Rollup merge of #157960 - aerooneqq:delegation-infers-in-generics, r=petrochenkov

delegation: add support for infers in generics

This PR adds support for generating generic params for lifetimes and types/consts infers (`'_`. `_`). We support only single infers, if they are nested then we do nothing and eventually an error will be emitted after inheriting this unsound signature (i.e., `reuse foo::<Vec<_>>` is not supported).

The basic idea is:
```rust
fn foo<'a, 'b: 'b, T, const N: usize>() {}

reuse foo::<'_, String, _> as bar;

// Desugaring:
fn bar<'b, const N: usize>() {
  foo::<'b, String, N>()
}
```

So we generated params and lifetimes for provided infers. Note that in case of lifetimes we may have early and late bound lifetimes in child segment. We process only early-bound lifetimes as they are present in signature function generics (`tcx.generics_of(sig_id)`). Moreover since this PR we started to explicitly propagate early-bound lifetimes in generated delegation's call, so the warning about specifying lifetimes when late-bound lifetimes are present is suppressed for delegation segments.

Next, we limit the number of processed infers with the number of generic params in the signature function, so if we have `fn foo<X, Y>() {}` and we wrote `reuse foo::<_, _, _, _>;` we will not generate 4 generic params in delegation, we will generate 2 (`X` and `Y`), two remaining infers will not be processed and this will result in an error.

Considering free-to-trait reuses this PR extends the number of supported cases, as now we always generate `Self` generic param when needed:
```rust
trait Trait<'a, X> {
    fn method<'b, const M: usize>(&self) where 'b:'b { }
    fn r#static<'b, Y, const B: bool>() { }
}

impl <'a, X> Trait<'a, X> for () { }

reuse Trait::<'_, _>::method::<'_, _> as foo;
reuse <_ as Trait<'_, _>>::method::<'_, _> as foo1;
reuse <() as Trait<'_, _>>::method::<'_, _> as foo2;
reuse <_ as Trait<'_, _>>::r#static::<_, _> as foo3;
reuse <() as Trait<'_, _>>::r#static::<_, _> as foo4;
reuse Trait::<'_, _>::r#static::<_, _> as foo5;

// Desugaring:
#[attr = Inline(Hint)]
fn foo<'a, 'b, Self, X, const M: _>(self: _) -> _ where 'a:'a,
    'b:'b { <Self as Trait::<'a, X>>::method::<'b, M>(self) }
#[attr = Inline(Hint)]
fn foo1<'a, 'b, Self, X, const M: _>(self: _) -> _ where 'a:'a,
    'b:'b { <Self as Trait::<'a, X>>::method::<'b, M>(self) }
#[attr = Inline(Hint)]
fn foo2<'a, 'b, X, const M: _>(self: _) -> _ where 'a:'a,
    'b:'b { <() as Trait::<'a, X>>::method::<'b, M>(self) }
#[attr = Inline(Hint)]
fn foo3<'a, Self, X, Y, const B: _>() -> _ where
    'a:'a { <Self as Trait::<'a, X>>::r#static::<Y, B>() }
#[attr = Inline(Hint)]
fn foo4<'a, X, Y, const B: _>() -> _ where
    'a:'a { <() as Trait::<'a, X>>::r#static::<Y, B>() }
#[attr = Inline(Hint)]
fn foo5<'a, Self, X, Y, const B: _>() -> _ where
    'a:'a { <Self as Trait::<'a, X>>::r#static::<Y, B>() }
```
Note that we generated `Self` in `foo5` reuse. With that done the error about self-type specification is removed.

Finally this PR greatly simplifies generic args for signature inheritance generation code.

Part of #118212.
r? @petrochenkov

Author: jhpratt

compiler/rustc_ast_lowering/src/delegation.rs

@@ -194,7 +194,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             return self.generate_delegation_error(span, delegation);
         }
 
-        let mut generics = self.uplift_delegation_generics(delegation, sig_id, is_method);
+        let mut generics = self.uplift_delegation_generics(delegation, sig_id);
 
         let (body_id, call_expr_id, unused_target_expr) = self.lower_delegation_body(
             delegation,
@@ -404,10 +404,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 hir::InferDelegationSig::Output(self.arena.alloc(hir::DelegationInfo {
                     call_expr_id,
                     call_path_res: self.get_resolution_id(call_path_node_id),
-                    child_args_segment_id: generics.child.args_segment_id,
-                    parent_args_segment_id: generics.parent.args_segment_id,
-                    self_ty_id: generics.self_ty_id,
-                    propagate_self_ty: generics.propagate_self_ty,
+                    child_seg_id: generics.child.args_segment_id,
+                    child_seg_id_for_sig: generics.child.segment_id_for_sig(),
+                    parent_seg_id_for_sig: generics.parent.segment_id_for_sig(),
+                    self_ty_propagation_kind: generics.self_ty_propagation_kind,
                     group_id: {
                         let id = match source {
                             DelegationSource::Single => None,
@@ -625,20 +625,40 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     }),
                 );
 
