Auto merge of rust-lang#134185 - compiler-errors:impl-trait-in-bindings, r=oli-obk

(Re-)Implement `impl_trait_in_bindings`

This reimplements the `impl_trait_in_bindings` feature for local bindings.

"`impl Trait` in bindings" serve as a form of *trait* ascription, where the type basically functions as an infer var but additionally registering the `impl Trait`'s trait bounds for the infer type. These trait bounds can be used to enforce that predicates hold, and can guide inference (e.g. for closure signature inference):

```rust
let _: impl Fn(&u8) -> &u8 = |x| x;
```

They are implemented as an additional set of bounds that are registered when the type is lowered during typeck, and then these bounds are tied to a given `CanonicalUserTypeAscription` for borrowck. We enforce these `CanonicalUserTypeAscription` bounds during borrowck to make sure that the `impl Trait` types are sensitive to lifetimes:

```rust
trait Static: 'static {}
impl<T> Static for T where T: 'static {}

let local = 1;
let x: impl Static = &local;
//~^ ERROR `local` does not live long enough
```

r? oli-obk

cc rust-lang#63065

---

Why can't we just use TAIT inference or something? Well, TAITs in bodies have the problem that they cannot reference lifetimes local to a body. For example:

```rust
type TAIT = impl Display;
let local = 0;
let x: TAIT = &local;
//~^ ERROR `local` does not live long enough
```

That's because TAITs requires us to do *opaque type inference* which is pretty strict, since we need to remap all of the lifetimes of the hidden type to universal regions. This is simply not possible here.

---

I consider this part of the "impl trait everywhere" experiment. I'm not certain if this needs yet another lang team experiment.

Author: bors

compiler/rustc_ast_lowering/src/block.rs

@@ -1,5 +1,6 @@
 use rustc_ast::{Block, BlockCheckMode, Local, LocalKind, Stmt, StmtKind};
 use rustc_hir as hir;
+use rustc_span::sym;
 use smallvec::SmallVec;
 
 use crate::{ImplTraitContext, ImplTraitPosition, LoweringContext};
@@ -82,11 +83,21 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         (self.arena.alloc_from_iter(stmts), expr)
     }
 
+    /// Return an `ImplTraitContext` that allows impl trait in bindings if
+    /// the feature gate is enabled, or issues a feature error if it is not.
+    fn impl_trait_in_bindings_ctxt(&self, position: ImplTraitPosition) -> ImplTraitContext {
+        if self.tcx.features().impl_trait_in_bindings() {
+            ImplTraitContext::InBinding
+        } else {
+            ImplTraitContext::FeatureGated(position, sym::impl_trait_in_bindings)
+        }
+    }
+
     fn lower_local(&mut self, l: &Local) -> &'hir hir::LetStmt<'hir> {
-        let ty = l
-            .ty
-            .as_ref()
-            .map(|t| self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Variable)));
+        // Let statements are allowed to have impl trait in bindings.
+        let ty = l.ty.as_ref().map(|t| {
+            self.lower_ty(t, self.impl_trait_in_bindings_ctxt(ImplTraitPosition::Variable))
+        });
         let init = l.kind.init().map(|init| self.lower_expr(init));
         let hir_id = self.lower_node_id(l.id);
         let pat = self.lower_pat(&l.pat);

compiler/rustc_ast_lowering/src/lib.rs

@@ -260,6 +260,13 @@ enum ImplTraitContext {
     /// equivalent to a new opaque type like `type T = impl Debug; fn foo() -> T`.
     ///
     OpaqueTy { origin: hir::OpaqueTyOrigin<LocalDefId> },
+
+    /// Treat `impl Trait` as a "trait ascription", which is like a type
+    /// variable but that also enforces that a set of trait goals hold.
+    ///
+    /// This is useful to guide inference for unnameable types.
+    InBinding,
+
     /// `impl Trait` is unstably accepted in this position.
     FeatureGated(ImplTraitPosition, Symbol),
     /// `impl Trait` is not accepted in this position.
@@ -1327,6 +1334,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         }
                         path
                     }
+                    ImplTraitContext::InBinding => {
+                        hir::TyKind::TraitAscription(self.lower_param_bounds(bounds, itctx))
+                    }
                     ImplTraitContext::FeatureGated(position, feature) => {
                         let guar = self
                             .tcx

compiler/rustc_borrowck/src/type_check/canonical.rs

@@ -275,7 +275,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         user_ty: ty::UserType<'tcx>,
         span: Span,
     ) {
-        let ty::UserType::Ty(user_ty) = user_ty else { bug!() };
+        let ty::UserTypeKind::Ty(user_ty) = user_ty.kind else { bug!() };
 
         // A fast path for a common case with closure input/output types.
         if let ty::Infer(_) = user_ty.kind() {

compiler/rustc_borrowck/src/type_check/input_output.rs

@@ -110,7 +110,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         ) {
             self.ascribe_user_type_skip_wf(
                 arg_decl.ty,
-                ty::UserType::Ty(user_ty),
+                ty::UserType::new(ty::UserTypeKind::Ty(user_ty)),
                 arg_decl.source_info.span,
             );
         }
@@ -119,7 +119,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         let output_decl = &body.local_decls[RETURN_PLACE];
         self.ascribe_user_type_skip_wf(
             output_decl.ty,
-            ty::UserType::Ty(user_provided_sig.output()),
+            ty::UserType::new(ty::UserTypeKind::Ty(user_provided_sig.output())),
             output_decl.source_info.span,
         );
     }

