including those currently
-using stable versions prior to 1.52.0 upgrade to 1.52.1.
+The Rust team has prepared a new release, 1.52.1, working around a bug in
+incremental compilation which was made into a compiler error in 1.52.0. We
+recommend all Rust users, including those currently using stable versions prior
+to 1.52.0, upgrade to 1.52.1 or disable incremental compilation. Guidance on how
+to do so is available below.
 
 If you have a previous version of Rust installed via rustup, getting Rust
 1.52.1 is as easy as:
@@ -22,44 +24,51 @@ from the appropriate page on our website.
 
 [install]: https://www.rust-lang.org/install.html
 
-# What's in 1.52.1 stable
-
-This point release contains a single change: it disables incremental
-compilation. Read on for more details as to why, and the next steps the Rust
-project is pursuing on this issue.
-
 # Summary
 
-The Rust teams are always excited to report on new features offered with each release. Sometimes, however, an important change that is not yet "fully baked" gets accidentally included in a release, and we need to issue a point release.
-
-There was an instance of this in the most recent release, 1.52.0, which added a new bit of internal-consistency checking, called "incremental compilation hash verification" (abbreviated `verify-ich`). This check is also called an "unstable fingerprint" check, because the diagnostic it currently prints look [like this](https://github.com/rust-lang/rust/issues/84336):
-
-```
-thread 'rustc' panicked at 'assertion failed: `(left == right)`
-  left: `Some(Fingerprint(4565771098143344972, 7869445775526300234))`,
-   right: `Some(Fingerprint(14934403843752251060, 623484215826468126))`: found unstable fingerprints for <massive text describing rustc internals elided>
-
-error: internal compiler error: unexpected panic
+This release works around broken builds on 1.52.0, which are caused by newly
+added verification. The bugs this verification detects are present in all Rust
+versions, and can trigger miscompilations in incremental builds, so downgrading
+to a prior stable version is not a fix.
 
-note: the compiler unexpectedly panicked. this is a bug.
-```
+Users are encouraged to upgrade to 1.52.1 or disable incremental in their local
+environment if on a prior versions: please see the [what you should do][part3]
+section for details on how to do so.
 
-This internal-consistency check, as stated in the diagnostic, yields an "Internal Compiler Error" (or ICE). In other words, it represents a bug in the internals of the Rust compiler itself. In *this* case, though, the ICE is revealing a bug that 1.) predates the 1.52.0 release and 2.) could result in miscompilation if it had not been caught by `verify-ich`.
-
-In other words: If you are seeing the above Internal Compiler Error, you may be tempted to respond by reverting to the 1.51 release. It is important to note that a downgrade is *not* the best response to this problem.
+Incremental compilation is off by default for release builds, so few
+production builds should be affected (only users who may have opted in).
 
 This post is going to:
 
+ 1. Explain [what the errors look like][part0],
  1. Explain [what the check does][part1], at a high level,
  2. Explain [how the check is presenting itself][part2] in the Rust 1.52.0 release,
  3. Tell you [what you should do][part3] if you see an unstable fingerprint on your project,
  4. Describe our plans for [how the Rust project will address][part4] the problems discussed here.
 
+[part0]: #what-does-the-error-look-like
 [part1]: #what-are-fingerprints-why-are-we-checking-them
 [part2]: #how-does-this-show-up
 [part3]: #what-should-a-rust-programmer-do-in-response
 [part4]: #what-is-the-rust-project-going-to-do-to-fix-this
 
+## What does the error look like?
+
+The error message looks something like this, with the key piece being the "found
+unstable fingerprints" text.
+
+```
+thread 'rustc' panicked at 'assertion failed: `(left == right)`
+  left: `Some(Fingerprint(4565771098143344972, 7869445775526300234))`,
+  right: `Some(Fingerprint(14934403843752251060, 623484215826468126))`: found unstable fingerprints for <massive text describing rustc internals elided>
+
+error: internal compiler error: unexpected panic
+
+note: the compiler unexpectedly panicked. this is a bug.
+```
+
+This is the error caused by the internal consistency check, and as stated in the diagnostic, it yields an "Internal Compiler Error" (or ICE). In other words, it represents a bug in the internals of the Rust compiler itself. In *this* case, the ICE is revealing a bug that predates the 1.52.0 release and could result in miscompilation if it had not been caught by `verify-ich`.
+
 ## What are fingerprints? Why are we checking them?
 
