GCC Rust Weekly Status Report 12

Thanks again to Open Source Security, inc and Embecosm for their ongoing support for this project.

Add check for duplicate overlapping impl-items

Rust allows for multiple impl blocks to give specialisation to a generic data type. But if the programmer adds a generic impl for a dup method it will become impossible to distinguish the method ‘bar’ in each of these specialised impl blocks when it comes to method resolution. Since you receiver could be Foo<_ (inference variable)> which could resolve to Foo or Foo. rustc –explain E0592

struct Foo<A>(A);

impl Foo<isize> {
    fn bar(self) -> isize {
        self.0
    }
}

impl Foo<char> {
    fn bar(self) -> char {
        self.0
    }
}

impl Foo<T> {
    fn bar(self) -> T {
        self.0
    }
}

Check for unconstrained type parameters

When we declare generic impl blocks but don’t use all of the type arguments within the generic data type we end up with unconstrained type parameters. This is important since we rely on the Self of the impl block to bind all of the substitutions relevant to the impl block. See rustc –explain E0207

struct Foo<T>(T, bool);

impl<X, Y> Foo<X> {
    fn test() -> Y {
        self.0
    }
}

Multiple candidate errors

Similar to the duplicate overlapping impl items each of theses specialised impl blocks are valid and there is no overlap here. The problem arises when the programmer tries to resolve this path but using the Path Foo::test without any generic argument specialisation, the resolver will find both ‘test’ impl item as candidates. This can be fixed if the programmer uses Foo::::test for example. rustc –explain E0034

struct Foo<A>(A);

impl Foo<isize> {
    fn test() -> i32 {
        123
    }
}

impl Foo<f32> {
    fn test() -> i32 {
        123
    }
}

fn main() {
    let a: i32 = Foo::test()
}

Rich locations

We have also merged a building block to take advantage of GCC’s rich location diagnostics for errors such as the multiple candidates error we had to emit multiple errors for each location. Now with rich locations we can add ranges/fixit hints at specific locations and emit one single error which makes diagnostics much easier to read for users of the compiler. There is alot of scope in terms of improving errors over time and rich locations are the first step in this direction.

Testsuite updates

Thanks to Thomas’s work we have jumped our test suite number of passes by including checking for warnings emitted by the compiler this will help a lot with detecting any regressions/changes in behaviour.

Completed Activities

1. Merged building block for using GCC rich locations: PR374
2. Merged canonical path work: PR358
3. Merged TuboFish work: PR358
4. Merged fix for crash in generic impl blocks with different type parameter names: PR377
5. Merged check for unconstrained type parameters: PR378
6. Merged testsuite to scan for warnings and errors: PR362

Overall Task Status

Category	Last Week	This Week	Delta
TODO	61	62	+1
In Progress	11	8	-3
Completed	110	118	+8

Test Cases

Category	Last Week	This Week	Delta
Passing	861	1921	+1060
XFAIL	26	36	+10
Failed	0	0	–
XPASS	0	0	–

Bugs

Category	Last Week	This Week	Delta
TODO	13	12	-1
In Progress	6	4	-2
Completed	4	31	+5

Milestones Progress

Milestone	Last Week	This Week	Delta	Start Date	Completion Date	Target
Data Structures 1 – Core	100%	100%	–	30th Nov 2020	27th Jan 2021	29th Jan 2021
Control Flow 1 – Core	100%	100%	–	28th Jan 2021	10th Feb 2021	26th Feb 2021
Data Structures 2 – Generics	72%	79%	+7%	11th Feb 2021	–	28th May 2021
Data Structures 3 – Traits	0%	0%	–	–	–	27th Aug 2021
Control Flow 2 – Pattern Matching	0%	0%	–	–	–	29th Oct 2021
Imports and Visibility	0%	0%	–	–	–	TBD

Risks

Risk	Impact (1-3)	Likelihood (0-10)	Risk (I*L)	Mitigation
Copyright assignments	2	5	10	Be up front on all PRs that the code is destined to be upstreamed to FSF
Rust Language Changes	3	7	21	Keep up to date with the Rust language on a regular basis

Planned Activities

• Continue default generic argument work
• Review contributor NeverType work updates