Rollup merge of #138176 - compiler-errors:rigid-sized-obl, r=lcnr

Prefer built-in sized impls (and only sized impls) for rigid types always

This PR changes the confirmation of `Sized` obligations to unconditionally prefer the built-in impl, even if it has nested obligations. This also changes all other built-in impls (namely, `Copy`/`Clone`/`DiscriminantKind`/`Pointee`) to *not* prefer built-in impls over param-env impls. This aligns the old solver with the behavior of the new solver.

---

In the old solver, we register many builtin candidates with the `BuiltinCandidate { has_nested: bool }` candidate kind. The precedence this candidate takes over other candidates is based on the `has_nested` field. We only prefer builtin impls over param-env candidates if `has_nested` is `false`

https://github.com/rust-lang/rust/blob/2b4694a69804f89ff9d47d1a427f72c876f7f44c/compiler/rustc_trait_selection/src/traits/select/mod.rs#L1804-L1866

Preferring param-env candidates when the builtin candidate has nested obligations *still* ends up leading to detrimental inference guidance, like:

```rust
fn hello<T>() where (T,): Sized {
    let x: (_,) = Default::default();
    // ^^ The `Sized` obligation on the variable infers `_ = T`.
    let x: (i32,) = x;
    // We error here, both a type mismatch and also b/c `T: Default` doesn't hold.
}
```

Therefore this PR adjusts the candidate precedence of `Sized` obligations by making them a distinct candidate kind and unconditionally preferring them over all other candidate kinds.

Special-casing `Sized` this way is necessary as there are a lot of traits with a `Sized` super-trait bound, so a `&'a str: From<T>` where-bound results in an elaborated `&'a str: Sized` bound. People tend to not add explicit where-clauses which overlap with builtin impls, so this tends to not be an issue for other traits.

We don't know of any tests/crates which need preference for other builtin traits. As this causes builtin impls to diverge from user-written impls we would like to minimize the affected traits. Otherwise e.g. moving impls for tuples to std by using variadic generics would be a breaking change. For other builtin impls it's also easier for the preference of builtin impls over where-bounds to result in issues.

---

There are two ways preferring builtin impls over where-bounds can be incorrect and undesirable:
- applying the builtin impl results in undesirable region constraints. E.g. if only `MyType<'static>` implements `Copy` then a goal like `(MyType<'a>,): Copy` would require `'a == 'static` so we must not prefer it over a `(MyType<'a>,): Copy` where-bound
   - this is mostly not an issue for `Sized` as all `Sized` impls are builtin and don't add any region constraints not already required for the type to be well-formed
   - however, even with `Sized` this is still an issue if a nested goal also gets proven via a where-bound: [playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=30377da5b8a88f654884ab4ebc72f52b)
- if the builtin impl has associated types, we should not prefer it over where-bounds when normalizing that associated type. This can result in normalization adding more region constraints than just proving trait bounds. https://github.com/rust-lang/rust/issues/133044
  - not an issue for `Sized` as it doesn't have associated types.

r? lcnr

3 files changed, 43 insertions, 15 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index 4cfd8149b1e..d15c9afef3a 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -86,10 +86,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // `Pointee` is automatically implemented for every type.
                 candidates.vec.push(BuiltinCandidate { has_nested: false });
             } else if tcx.is_lang_item(def_id, LangItem::Sized) {
-                // Sized is never implementable by end-users, it is
-                // always automatically computed.
-                let sized_conditions = self.sized_conditions(obligation);
-                self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates);
+                self.assemble_builtin_sized_candidate(obligation, &mut candidates);
             } else if tcx.is_lang_item(def_id, LangItem::Unsize) {
                 self.assemble_candidates_for_unsizing(obligation, &mut candidates);
             } else if tcx.is_lang_item(def_id, LangItem::Destruct) {
@@ -1061,6 +1058,27 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// Assembles the trait which are built-in to the language itself:
     /// `Copy`, `Clone` and `Sized`.
     #[instrument(level = "debug", skip(self, candidates))]
+    fn assemble_builtin_sized_candidate(
+        &mut self,
+        obligation: &PolyTraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) {
+        match self.sized_conditions(obligation) {
+            BuiltinImplConditions::Where(nested) => {
+                candidates
+                    .vec
+                    .push(SizedCandidate { has_nested: !nested.skip_binder().is_empty() });
+            }
+            BuiltinImplConditions::None => {}
+            BuiltinImplConditions::Ambiguous => {
+                candidates.ambiguous = true;
+            }
+        }
+    }
+
+    /// Assembles the trait which are built-in to the language itself:
+    /// e.g. `Copy` and `Clone`.
+    #[instrument(level = "debug", skip(self, candidates))]
     fn assemble_builtin_bound_candidates(
         &mut self,
         conditions: BuiltinImplConditions<'tcx>,
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index a66c958c109..630241725fd 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -40,6 +40,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         candidate: SelectionCandidate<'tcx>,
     ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
         let mut impl_src = match candidate {
+            SizedCandidate { has_nested } => {
+                let data = self.confirm_builtin_candidate(obligation, has_nested);
+                ImplSource::Builtin(BuiltinImplSource::Misc, data)
+            }
+
             BuiltinCandidate { has_nested } => {
                 let data = self.confirm_builtin_candidate(obligation, has_nested);
                 ImplSource::Builtin(BuiltinImplSource::Misc, data)
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index e439df76cd4..0679dbf1296 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -1801,17 +1801,21 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             return Some(candidates.pop().unwrap().candidate);
         }
 
-        // We prefer trivial builtin candidates, i.e. builtin impls without any nested
-        // requirements, over all others. This is a fix for #53123 and prevents winnowing
-        // from accidentally extending the lifetime of a variable.
-        let mut trivial_builtin = candidates
-            .iter()
-            .filter(|c| matches!(c.candidate, BuiltinCandidate { has_nested: false }));
-        if let Some(_trivial) = trivial_builtin.next() {
-            // There should only ever be a single trivial builtin candidate
+        // We prefer `Sized` candidates over everything.
+        let mut sized_candidates =
+            candidates.iter().filter(|c| matches!(c.candidate, SizedCandidate { has_nested: _ }));
+        if let Some(sized_candidate) = sized_candidates.next() {
+            // There should only ever be a single sized candidate
             // as they would otherwise overlap.
-            debug_assert_eq!(trivial_builtin.next(), None);
-            return Some(BuiltinCandidate { has_nested: false });
+            debug_assert_eq!(sized_candidates.next(), None);
+            // Only prefer the built-in `Sized` candidate if its nested goals are certain.
+            // Otherwise, we may encounter failure later on if inference causes this candidate
+            // to not hold, but a where clause would've applied instead.
+            if sized_candidate.evaluation.must_apply_modulo_regions() {
+                return Some(sized_candidate.candidate.clone());
+            } else {
+                return None;
+            }
         }
 
         // Before we consider where-bounds, we have to deduplicate them here and also
@@ -1940,7 +1944,8 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             // Don't use impl candidates which overlap with other candidates.
             // This should pretty much only ever happen with malformed impls.
             if candidates.iter().all(|c| match c.candidate {
-                BuiltinCandidate { has_nested: _ }
+                SizedCandidate { has_nested: _ }
+                | BuiltinCandidate { has_nested: _ }
                 | TransmutabilityCandidate
                 | AutoImplCandidate
                 | ClosureCandidate { .. }