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Do not resolve associated const when there is no provided value #99449

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Jul 24, 2022
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Do not resolve associated const when there is no provided value #99449

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15 changes: 12 additions & 3 deletions compiler/rustc_trait_selection/src/traits/const_evaluatable.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -185,14 +185,20 @@ pub fn is_const_evaluatable<'cx, 'tcx>(
}
let concrete = infcx.const_eval_resolve(param_env, uv.expand(), Some(span));
match concrete {
Err(ErrorHandled::TooGeneric) => Err(if !uv.has_infer_types_or_consts() {
Err(ErrorHandled::TooGeneric) => Err(if uv.has_infer_types_or_consts() {
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TooGeneric here is a bit of a misnomer, since it really should be "not concrete" or something.

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🤔 it's weird that we have to actually deal with TooGeneric here 🤔

this should already be guarded against by the uniify_failure_kind. Do any tests change if you always use

let guar = infcx.tcx.sess.delay_span_bug(
                    span,
                    format!("Missing value for constant, but no error reported?"),
                );
                NotConstEvaluatable::Error(guar)

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@compiler-errors compiler-errors Jul 19, 2022 •
edited
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I changed this to always delay a bug, and not specially deal with TooGeneric in the case of infer/param types:

        match concrete {
            Err(ErrorHandled::TooGeneric) => Err({
                let guar = infcx.tcx.sess.delay_span_bug(
                    span,
                    format!("Missing value for constant, but no error reported?"),
                );
                NotConstEvaluatable::Error(guar)
            }),

and I get two ICEs, see this gist: https://gist.github.com/compiler-errors/146cd973d15ddc7fcfee6ff28a9ac5e0

Was that what you were asking me to try, @lcnr?

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yeah, so InferCtxt::const_eval_resolve is not not doing what we need for this

rust/compiler/rustc_infer/src/infer/mod.rs

Lines 1664 to 1671 in 03d488b

if substs.has_infer_types_or_consts() {
let ac = AbstractConst::new(self.tcx, unevaluated.shrink());
if let Ok(None) = ac {
substs = InternalSubsts::identity_for_item(self.tcx, unevaluated.def.did);
} else {
return Err(ErrorHandled::TooGeneric);
}
}

I guess what we should do is:
match on AbstractConst::new(self.tcx, unevaluated.shrink());

Ok(None) is already correct.
Err(emitted) -> NotConstEvaluatable::Error(emitted)
Ok(Some(x)) -> the inference vars aren't used in the anon const, replace them with placeholders

feel free to do that in this PR, otherwise r=me

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@lcnr: I'm gonna do it in a follow-up PR, since this solution doesn't really rely on that change, does it?

NotConstEvaluatable::MentionsInfer
} else if uv.has_param_types_or_consts() {
infcx
.tcx
.sess
.delay_span_bug(span, &format!("unexpected `TooGeneric` for {:?}", uv));
NotConstEvaluatable::MentionsParam
} else {
NotConstEvaluatable::MentionsInfer
let guar = infcx.tcx.sess.delay_span_bug(
span,
format!("Missing value for constant, but no error reported?"),
);
NotConstEvaluatable::Error(guar)
}),
Err(ErrorHandled::Linted) => {
let reported = infcx
Expand Down Expand Up @@ -240,8 +246,11 @@ pub fn is_const_evaluatable<'cx, 'tcx>(

Err(ErrorHandled::TooGeneric) => Err(if uv.has_infer_types_or_consts() {
NotConstEvaluatable::MentionsInfer
} else {
} else if uv.has_param_types_or_consts() {
NotConstEvaluatable::MentionsParam
} else {
let guar = infcx.tcx.sess.delay_span_bug(span, format!("Missing value for constant, but no error reported?"));
NotConstEvaluatable::Error(guar)
}),
Err(ErrorHandled::Linted) => {
let reported =
Expand Down
5 changes: 5 additions & 0 deletions compiler/rustc_ty_utils/src/instance.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -280,6 +280,11 @@ fn resolve_associated_item<'tcx>(
return Ok(None);
}

// If the item does not have a value, then we cannot return an instance.
compiler-errors marked this conversation as resolved.
Show resolved Hide resolved
if !leaf_def.item.defaultness.has_value() {
return Ok(None);
}

let substs = tcx.erase_regions(substs);

