VTable-related miscompilation with incremental compilation #89598
Description
PR #86475 introduced a global cache for vtable-layouts into the tcx which circumvents incremental compilation's dependency tracking. The effect is that object files might not get updated when methods within a trait get shuffled around, thus leading to miscompilations very similar to the one observed in #82920, i.e. trait object method calls will invoke the wrong method.
The following test (when added to src/test/incremental) shows this behavior:
// revisions:rpass1 rpass2
trait Foo {
#[cfg(rpass1)]
fn method1(&self) -> u32;
fn method2(&self) -> u32;
#[cfg(rpass2)]
fn method1(&self) -> u32;
}
impl Foo for u32 {
fn method1(&self) -> u32 { 17 }
fn method2(&self) -> u32 { 42 }
}
fn main() {
let x: &dyn Foo = &0u32;
// This fails in `rpass2` because `method2` gets called instead of `method1`.
assert_eq!(mod1::foo(x), 17);
}
mod mod1 {
pub fn foo(x: &dyn super::Foo) -> u32 {
x.method1()
}
}Global caches without dependency tracking are a sure way of introducing subtle incr. comp. bugs. Queries must be used instead.
I'm going to mark this as P-critical. We should fix this asap.