Unrolled build for #143519

Rollup merge of #143519 - mu001999-contrib:dead-code/impl-items, r=petrochenkov

Check assoc consts and tys later like assoc fns

This PR
1. checks assoc consts and tys later like assoc fns
2. marks assoc consts appear in poly-trait-ref live

For assoc consts, considering
```rust
#![deny(dead_code)]

trait Tr { // ERROR trait `Tr` is never used
    const I: Self;
}

struct Foo; //~ ERROR struct `Foo` is never constructed

impl Tr for Foo {
    const I: Self = Foo;
}

fn main() {}
```

Current this will produce unused `I` instead of unused `Tr` and `Foo` ([play](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=e0490d4a2d522cb70437b26e514a3d9c)), because `const I: Self = Foo;` will be added into the worklist at first:
```
error: associated constant `I` is never used
 --> src/main.rs:4:11
  |
3 | trait Tr { // ERROR trait `Tr` is never used
  |       -- associated constant in this trait
4 |     const I: Self;
  |           ^
  |
note: the lint level is defined here
 --> src/main.rs:1:9
  |
1 | #![deny(dead_code)]
  |         ^^^^^^^^^

error: could not compile `playground` (bin "playground") due to 1 previous error
```

This also happens to assoc tys, see the [new test](https://github.com/rust-lang/rust/compare/master...mu001999-contrib:rust:dead-code/impl-items?expand=1#diff-bf45fa403934a31c9d610a073ed2603d885e7e81572e8edf38b7f4e08a1f3531)

Fixes #126729

r? `````@petrochenkov`````

Author: rust-timer

compiler/rustc_passes/src/dead.rs

@@ -18,7 +18,7 @@ use rustc_hir::{self as hir, ImplItem, ImplItemKind, Node, PatKind, QPath, TyKin
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::middle::privacy::Level;
 use rustc_middle::query::Providers;
-use rustc_middle::ty::{self, TyCtxt};
+use rustc_middle::ty::{self, AssocTag, TyCtxt};
 use rustc_middle::{bug, span_bug};
 use rustc_session::lint::builtin::DEAD_CODE;
 use rustc_session::lint::{self, LintExpectationId};
@@ -115,7 +115,10 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
 
     fn handle_res(&mut self, res: Res) {
         match res {
-            Res::Def(DefKind::Const | DefKind::AssocConst | DefKind::TyAlias, def_id) => {
+            Res::Def(
+                DefKind::Const | DefKind::AssocConst | DefKind::AssocTy | DefKind::TyAlias,
+                def_id,
+            ) => {
                 self.check_def_id(def_id);
             }
             _ if self.in_pat => {}
@@ -482,7 +485,7 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
     ) -> bool {
         let trait_def_id = match self.tcx.def_kind(local_def_id) {
             // assoc impl items of traits are live if the corresponding trait items are live
-            DefKind::AssocFn => self
+            DefKind::AssocConst | DefKind::AssocTy | DefKind::AssocFn => self
                 .tcx
                 .associated_item(local_def_id)
                 .trait_item_def_id
@@ -647,6 +650,31 @@ impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> {
 
         self.in_pat = in_pat;
     }
+
+    fn visit_trait_ref(&mut self, t: &'tcx hir::TraitRef<'tcx>) {
+        if let Some(trait_def_id) = t.path.res.opt_def_id()
+            && let Some(segment) = t.path.segments.last()
+            && let Some(args) = segment.args
+        {
+            for constraint in args.constraints {
+                if let Some(local_def_id) = self
+                    .tcx
+                    .associated_items(trait_def_id)
+                    .find_by_ident_and_kind(
+                        self.tcx,
+                        constraint.ident,
+                        AssocTag::Const,
+                        trait_def_id,
+                    )
+                    .and_then(|item| item.def_id.as_local())
+                {
+                    self.worklist.push((local_def_id, ComesFromAllowExpect::No));
+                }
+            }
+        }
+
+        intravisit::walk_trait_ref(self, t);
+    }
 }
 