-                hir::QPath::Resolved(ty, self.arena.alloc(new_path))
-            }
-            hir::QPath::TypeRelative(ty, segment) => {
-                let segment = self.process_segment(span, segment, &mut generics.child);
+                // Explicitly create `Self` self-type in case of infers or static
+                // free-to-trait reuses.
+                let ty = match generics.self_ty_propagation_kind {
+                    Some(hir::DelegationSelfTyPropagationKind::SelfParam) => {
+                        let self_param = generics.parent.generics.find_self_param();
+                        let path = self.create_generic_arg_path(self_param);
+                        let kind = hir::TyKind::Path(path);
+
+                        let ty = match ty {
+                            Some(ty) => hir::Ty { kind, ..ty.clone() },
+                            None => hir::Ty { kind, hir_id: self.next_id(), span },
+                        };
+
+                        Some(&*self.arena.alloc(ty))
+                    }
+                    _ => ty,
+                };
 
-                hir::QPath::TypeRelative(ty, self.arena.alloc(segment))
+                hir::QPath::Resolved(ty, self.arena.alloc(new_path))
             }
+            hir::QPath::TypeRelative(..) => unreachable!("until inherent methods are supported"),
         };
 
-        generics.self_ty_id = match new_path {
-            hir::QPath::Resolved(ty, _) => ty,
-            hir::QPath::TypeRelative(ty, _) => Some(ty),
+        if let Some(hir::DelegationSelfTyPropagationKind::SelfTy(id)) =
+            generics.self_ty_propagation_kind.as_mut()
+        {
+            *id = match new_path {
+                hir::QPath::Resolved(ty, _) => {
+                    ty.expect("must contain self type as `SelfTy` propagation kind is specified")
+                }
+                hir::QPath::TypeRelative(ty, _) => ty,
+            }
+            .hir_id;
         }
-        .map(|ty| ty.hir_id);
 
         let callee_path = self.arena.alloc(self.mk_expr(hir::ExprKind::Path(new_path), span));
         let args = self.arena.alloc_from_iter(args);
@@ -662,25 +682,38 @@ impl<'hir> LoweringContext<'_, 'hir> {
         segment: &hir::PathSegment<'hir>,
         result: &mut GenericsGenerationResult<'hir>,
     ) -> hir::PathSegment<'hir> {
-        let details = result.generics.args_propagation_details();
-
-        // Always uplift generic params, because if they are not empty then they
-        // should be generated in delegation.
-        let generics = result.generics.into_hir_generics(self, span);
-        let segment = if details.should_propagate {
-            let args = generics.into_generic_args(self, span);
-
-            // Needed for better error messages (`trait-impl-wrong-args-count.rs` test).
-            let args = if args.is_empty() { None } else { Some(args) };
-
-            hir::PathSegment { args, ..segment.clone() }
-        } else {
-            segment.clone()
-        };
+        let infer_indices = result.generics.infer_indices();
+        result.generics.into_hir_generics(self, span);
+
+        let mut segment = segment.clone();
+        let mut args_iter = result.generics.create_args_iterator();
+
+        let new_args = segment
+            .args
+            .filter(|args| !args.is_empty())
+            .map(|args| {
+                self.arena.alloc_from_iter(args.args.iter().enumerate().map(|(idx, arg)| {
+                    if infer_indices.contains(&idx) {
+                        args_iter.next(self, |_| arg.hir_id()).expect("arg must exist for infer")
+                    } else {
+                        *arg
+                    }
+                }))
+            })
+            .unwrap_or_else(|| self.arena.alloc_from_iter(args_iter.consume_all(self)));
+
+        // Needed for better error messages (`trait-impl-wrong-args-count.rs` test).
+        segment.args = (!new_args.is_empty()).then(|| {
+            &*self.arena.alloc(hir::GenericArgs {
+                args: new_args,
+                constraints: &[],
+                parenthesized: hir::GenericArgsParentheses::No,
+                span_ext: segment.args.map_or(span, |args| args.span_ext),
+            })
+        });
 
-        if details.use_args_in_sig_inheritance {
-            result.args_segment_id = Some(segment.hir_id);
-        }
+        result.args_segment_id = segment.hir_id;
+        result.use_for_sig_inheritance = !result.generics.is_trait_impl();
 
         segment
     }