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Unsoundness: Rust since 1.78 accepts invalid ? #132463
Description
Code
I tried this code:
pub trait Frob<T: ?Frob> {}I expected to see this happen: rustc rejects this code
Instead, this happened: rustc since 1.78 accepts the code
Version it worked on
It most recently worked on: Rust 1.77
% rustc -Awarnings --crate-type=lib test.rs
error[E0107]: missing generics for trait `Frob`
--> src/lib.rs:1:20
|
1 | pub trait Frob<T: ?Frob> {}
| ^^^^ expected 1 generic argument
|
note: trait defined here, with 1 generic parameter: `T`
--> src/lib.rs:1:11
|
1 | pub trait Frob<T: ?Frob> {}
| ^^^^ -
help: add missing generic argument
|
1 | pub trait Frob<T: ?Frob<T>> {}
| +++
error: aborting due to 1 previous error
For more information about this error, try `rustc --explain E0107`.
%
% rustc --version --verbose
rustc 1.77.0-nightly (e51e98dde 2023-12-31)
binary: rustc
commit-hash: e51e98dde6a60637b6a71b8105245b629ac3fe77
commit-date: 2023-12-31
host: x86_64-unknown-linux-gnu
release: 1.77.0-nightly
LLVM version: 17.0.6
%Version with regression
% rustc -Awarnings --crate-type=lib test.rs
%
% rustc --version --verbose
rustc 1.84.0-nightly (a0d98ff0e 2024-10-31)
binary: rustc
commit-hash: a0d98ff0e5b6e1f2c63fd26f68484792621b235c
commit-date: 2024-10-31
host: x86_64-unknown-linux-gnu
release: 1.84.0-nightly
LLVM version: 19.1.1
%
Activity
compiler-errors commentedon Nov 1, 2024
This is not unsound. This is just nonsense code being accepted by the compiler. There is no meaning to
?on arbitrary traits, so ignoring the bound outright is obviously not desirable, but totally fine for the compiler to do.I fixed this in #132209.