Lint unused associated types

[mu001999]

This PR lints unused associated types, like associated consts and fns.

Different from assoc consts and fns, associated types correspond to QPath::TypeRelative, and Res::Err will be obtained in HIR. Therefore, it is necessary to traverse rustc_middle::ty::Ty to help propagate liveness to the corresponding associated type.

Fixes #127673
Fixes #110332

[rustbot]

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[BoxyUwU]

does this handle assoc types used only from item signatures and not from within bodies?

[rustbot]

Some changes occurred in tests/ui/sanitizer

cc @rcvalle

[fmease]

does this handle assoc types used only from item signatures and not from within bodies?

This also handles assoc types used from within bodies, by checking type of expr,

She meant the exact opposite. Assoc tys only used in non-bodies (aka "item signatures", i.e., places where we don't infer types, i.e., not fn or const bodies, but e.g., inside ADTs, type aliases, impl headers, etc.).

[mu001999]

@fmease Hi, I add some tests for such cases but I'm not sure they are right or good enough. Could you have a look or give some examples? Thanks!

[fmease]

Personally speaking, I was thinking of

trait Trait { type Ty; }

impl Trait for () { type Ty = (); }

pub struct Wrap(<() as Trait>::Ty); // <- use of QPath::Resolved `Ty` in a non-body only

impl Wrap {
    pub fn live(self) { _ = self.0; }
}

or

trait Trait { type Ty; }

impl Trait for () { type Ty = (); }

pub struct Wrap(Inner<()>);
struct Inner<T: Trait>(T::Ty); // <- use of QPath::TypeRelative `Ty` in a non-body only

impl Wrap {
    pub fn live(self) { _ = self.0; }
}

I'm sure my examples can be simplified. Also, I haven't really thought much about all of this yet as I'm just drive-by reviewing, so feel free to correct me anywhere :)

[mu001999]

@fmease Added and passed.

But current implementation for struct fields is special. Simply speaking, types of fields in used struct will be marked live directly whether the field self is used or not.

The detail is described in https://github.com/rust-lang/rust/pull/148654/files#diff-719b4f36bbbf3b3bf48683429408f5d27e5633f70c1144a93554bd536d5bf655R753-R764

[mu001999]

does this handle assoc types used only from item signatures and not from within bodies?

Yes, such assoc types are handled by visiting predicates of the source items.

[mu001999]

@fmease I test and find this also fixes #110332

[fee1-dead]

I am not confident enough that this code works well enough, I am worried about the false positives. E.g., what about projected types on impls:

pub struct A;

trait B {
    type Assoc;
}

impl B for A {
    type Assoc = A;
}

trait C {}

impl C for <A as B>::Assoc {}

What about types used in free-consts, e.g. const X: <A as B>::Assoc = A;?

This either needs to have solid coverage or land as an allowed-by-default lint for now.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[mu001999]

@fee1-dead Added test cases for projectd types on impl and free const.