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -31,7 +31,7 @@ use rustc_middle::ty::visit::TypeVisitableExt;
 use rustc_middle::ty::{
     self, Binder, CanonicalUserTypeAnnotation, CanonicalUserTypeAnnotations, CoroutineArgsExt,
     Dynamic, GenericArgsRef, OpaqueHiddenType, OpaqueTypeKey, RegionVid, Ty, TyCtxt, UserArgs,
-    UserType, UserTypeAnnotationIndex,
+    UserTypeAnnotationIndex,
 };
 use rustc_middle::{bug, span_bug};
 use rustc_mir_dataflow::ResultsCursor;
@@ -370,7 +370,10 @@ impl<'a, 'b, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'tcx> {
                 } else {
                     self.cx.ascribe_user_type(
                         constant.const_.ty(),
-                        UserType::TypeOf(uv.def, UserArgs { args: uv.args, user_self_ty: None }),
+                        ty::UserType::new(ty::UserTypeKind::TypeOf(uv.def, UserArgs {
+                            args: uv.args,
+                            user_self_ty: None,
+                        })),
                         locations.span(self.cx.body),
                     );
                 }
@@ -991,9 +994,10 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         for user_annotation in self.user_type_annotations {
             let CanonicalUserTypeAnnotation { span, ref user_ty, inferred_ty } = *user_annotation;
             let annotation = self.instantiate_canonical(span, user_ty);
-            if let ty::UserType::TypeOf(def, args) = annotation
+            if let ty::UserTypeKind::TypeOf(def, args) = annotation.kind
                 && let DefKind::InlineConst = tcx.def_kind(def)
             {
+                assert!(annotation.bounds.is_empty());
                 self.check_inline_const(inferred_ty, def.expect_local(), args, span);
             } else {
                 self.ascribe_user_type(inferred_ty, annotation, span);

compiler/rustc_feature/src/removed.rs

@@ -126,9 +126,6 @@ declare_features! (
             better implied higher-ranked implied bounds support"
         )
     ),
-    /// Allows `impl Trait` in bindings (`let`, `const`, `static`).
-    (removed, impl_trait_in_bindings, "1.55.0", Some(63065),
-     Some("the implementation was not maintainable, the feature may get reintroduced once the current refactorings are done")),
     (removed, import_shadowing, "1.0.0", None, None),
     /// Allows in-band quantification of lifetime bindings (e.g., `fn foo(x: &'a u8) -> &'a u8`).
     (removed, in_band_lifetimes, "1.23.0", Some(44524),

compiler/rustc_feature/src/unstable.rs

@@ -517,6 +517,8 @@ declare_features! (
     (unstable, if_let_guard, "1.47.0", Some(51114)),
     /// Allows `impl Trait` to be used inside associated types (RFC 2515).
     (unstable, impl_trait_in_assoc_type, "1.70.0", Some(63063)),
+    /// Allows `impl Trait` in bindings (`let`).
+    (unstable, impl_trait_in_bindings, "1.64.0", Some(63065)),
     /// Allows `impl Trait` as output type in `Fn` traits in return position of functions.
     (unstable, impl_trait_in_fn_trait_return, "1.64.0", Some(99697)),
     /// Allows associated types in inherent impls.

compiler/rustc_hir/src/hir.rs

@@ -2906,6 +2906,8 @@ pub enum TyKind<'hir> {
     Path(QPath<'hir>),
     /// An opaque type definition itself. This is only used for `impl Trait`.
     OpaqueDef(&'hir OpaqueTy<'hir>),
+    /// A trait ascription type, which is `impl Trait` within a local binding.
+    TraitAscription(GenericBounds<'hir>),
     /// A trait object type `Bound1 + Bound2 + Bound3`
     /// where `Bound` is a trait or a lifetime.
     TraitObject(&'hir [PolyTraitRef<'hir>], &'hir Lifetime, TraitObjectSyntax),

compiler/rustc_hir/src/intravisit.rs

@@ -900,6 +900,9 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty<'v>) -> V::Resul
         TyKind::OpaqueDef(opaque) => {
             try_visit!(visitor.visit_opaque_ty(opaque));
         }
+        TyKind::TraitAscription(bounds) => {
+            walk_list!(visitor, visit_param_bound, bounds);
+        }
         TyKind::Array(ref ty, ref length) => {
             try_visit!(visitor.visit_ty(ty));
             try_visit!(visitor.visit_const_arg(length));

compiler/rustc_hir_analysis/src/collect.rs

@@ -29,7 +29,7 @@ use rustc_errors::{
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::{self, Visitor, walk_generics};
-use rustc_hir::{self as hir, GenericParamKind, Node};
+use rustc_hir::{self as hir, GenericParamKind, HirId, Node};
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::ObligationCause;
 use rustc_middle::hir::nested_filter;
@@ -437,6 +437,15 @@ impl<'tcx> HirTyLowerer<'tcx> for ItemCtxt<'tcx> {
         ty::Const::new_error_with_message(self.tcx(), span, "bad placeholder constant")
     }
 
+    fn register_trait_ascription_bounds(
+        &self,
+        _: Vec<(ty::Clause<'tcx>, Span)>,
+        _: HirId,
+        span: Span,
+    ) {
+        self.dcx().span_delayed_bug(span, "trait ascription type not allowed here");
+    }
+
     fn probe_ty_param_bounds(
         &self,
         span: Span,