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Forbid elided lifetimes within const generic parameter types #68143

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Jan 14, 2020
Merged

Forbid elided lifetimes within const generic parameter types #68143

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b4fddf0
Forbid elided lifetimes within const generic parameter types.
skinnyBat Jan 12, 2020
9e46ddc
Added comment about behaviour.
skinnyBat Jan 12, 2020
82b90bd
Update test benchmark file
skinnyBat Jan 12, 2020
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14 changes: 8 additions & 6 deletions src/librustc_ast_lowering/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2121,12 +2121,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {

(hir::ParamName::Plain(param.ident), kind)
}
GenericParamKind::Const { ref ty } => (
hir::ParamName::Plain(param.ident),
hir::GenericParamKind::Const {
ty: self.lower_ty(&ty, ImplTraitContext::disallowed()),
},
),
GenericParamKind::Const { ref ty } => {
let ty = self
.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
this.lower_ty(&ty, ImplTraitContext::disallowed())
});

(hir::ParamName::Plain(param.ident), hir::GenericParamKind::Const { ty })
}
};

hir::GenericParam {
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24 changes: 24 additions & 0 deletions src/test/ui/const-generics/const-param-elided-lifetime.rs
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@@ -0,0 +1,24 @@
// Elided lifetimes within the type of a const generic parameters is disallowed. This matches the
// behaviour of trait bounds where `fn foo<T: Ord<&u8>>() {}` is illegal. Though we could change
// elided lifetimes within the type of a const generic parameters to be 'static, like elided
// lifetimes within const/static items.

#![feature(const_generics)]
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Could you give a brief comment at the top explaining why the behavior is how it is? (Contrast and compare with e.g. fn foo<T: Ord<&u8>>() {}.)

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I have added that comment. Though I'm not sure what the behaviour should be in the long term, it was the easiest fix(for the ICE) to just disallow elided lifetimes.

I think the only lifetime const generic arguments can have is 'static, so maybe we should do ''static lifetime elision'(https://doc.rust-lang.org/reference/lifetime-elision.html#static-lifetime-elision), like what happens for const/static items.

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It's better to make this an error for now.
Then we'll be able to introduce a 'static default more explicitly than in some bugfix PR, when const generics are stable and we have more experience with them.
(The 'static default for const items actually went through an RFC.)

//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash

struct A<const N: &u8>;
//~^ ERROR `&` without an explicit lifetime name cannot be used here
trait B {}

impl<const N: &u8> A<N> { //~ ERROR `&` without an explicit lifetime name cannot be used here
fn foo<const M: &u8>(&self) {}
//~^ ERROR `&` without an explicit lifetime name cannot be used here
}

impl<const N: &u8> B for A<N> {}
//~^ ERROR `&` without an explicit lifetime name cannot be used here

fn bar<const N: &u8>() {}
//~^ ERROR `&` without an explicit lifetime name cannot be used here

fn main() {}
40 changes: 40 additions & 0 deletions src/test/ui/const-generics/const-param-elided-lifetime.stderr
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@@ -0,0 +1,40 @@
error[E0637]: `&` without an explicit lifetime name cannot be used here
--> $DIR/const-param-elided-lifetime.rs:9:19
|
LL | struct A<const N: &u8>;
| ^ explicit lifetime name needed here

error[E0637]: `&` without an explicit lifetime name cannot be used here
--> $DIR/const-param-elided-lifetime.rs:13:15
|
LL | impl<const N: &u8> A<N> {
| ^ explicit lifetime name needed here

error[E0637]: `&` without an explicit lifetime name cannot be used here
--> $DIR/const-param-elided-lifetime.rs:14:21
|
LL | fn foo<const M: &u8>(&self) {}
| ^ explicit lifetime name needed here

error[E0637]: `&` without an explicit lifetime name cannot be used here
--> $DIR/const-param-elided-lifetime.rs:18:15
|
LL | impl<const N: &u8> B for A<N> {}
| ^ explicit lifetime name needed here

error[E0637]: `&` without an explicit lifetime name cannot be used here
--> $DIR/const-param-elided-lifetime.rs:21:17
|
LL | fn bar<const N: &u8>() {}
| ^ explicit lifetime name needed here

warning: the feature `const_generics` is incomplete and may cause the compiler to crash
--> $DIR/const-param-elided-lifetime.rs:6:12
|
LL | #![feature(const_generics)]
| ^^^^^^^^^^^^^^
|
= note: `#[warn(incomplete_features)]` on by default

error: aborting due to 5 previous errors