Overflow evaluating the requirement [stable/nightly] #89275
Labels
A-diagnostics
Area: Messages for errors, warnings, and lints
A-inference
Area: Type inference
A-trait-system
Area: Trait system
D-confusing
Diagnostics: Confusing error or lint that should be reworked.
D-newcomer-roadblock
Diagnostics: Confusing error or lint; hard to understand for new users.
T-compiler
Relevant to the compiler team, which will review and decide on the PR/issue.
Comments
ZippyMagician
changed the title
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It's worth noting that if you use the turbofish syntax the compiler gives the correct error message: |
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For a little more context, this issue was filed from a discussion on reddit. |
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It's possible to trigger this issue with use num_traits as _;
struct Ratio<T>(T);
pub trait Pow {
fn pow(self);
}
impl<'a, T> Pow for &'a Ratio<T>
where
&'a T: Pow,
{
fn pow(self) {
unimplemented!()
}
}
struct Foo;
impl Foo {
fn downcast<W>(&self) -> &W {
todo!()
}
}
struct Other;
fn main() {
let other: &mut Other = Foo.downcast();
} |
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I am having a similar issue with |
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For info here is a backtrace of the compiler during compilation of this Minimal reproduction (with recursion limit set to 8 for more brevity) BacktraceI believe we start to go awry at around line 68 - it looks like it's perhaps inferring the wrong return type for the method downcast() (line 71) and recursing from there but I'm fairly new to debugging the compiler and I could be extremely off on my guesswork. I will try running a backtrace on some of the other reproductions of this and see if they're similar |
estebank
added
A-diagnostics
|
Version with no dependencies: #![recursion_limit = "5"] // To reduce noise
struct Ratio<T>(T);
pub trait Pow {
fn pow(self) -> Self;
}
impl<'a, T> Pow for &'a Ratio<T>
where
&'a T: Pow,
{
fn pow(self) -> Self {
self
}
}
fn downcast<'a, W: ?Sized>() -> &'a W {
todo!()
}
struct Other;
fn main() {
let other: &mut Other = downcast();
} |
|
From chatting to some people on the rust Zulip this is happening because the error reporting system is trying to find a valid conversion method to put as a suggestion in the mutability error. It's iterating over every trait implementation in scope to check for a conversion method on &_ so either we need a way to tell the error reporting system not to report new errors if it encounters one or fix which span the error is reported on |
This was referenced Oct 5, 2021
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Given the following code snippet:
One would expect the code to error, explaining that the types differ in mutability (we will come back to this in a moment).
However, it instead leaves a long and confusing error:
If you follow the suggestion of increasing the recursion limit each time, it will eventually segfault once you have
#![recursion_limit="2048"]Furthermore, removing the import of
num-rationalleads to the program erroring as expectedMeta
rustc --version --verbose:This error exists on stable, too.
Backtrace
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