// Check if we just resolved an associated `const` declaration from
Expand Down
1 change: 0 additions & 1 deletion src/test/ui/const-generics/issues/issue-86530.rs
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Expand Up @@ -15,7 +15,6 @@ where
fn unit_literals() {
z(" ");
//~^ ERROR: the trait bound `&str: X` is not satisfied
//~| ERROR: unconstrained generic constant
}

fn main() {}
18 changes: 1 addition & 17 deletions src/test/ui/const-generics/issues/issue-86530.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -15,22 +15,6 @@ LL | where
LL | T: X,
| ^ required by this bound in `z`

error: unconstrained generic constant
--> $DIR/issue-86530.rs:16:5
|
LL | z(" ");
| ^
|
= help: try adding a `where` bound using this expression: `where [(); T::Y]:`
note: required by a bound in `z`
--> $DIR/issue-86530.rs:11:10
|
LL | fn z<T>(t: T)
| - required by a bound in this
...
LL | [(); T::Y]: ,
| ^^^^ required by this bound in `z`

error: aborting due to 2 previous errors
error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
15 changes: 15 additions & 0 deletions src/test/ui/const-generics/issues/issue-98629.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
#![feature(const_trait_impl)]

trait Trait {
const N: usize;
}

impl const Trait for i32 {}
//~^ ERROR not all trait items implemented, missing: `N`

fn f()
where
[(); <i32 as Trait>::N]:,
{}

fn main() {}
12 changes: 12 additions & 0 deletions src/test/ui/const-generics/issues/issue-98629.stderr
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Original file line number Diff line number Diff line change
@@ -0,0 +1,12 @@
error[E0046]: not all trait items implemented, missing: `N`
--> $DIR/issue-98629.rs:7:1
|
LL | const N: usize;
| -------------- `N` from trait
...
LL | impl const Trait for i32 {}
| ^^^^^^^^^^^^^^^^^^^^^^^^ missing `N` in implementation

error: aborting due to previous error

For more information about this error, try `rustc --explain E0046`.
1 change: 0 additions & 1 deletion src/test/ui/issues/issue-77919.rs
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@@ -1,6 +1,5 @@
fn main() {
[1; <Multiply<Five, Five>>::VAL];
//~^ ERROR: constant expression depends on a generic parameter
}
trait TypeVal<T> {
const VAL: T;
Expand Down
16 changes: 4 additions & 12 deletions src/test/ui/issues/issue-77919.stderr
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@@ -1,5 +1,5 @@
error[E0412]: cannot find type `PhantomData` in this scope
--> $DIR/issue-77919.rs:10:9
--> $DIR/issue-77919.rs:9:9
|
LL | _n: PhantomData,
| ^^^^^^^^^^^ not found in this scope
Expand All @@ -10,31 +10,23 @@ LL | use std::marker::PhantomData;
|

error[E0412]: cannot find type `VAL` in this scope
--> $DIR/issue-77919.rs:12:63
--> $DIR/issue-77919.rs:11:63
|
LL | impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}
| - ^^^ not found in this scope
| |
| help: you might be missing a type parameter: `, VAL`

error[E0046]: not all trait items implemented, missing: `VAL`
--> $DIR/issue-77919.rs:12:1
--> $DIR/issue-77919.rs:11:1
|
LL | const VAL: T;
| ------------ `VAL` from trait
...
LL | impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `VAL` in implementation

error: constant expression depends on a generic parameter
--> $DIR/issue-77919.rs:2:9
|
LL | [1; <Multiply<Five, Five>>::VAL];
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: this may fail depending on what value the parameter takes

error: aborting due to 4 previous errors
error: aborting due to 3 previous errors

Some errors have detailed explanations: E0046, E0412.
For more information about an error, try `rustc --explain E0046`.
10 changes: 1 addition & 9 deletions src/tools/clippy/tests/ui/crashes/ice-6252.stderr
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Expand Up @@ -30,15 +30,7 @@ LL | const VAL: T;
LL | impl<N, M> TypeVal<usize> for Multiply<N, M> where N: TypeVal<VAL> {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `VAL` in implementation

error: constant expression depends on a generic parameter
--> $DIR/ice-6252.rs:13:9
|
LL | [1; <Multiply<Five, Five>>::VAL];
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: this may fail depending on what value the parameter takes

error: aborting due to 4 previous errors
error: aborting due to 3 previous errors

Some errors have detailed explanations: E0046, E0412.
For more information about an error, try `rustc --explain E0046`.