 The Rust compiler has support for "incremental compilation", which has been described in a [2016 blog post][]. When incremental compilation is turned on, the compiler breaks the input source into pieces, and tracks how those input pieces influence the final build product. Then, when the inputs change, it detects this and reuses artifacts from previous builds, striving to expend effort solely on building the parts that need to respond to the changes to the input source code.
@@ -70,14 +79,18 @@ Fingerprints are part of our architecture for detecting when inputs change. More
 
 [rustc-dev-guide-fingerprints]: https://rustc-dev-guide.rust-lang.org/queries/incremental-compilation-in-detail.html#checking-query-results-for-changes-hashstable-and-fingerprints
 
-The `verify-ich` check is a safeguard asserting internal inconsistency of the fingerprints.
-The compiler stores fingerprints for both cached and uncached values.
-Every time we compute an uncached value, we double-check that its newly computed fingerprint against the finger print stored in the cache.
-There are multiple ways that a fingerprint mismatch could arise, but they all represent bugs within the Rust compiler itself.
+The `verify-ich` check is a safeguard asserting internal consistency of the
+fingerprints. Sometimes the compiler is forced to rerun a query, and expects
+that the output is the same as from a prior incremental compilation session. The
+newly enabled verification checks that the value is indeed as expected, rather
+than assuming so. In some cases, due to bugs in the compiler's implementation,
+this was not actually the case.
 
-## History of deployment
+## History
 
-We [initially added][pr-45867] `verify-ich` as a tool to use when developing rustc itself, back in 2017; it was solely provided via an unstable `-Z` flag, only available to nightly and development builds.
+We [initially added][pr-45867] `verify-ich` as a tool to use when developing
+rustc itself, back in 2017. It was solely provided via an unstable `-Z` flag,
+only available to nightly and development builds.
 
 More recently, in March, we encountered a [miscompilation][issue-82920] that led us to [turn on `verify-ich` by default][pr-83007]. The Rust compiler team decided it was better to catch fingerprint problems and abort compilation, rather than allow for potential miscompilations (and subsequent misbehavior) to sneak into Rust programmer's binaries.
 
@@ -87,11 +100,13 @@ More recently, in March, we encountered a [miscompilation][issue-82920] that led
 
 When we first turned on `verify-ich` by default, there was a steady stream of
 issues filed by users of the nightly (and beta) toolchains, and steady progress
-has been made on identifying fixes, a number of which have already landed. This
-last week, we noted incorrectly that the error would be shipping to stable
-next cycle in 1.53.0, and we started [making plans][issue-84970] to improve the
+has been made on identifying fixes, a number of which have already landed.
+
+In the past week, we had started [making plans][issue-84970] to improve the
 user-experience, so that the diagnostic issued by the check would do a better
-job of telling the programmer what to do in response.
+job of telling the programmer what to do in response. Unfortunately, this was
+done under the assumption that the new verification would ship in 1.53, not
+1.52.
 
