Inference failure triggered by null proc macro application to totally unused and irrelevant struct
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Description
Steps to reproduce
git clone https://gitlab.torproject.org/Diziet/arti -b report-inference-failure
cd arti
cargo +nightly check --locked -p tor-socksproto --all-features --all-targets
I'm afraid I haven't minimised this, for reasons that will become obvious.
Expected behaviour
It compiles.
Actual behaviour:
error[E0282]: type annotations needed
--> crates/tor-socksproto/src/handshake/client.rs:332:9
|
332 | assert_eq!(action.reply, &[]);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type
|
= note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
Why is this surprising
This works:
git checkout report-inference-failure~
cargo +nightly check --locked -p tor-socksproto --all-features --all-targets
The commit that makes things break, 778c2753469c020bf2fde9d43f182c8b1b02d0af, applies amplify::Getters to an unused struct - one which is therefore not related to the one where inference is failing. Indeed, deleting the whole struct definition leaves a tree that compiles.
I looked at the output of cargo expand. As expected, the diff is trivial and not a plausible cause of the inference failure: with the specified attributes, the maro generates no methods - just one empty impl block. Adding that emply impl block by hand compiles.
This is all extremely mysterious. I feel I must be missing something but I struggle to guess what.
Source diff that makes it break
diff --git a/crates/tor-socksproto/src/handshake/framework.rs b/crates/tor-socksproto/src/handshake/framework.rs
index 58318bbcf..a451d7e69 100644
--- a/crates/tor-socksproto/src/handshake/framework.rs
+++ b/crates/tor-socksproto/src/handshake/framework.rs
@@ -64,9 +64,10 @@ pub struct Done<'b, O> {
/// dispose of the [`RecvStep`] (which mutably borrows the `Buffer`)
/// while reading,
/// at the cost of slightly less correctness checking by the compiler.
-#[derive(Debug)]
+#[derive(Debug, amplify::Getters)]
pub struct RecvStep<'b> {
/// The buffer
+ #[getter(skip)]
buffer: &'b mut Buffer,
/// Lower bound on the number of bytes that the handshake needs to read to complete.
@@ -78,6 +79,7 @@ pub struct RecvStep<'b> {
/// [`tor_bytes::IncompleteMessage.deficit`.
// XXXX should this be NonZeroUsize instead? Or maybe even usize?
// Note that the getter exposes the type.
+ #[getter(skip)]
deficit: Sensitive<NonZeroUsize>,
}
Diff in the output of cargo expand --locked -p tor-socksproto
--- a 2024-09-17 18:43:13.903354184 +0100
+++ b 2024-09-17 18:43:58.655551951 +0100
@@ -413,6 +413,7 @@
/// at the cost of slightly less correctness checking by the compiler.
pub struct RecvStep<'b> {
/// The buffer
+ #[getter(skip)]
buffer: &'b mut Buffer,
/// Lower bound on the number of bytes that the handshake needs to read to complete.
///
@@ -421,6 +422,7 @@
///
/// The returned value has the same semantics as
/// [`tor_bytes::IncompleteMessage.deficit`.
+ #[getter(skip)]
deficit: Sensitive<NonZeroUsize>,
}
#[automatically_derived]
@@ -437,6 +439,8 @@
)
}
}
+ #[automatically_derived]
+ impl<'b> RecvStep<'b> {}
/// `Handshake` structs that have a state that can be `Failed`
///
/// Derive this with
Meta
rustc --version --verbose:
rustcargo@zealot:/volatile/rustcargo/d/arti$ rustc +nightly -vV
rustc 1.83.0-nightly (c52c23b6f 2024-09-16)
binary: rustc
commit-hash: c52c23b6f44cd19718721a5e3b2eeb169e9c96ff
commit-date: 2024-09-16
host: x86_64-unknown-linux-gnu
release: 1.83.0-nightly
LLVM version: 19.1.0
rustcargo@zealot:/volatile/rustcargo/d/arti$
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Reproduce the failure with cargo +nightly check --locked -p tor-socksproto --all-features --all-targets, then compare crates/tor-socksproto/src/handshake/framework.rs and handshake/client.rs before and after commit 778c2753469c020bf2fde9d43f182c8b1b02d0af. Use the reported cargo expand output to investigate why the empty derived impl affects inference; done means the command compiles without E0282.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Needs clarification
- Newbie friendliness
- 35/100