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missing_const_for_fn: consider constness of instance #14759

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missing_const_for_fn: consider constness of instance #14759

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9 changes: 7 additions & 2 deletions clippy_utils/src/qualify_min_const_fn.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -18,7 +18,7 @@ use rustc_middle::mir::{
};
use rustc_middle::traits::{BuiltinImplSource, ImplSource, ObligationCause};
use rustc_middle::ty::adjustment::PointerCoercion;
use rustc_middle::ty::{self, GenericArgKind, TraitRef, Ty, TyCtxt};
use rustc_middle::ty::{self, GenericArgKind, Instance, TraitRef, Ty, TyCtxt};
use rustc_span::Span;
use rustc_span::symbol::sym;
use rustc_trait_selection::traits::{ObligationCtxt, SelectionContext};
Expand Down Expand Up @@ -349,7 +349,12 @@ fn check_terminator<'tcx>(
}
| TerminatorKind::TailCall { func, args, fn_span: _ } => {
let fn_ty = func.ty(body, cx.tcx);
if let ty::FnDef(fn_def_id, _) = *fn_ty.kind() {
if let ty::FnDef(fn_def_id, fn_substs) = *fn_ty.kind() {
let fn_def_id = match Instance::try_resolve(cx.tcx, cx.typing_env(), fn_def_id, fn_substs) {
Ok(Some(fn_inst)) => fn_inst.def_id(),
Ok(None) => return Err((span, format!("cannot resolve instance for {func:?}").into())),
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Not being able to resolve to an instance doesn't mean the function can't be const. Not sure what the syntax is but there should be some way to constrain the type to implement the trait as a const trait.

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I don't see how to do that.

I think this is still better to get false negatives rather than false positives, especially when none of the tested cases fails because of this change. There were also no lintcheck misses.

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I think the current form of const traits is recentish so I wouldn't expect many uses. Can you just add a comment about this doesn't handle where predicates?

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I think the current form of const traits is recentish so I wouldn't expect many uses. Can you just add a comment about this doesn't handle where predicates?

I don't understand how where predicates could be used, and where in this situation, so I have a hard time understanding where to put this comment and what to put in it.

Err(_) => return Err((span, format!("error during instance resolution of {func:?}").into())),
};
if !is_stable_const_fn(cx, fn_def_id, msrv) {
return Err((
span,
Expand Down
28 changes: 28 additions & 0 deletions tests/ui/missing_const_for_fn/const_trait.fixed
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@@ -0,0 +1,28 @@
#![feature(const_trait_impl)]
#![warn(clippy::missing_const_for_fn)]

// Reduced test case from https://github.com/rust-lang/rust-clippy/issues/14658

#[const_trait]
trait ConstTrait {
fn method(self);
}

impl ConstTrait for u32 {
fn method(self) {}
}

impl const ConstTrait for u64 {
fn method(self) {}
}

fn cannot_be_const() {
0u32.method();
}

//~v missing_const_for_fn
const fn can_be_const() {
0u64.method();
}

fn main() {}
28 changes: 28 additions & 0 deletions tests/ui/missing_const_for_fn/const_trait.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,28 @@
#![feature(const_trait_impl)]
#![warn(clippy::missing_const_for_fn)]

// Reduced test case from https://github.com/rust-lang/rust-clippy/issues/14658

#[const_trait]
trait ConstTrait {
fn method(self);
}

impl ConstTrait for u32 {
fn method(self) {}
}

impl const ConstTrait for u64 {
fn method(self) {}
}

fn cannot_be_const() {
0u32.method();
}

//~v missing_const_for_fn
fn can_be_const() {
0u64.method();
}

fn main() {}
17 changes: 17 additions & 0 deletions tests/ui/missing_const_for_fn/const_trait.stderr
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@@ -0,0 +1,17 @@
error: this could be a `const fn`
--> tests/ui/missing_const_for_fn/const_trait.rs:24:1
|
LL | / fn can_be_const() {
LL | | 0u64.method();
LL | | }
| |_^
|
= note: `-D clippy::missing-const-for-fn` implied by `-D warnings`
= help: to override `-D warnings` add `#[allow(clippy::missing_const_for_fn)]`
help: make the function `const`
|
LL | const fn can_be_const() {
| +++++

error: aborting due to 1 previous error

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