 [issue-84970]: https://github.com/rust-lang/rust/issues/84970
 
@@ -110,20 +125,27 @@ Essentially, for some crates, certain sequences of edit-compile cycles will caus
 Another recent example looks [like this](https://github.com/rust-lang/rust/issues/85039):
 
 ```
-thread 'rustc' panicked at 'found unstable fingerprints for predicates_of(<massive text describing rustc internals elided>)', /rustc/88f19c6dab716c6281af7602e30f413e809c5974/compiler/rustc_query_system/src/query/plumbing.rs:593:5
+thread 'rustc' panicked at 'found unstable fingerprints for predicates_of(<massive text describing rustc internals elided>)', /rustc/.../compiler/rustc_query_system/src/query/plumbing.rs:593:5
 ```
 
 They all arise from inconsistencies when comparing the incremental-compilation cache stored on disk against the values computed during a current `rustc` invocation, which means they all arise from using incremental compilation.
 
-There are three ways that you may have incremental compilation turned on: You may have set the [environment variable][env-vars] `CARGO_INCREMENTAL=1`, or you may have enabled the `build.incremental` [setting in your Cargo.toml][cargo-toml], or you may be building with the `dev` or `test` [profiles][], which default to having incremental compilation enabled.
+There are several ways that you may have incremental compilation turned on:
+
+1. You may be building with the `dev` or `test` [profiles][] which default to having incremental compilation enabled.
+2. You may have set the [environment variable][env-vars] `CARGO_INCREMENTAL=1`
+3. You may have enabled the `build.incremental` [setting in your Cargo config][cargo-config]
+4. You may have enabled the `incremental` [setting in your Cargo.toml][cargo-toml] for a given profile
 
 [env-vars]: https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-reads
-[cargo-toml]: https://doc.rust-lang.org/cargo/reference/config.html#buildincremental
+[cargo-config]: https://doc.rust-lang.org/cargo/reference/config.html#buildincremental
+[cargo-toml]: https://doc.rust-lang.org/cargo/reference/profiles.html#incremental
 [profiles]: https://doc.rust-lang.org/cargo/reference/profiles.html
 
-Incremental is disabled by default for the release profile, which should mean
-that unless you have enabled it yourself, these errors do not affect release
-builds, as they are only present in incremental builds.
+If your project has not adjusted the defaults, then when running `cargo build
+--release` or otherwise in the `release` profile configuration incremental is
+disabled on all Rust versions, and these issues should not affect your release
+builds.
 
 ## What should a Rust programmer do in response
 
@@ -144,9 +166,18 @@ We do *not* recommend that users of 1.52.0 downgrade to an earlier version of Ru
 If a user is willing to deal with the incremental verification ICE's, and wishes
 to opt back into the 1.52.0 behavior, they may set `RUSTC_FORCE_INCREMENTAL` to
 `1` in their environment. The Rust compiler will then respect the
-`-Cincremental` option passed by Cargo, and things will work as before. Note
-that this flag does not enable incremental if it has not already been separately
-enabled (whether by Cargo or otherwise).
+`-Cincremental` option passed by Cargo, and things will work as before, though
+with the added verification. Note that this flag does not enable incremental if
+it has not already been separately enabled (whether by Cargo or otherwise).
+
+If you are currently using a toolchain prior to 1.52.0, and wish to continue
+doing so, we recommend that you disable incremental compilation to avoid hitting
+silent miscompilations.
+
+On all Rust builds since incremental has landed, it has been a major
+improvement to compile times for many users, and has only improved over time. We
+acknowledge that the workarounds presented here and recommendations are painful,
+and will be working hard to ensure the situation is as temporary as possible.
 
 ## What is the Rust project going to do to fix this
 
@@ -156,7 +187,9 @@ We have issued 1.52.1 today which:
 
 * Disables incremental compilation in the Rust compiler (unless asked for by a
   new environment variable, `RUSTC_FORCE_INCREMENTAL=1`).
-* Improves diagnostic output for the new verification if incremental compilation is enabled, indicating how to work around the bugs.
+* Improves diagnostic output for the new verification if incremental compilation is enabled,
+  indicating how to work around the bugs by purging incremental state or
+  disabling incremental.
 
 This is intended to be a mitigation that helps the majority of Rust users have
 an upgrade path to a safe Rust compiler which does not have the risk of