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r? @nagisa

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See #94188 for some prior work in this direction. I'll probably create an MCP for removing the HashStable implementation from HashMap and HashSet.

#98890 was not caused by a FxHashMap getting returned by a query. It was caused by a FxHashMap getting iterated upon. I think the proper fix for this class of bugs is the internal lint forbidding iteration over hash-maps.

In the MCP write-up I'll explain in more detail how I think things fit together. But in short, I think it's preferable to just not implement HashStable for HashMap and HashSet (and BTreeMap and BTreeSet). They don't fulfill the contract that any observable difference results in a different Fingerprint -- so I think it's hard to argue that they should nonetheless implement HashStable.

I think the proper fix for this class of bugs is the internal lint forbidding iteration over hash-maps.

I think the solution is a combination of both. If we disallow types in query results that don't have a proper HashStable impl, then indeterminism coming from iterating over a HashSet will result in spurious cache invalidation instead of potential soundness errors -- so that's an improvement. But we don't want the spurious invalidations either, so the lint will be useful to avoid those too.

@bors r+

📌 Commit 2d2ac32 has been approved by nagisa

It is now in the queue for this repository.

failed in rollup ci

@bors r-

☔ The latest upstream changes (presumably #99451) made this pull request unmergeable. Please resolve the merge conflicts.

Rebased.

Don't include this in rollups since it can easily break if another PR adds a use rustc_data_structures::stable_map::FxHashMap before this is being tested.

@bors r=nagisa rollup=never

📌 Commit 88f6c6d has been approved by nagisa

It is now in the queue for this repository.

⌛ Testing commit 88f6c6d with merge be9cfb3...

☀️ Test successful - checks-actions
Approved by: nagisa
Pushing be9cfb3 to master...

Finished benchmarking commit (be9cfb3): comparison url.

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

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If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

The job x86_64-gnu-llvm-12 failed! Check out the build log: (web) (plain)

   Compiling rustc_expand v0.0.0 (/checkout/compiler/rustc_expand)
error[E0282]: type annotations needed
   --> compiler/rustc_middle/src/mir/terminator.rs:294:42
    |
294 |             1 => write!(fmt, " -> {:?}", self.successors().next().unwrap()),
    |                  |                       |
    |                  |                       |
    |                  |                       cannot infer type of the type parameter `T` declared on the associated function `new_debug`
    |
   ::: /checkout/library/core/src/macros/mod.rs:498:1
    |
498 | macro_rules! write {
498 | macro_rules! write {
    | ------------------ in this expansion of `write!` (#1)
499 |     ($dst:expr, $($arg:tt)*) => {{
500 |         let result = $dst.write_fmt($crate::format_args!($($arg)*));
...
879 |     macro_rules! format_args {
    |     ------------------------ in this expansion of `$crate::format_args!` (#2)
    |
    |
help: consider specifying the generic argument
    |
294 |             1 => write!(fmt, " -> {:?}", self.successors().next().unwrap()::<T>),

   Compiling rustc_builtin_macros v0.0.0 (/checkout/compiler/rustc_builtin_macros)
error[E0282]: type annotations needed
    --> compiler/rustc_middle/src/ty/mod.rs:1786:21
    --> compiler/rustc_middle/src/ty/mod.rs:1786:21
     |
1786 |         for attr in tcx.get_attrs(did, sym::repr) {

error[E0282]: type annotations needed
    --> compiler/rustc_middle/src/ty/mod.rs:2171:35
     |
     |
2171 |         self.get_attrs(did, attr).next()
     |
help: try using a fully qualified path to specify the expected types
     |
     |
2171 |         <Self as Iterator>::next(&mut self.get_attrs(did, attr))

error[E0282]: type annotations needed
    --> compiler/rustc_middle/src/ty/mod.rs:2179:39
     |
     |
2179 |             self.get_attrs(did, attr).next().is_some()
     |
help: try using a fully qualified path to specify the expected types
     |
     |
2179 |             <Self as Iterator>::next(&mut self.get_attrs(did, attr)).is_some()

For more information about this error, try `rustc --explain E0282`.
error: could not compile `rustc_middle` due to 4 previous errors
warning: build failed, waiting for other jobs to finish...