 fn has_allow_dead_code_or_lang_attr(
@@ -744,18 +772,12 @@ fn check_item<'tcx>(
                 {
                     worklist.push((local_def_id, comes_from_allow));
                 } else if of_trait {
-                    // FIXME: This condition can be removed
-                    // if we support dead check for assoc consts and tys.
-                    if !matches!(tcx.def_kind(local_def_id), DefKind::AssocFn) {
-                        worklist.push((local_def_id, ComesFromAllowExpect::No));
-                    } else {
-                        // We only care about associated items of traits,
-                        // because they cannot be visited directly,
-                        // so we later mark them as live if their corresponding traits
-                        // or trait items and self types are both live,
-                        // but inherent associated items can be visited and marked directly.
-                        unsolved_items.push((id, local_def_id));
-                    }
+                    // We only care about associated items of traits,
+                    // because they cannot be visited directly,
+                    // so we later mark them as live if their corresponding traits
+                    // or trait items and self types are both live,
+                    // but inherent associated items can be visited and marked directly.
+                    unsolved_items.push((id, local_def_id));
                 }
             }
         }
@@ -791,15 +813,14 @@ fn check_trait_item(
     worklist: &mut Vec<(LocalDefId, ComesFromAllowExpect)>,
     id: hir::TraitItemId,
 ) {
-    use hir::TraitItemKind::{Const, Fn};
-    if matches!(tcx.def_kind(id.owner_id), DefKind::AssocConst | DefKind::AssocFn) {
-        let trait_item = tcx.hir_trait_item(id);
-        if matches!(trait_item.kind, Const(_, Some(_)) | Fn(..))
-            && let Some(comes_from_allow) =
-                has_allow_dead_code_or_lang_attr(tcx, trait_item.owner_id.def_id)
-        {
-            worklist.push((trait_item.owner_id.def_id, comes_from_allow));
-        }
+    use hir::TraitItemKind::{Const, Fn, Type};
+
+    let trait_item = tcx.hir_trait_item(id);
+    if matches!(trait_item.kind, Const(_, Some(_)) | Type(_, Some(_)) | Fn(..))
+        && let Some(comes_from_allow) =
+            has_allow_dead_code_or_lang_attr(tcx, trait_item.owner_id.def_id)
+    {
+        worklist.push((trait_item.owner_id.def_id, comes_from_allow));
     }
 }
 
@@ -1163,6 +1184,7 @@ impl<'tcx> DeadVisitor<'tcx> {
         }
         match self.tcx.def_kind(def_id) {
             DefKind::AssocConst
+            | DefKind::AssocTy
             | DefKind::AssocFn
             | DefKind::Fn
             | DefKind::Static { .. }

tests/ui/associated-consts/equality-unused-issue-126729.rs

@@ -0,0 +1,44 @@
+//@ check-pass
+
+#![feature(associated_const_equality)]
+#![deny(dead_code)]
+
+trait Tr {
+    const I: i32;
+}
+
+impl Tr for () {
+    const I: i32 = 1;
+}
+
+fn foo() -> impl Tr<I = 1> {}
+
+trait Tr2 {
+    const J: i32;
+    const K: i32;
+}
+
+impl Tr2 for () {
+    const J: i32 = 1;
+    const K: i32 = 1;
+}
+
+fn foo2() -> impl Tr2<J = 1, K = 1> {}
+
+mod t {
+    pub trait Tr3 {
+        const L: i32;
+    }
+
+    impl Tr3 for () {
+        const L: i32 = 1;
+    }
+}
+
+fn foo3() -> impl t::Tr3<L = 1> {}
+
+fn main() {
+    foo();
+    foo2();
+    foo3();
+}

tests/ui/const-generics/cross_crate_complex.rs

@@ -11,6 +11,7 @@ async fn foo() {
     async_in_foo(async_out_foo::<4>().await).await;
 }
 
+#[allow(dead_code)]
 struct Faz<const N: usize>;
 
 impl<const N: usize> Foo<N> for Faz<N> {}

tests/ui/generic-associated-types/missing-bounds.fixed

@@ -1,5 +1,7 @@
 //@ run-rustfix
 
+#![allow(dead_code)]
+
 use std::ops::Add;
 
 struct A<B>(B);

tests/ui/generic-associated-types/missing-bounds.rs

@@ -1,5 +1,7 @@
 //@ run-rustfix
 
+#![allow(dead_code)]
+
 use std::ops::Add;
 
 struct A<B>(B);

tests/ui/generic-associated-types/missing-bounds.stderr

@@ -1,5 +1,5 @@
 error: equality constraints are not yet supported in `where` clauses
-  --> $DIR/missing-bounds.rs:37:33
+  --> $DIR/missing-bounds.rs:39:33
    |
 LL | impl<B: Add> Add for E<B> where <B as Add>::Output = B {
    |                                 ^^^^^^^^^^^^^^^^^^^^^^ not supported
@@ -12,7 +12,7 @@ LL + impl<B: Add> Add for E<B> where B: Add<Output = B> {
    |
 
