Auto merge of rust-lang#138176 - compiler-errors:rigid-sized-obl, r=<try>

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 obligation, even if it has nested obligations.

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. If it's false, then we prefer it over param-env candidates:

https://github.com/rust-lang/rust/blob/2b4694a69804f89ff9d47d1a427f72c876f7f44c/compiler/rustc_trait_selection/src/traits/select/mod.rs#L1804-L1815

Otherwise we prefer param-env candidates over it:
https://github.com/rust-lang/rust/blob/2b4694a69804f89ff9d47d1a427f72c876f7f44c/compiler/rustc_trait_selection/src/traits/select/mod.rs#L1847-L1866

I assume that the motivation for this behavior is to prefer built-in candidates if they have no nested obligations, but we can't as confidently prefer built-in candidates when they have where-clauses since we have no guarantee that the nested obligations for the built-in candidate are actually satisfyable (especially considering regions).

However, for `Sized` candidates in particular, unlike the example above we never end up bottoming-out in a user-written impl, since *all* `Sized` impls are built-in. Thus, we don't have this problem, and preferring param-env candidates actually 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.

r? lcnr

Author: bors

compiler/rustc_middle/src/traits/select.rs

@@ -95,10 +95,16 @@ pub type EvaluationCache<'tcx, ENV> = Cache<(ENV, ty::PolyTraitPredicate<'tcx>),
 /// parameter environment.
 #[derive(PartialEq, Eq, Debug, Clone, TypeVisitable)]
 pub enum SelectionCandidate<'tcx> {
+    /// A built-in implementation for the `Sized` trait. This is preferred
+    /// over all other candidates.
+    SizedCandidate {
+        has_nested: bool,
+    },
+
     /// A builtin implementation for some specific traits, used in cases
     /// where we cannot rely an ordinary library implementations.
     ///
-    /// The most notable examples are `sized`, `Copy` and `Clone`. This is also
+    /// The most notable examples are `Copy` and `Clone`. This is also
     /// used for the `DiscriminantKind` and `Pointee` trait, both of which have
     /// an associated type.
     BuiltinCandidate {

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) {
@@ -1059,6 +1056,27 @@ 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>,

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)

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1801,17 +1801,14 @@ 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);
+            return Some(sized_candidate.candidate.clone());
         }
 
         // Before we consider where-bounds, we have to deduplicate them here and also
@@ -1940,7 +1937,8 @@ 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 { .. }