 error[E0308]: mismatched types
-  --> $DIR/missing-bounds.rs:11:11
+  --> $DIR/missing-bounds.rs:13:11
    |
 LL | impl<B> Add for A<B> where B: Add {
    |      - expected this type parameter
@@ -25,14 +25,14 @@ LL |         A(self.0 + rhs.0)
    = note: expected type parameter `B`
              found associated type `<B as Add>::Output`
 help: the type constructed contains `<B as Add>::Output` due to the type of the argument passed
-  --> $DIR/missing-bounds.rs:11:9
+  --> $DIR/missing-bounds.rs:13:9
    |
 LL |         A(self.0 + rhs.0)
    |         ^^--------------^
    |           |
    |           this argument influences the type of `A`
 note: tuple struct defined here
-  --> $DIR/missing-bounds.rs:5:8
+  --> $DIR/missing-bounds.rs:7:8
    |
 LL | struct A<B>(B);
    |        ^
@@ -42,7 +42,7 @@ LL | impl<B> Add for A<B> where B: Add<Output = B> {
    |                                  ++++++++++++
 
 error[E0308]: mismatched types
-  --> $DIR/missing-bounds.rs:21:14
+  --> $DIR/missing-bounds.rs:23:14
    |
 LL | impl<B: Add> Add for C<B> {
    |      - expected this type parameter
@@ -55,7 +55,7 @@ LL |         Self(self.0 + rhs.0)
    = note: expected type parameter `B`
              found associated type `<B as Add>::Output`
 note: tuple struct defined here
-  --> $DIR/missing-bounds.rs:15:8
+  --> $DIR/missing-bounds.rs:17:8
    |
 LL | struct C<B>(B);
    |        ^
@@ -65,7 +65,7 @@ LL | impl<B: Add<Output = B>> Add for C<B> {
    |            ++++++++++++
 
 error[E0369]: cannot add `B` to `B`
-  --> $DIR/missing-bounds.rs:31:21
+  --> $DIR/missing-bounds.rs:33:21
    |
 LL |         Self(self.0 + rhs.0)
    |              ------ ^ ----- B
@@ -78,7 +78,7 @@ LL | impl<B: std::ops::Add<Output = B>> Add for D<B> {
    |       +++++++++++++++++++++++++++
 
 error[E0308]: mismatched types
-  --> $DIR/missing-bounds.rs:42:14
+  --> $DIR/missing-bounds.rs:44:14
    |
 LL | impl<B: Add> Add for E<B> where <B as Add>::Output = B {
    |      - expected this type parameter
@@ -91,7 +91,7 @@ LL |         Self(self.0 + rhs.0)
    = note: expected type parameter `B`
              found associated type `<B as Add>::Output`
 note: tuple struct defined here
-  --> $DIR/missing-bounds.rs:35:8
+  --> $DIR/missing-bounds.rs:37:8
    |
 LL | struct E<B>(B);
    |        ^

tests/ui/lint/dead-code/unused-trait-with-assoc-const.rs

@@ -0,0 +1,13 @@
+#![deny(dead_code)]
+
+trait Tr { //~ ERROR trait `Tr` is never used
+    const I: Self;
+}
+
+struct Foo; //~ ERROR struct `Foo` is never constructed
+
+impl Tr for Foo {
+    const I: Self = Foo;
+}
+
+fn main() {}

tests/ui/lint/dead-code/unused-trait-with-assoc-const.stderr

@@ -0,0 +1,20 @@
+error: trait `Tr` is never used
+  --> $DIR/unused-trait-with-assoc-const.rs:3:7
+   |
+LL | trait Tr {
+   |       ^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-trait-with-assoc-const.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: struct `Foo` is never constructed
+  --> $DIR/unused-trait-with-assoc-const.rs:7:8
+   |
+LL | struct Foo;
+   |        ^^^
+
+error: aborting due to 2 previous errors
+

tests/ui/lint/dead-code/unused-trait-with-assoc-ty.rs

@@ -0,0 +1,11 @@
+#![deny(dead_code)]
+
+struct T1; //~ ERROR struct `T1` is never constructed
+
+trait Foo { type Unused; } //~ ERROR trait `Foo` is never used
+impl Foo for T1 { type Unused = Self; }
+
+pub trait Bar { type Used; }
+impl Bar for T1 { type Used = Self; }
+
+fn main() {}

tests/ui/lint/dead-code/unused-trait-with-assoc-ty.stderr

@@ -0,0 +1,20 @@
+error: struct `T1` is never constructed
+  --> $DIR/unused-trait-with-assoc-ty.rs:3:8
+   |
+LL | struct T1;
+   |        ^^
+   |
+note: the lint level is defined here
+  --> $DIR/unused-trait-with-assoc-ty.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: trait `Foo` is never used
+  --> $DIR/unused-trait-with-assoc-ty.rs:5:7
+   |
+LL | trait Foo { type Unused; }
+   |       ^^^
+
+error: aborting due to 2 previous errors
+