Auto merge of #12982 - jridgewell:into_future, r=Veykril

Implement IntoFuture type inference

One of my projects is using [IntoFuture](https://doc.rust-lang.org/std/future/trait.IntoFuture.html) to make our async code a little less verbose. However, rust-analyzer can't infer the output type of an await expression if the value uses `IntoFuture` to convert into another type. So we're getting `{unknown}` types everywhere since switching.

`foo.await` itself [desugars](e4417cf020/compiler/rustc_ast_lowering/src/expr.rs (L644-L658)) into a `match into_future(foo) {}`, with every `Future` impl getting a [default](e4417cf020/library/core/src/future/into_future.rs (L131-L139)) `IntoFuture` implementation. I'm not sure if we want to disable the old `future_trait` paths, since this only recently [stabilize](https://github.com/rust-lang/rust/pull/98718).

crates/hir/src/lib.rs

@@ -2778,20 +2778,32 @@ impl Type {
         self.ty.is_unknown()
     }
 
-    /// Checks that particular type `ty` implements `std::future::Future`.
+    /// Checks that particular type `ty` implements `std::future::IntoFuture` or
+    /// `std::future::Future`.
     /// This function is used in `.await` syntax completion.
-    pub fn impls_future(&self, db: &dyn HirDatabase) -> bool {
-        let std_future_trait = db
-            .lang_item(self.env.krate, SmolStr::new_inline("future_trait"))
-            .and_then(|it| it.as_trait());
-        let std_future_trait = match std_future_trait {
+    pub fn impls_into_future(&self, db: &dyn HirDatabase) -> bool {
+        let trait_ = db
+            .lang_item(self.env.krate, SmolStr::new_inline("into_future"))
+            .and_then(|it| {
+                let into_future_fn = it.as_function()?;
+                let assoc_item = as_assoc_item(db, AssocItem::Function, into_future_fn)?;
+                let into_future_trait = assoc_item.containing_trait_or_trait_impl(db)?;
+                Some(into_future_trait.id)
+            })
+            .or_else(|| {
+                let future_trait =
+                    db.lang_item(self.env.krate, SmolStr::new_inline("future_trait"))?;
+                future_trait.as_trait()
+            });
+
+        let trait_ = match trait_ {
             Some(it) => it,
             None => return false,
         };
 
         let canonical_ty =
             Canonical { value: self.ty.clone(), binders: CanonicalVarKinds::empty(Interner) };
-        method_resolution::implements_trait(&canonical_ty, db, self.env.clone(), std_future_trait)
+        method_resolution::implements_trait(&canonical_ty, db, self.env.clone(), trait_)
     }
 
     /// Checks that particular type `ty` implements `std::ops::FnOnce`.

crates/hir/src/source_analyzer.rs

@@ -27,6 +27,7 @@ use hir_def::{
 use hir_expand::{
     builtin_fn_macro::BuiltinFnLikeExpander,
     hygiene::Hygiene,
+    mod_path::path,
     name,
     name::{AsName, Name},
     HirFileId, InFile,
@@ -269,14 +270,35 @@ impl SourceAnalyzer {
         db: &dyn HirDatabase,
         await_expr: &ast::AwaitExpr,
     ) -> Option<FunctionId> {
-        let ty = self.ty_of_expr(db, &await_expr.expr()?.into())?;
+        let mut ty = self.ty_of_expr(db, &await_expr.expr()?.into())?.clone();
 
-        let op_fn = db
+        let into_future_trait = self
+            .resolver
+            .resolve_known_trait(db.upcast(), &path![core::future::IntoFuture])
+            .map(Trait::from);
+
+        if let Some(into_future_trait) = into_future_trait {
+            let type_ = Type::new_with_resolver(db, &self.resolver, ty.clone());
+            if type_.impls_trait(db, into_future_trait, &[]) {
+                let items = into_future_trait.items(db);
+                let into_future_type = items.into_iter().find_map(|item| match item {
+                    AssocItem::TypeAlias(alias)
+                        if alias.name(db) == hir_expand::name![IntoFuture] =>
+                    {
+                        Some(alias)
+                    }
+                    _ => None,
+                })?;
+                let future_trait = type_.normalize_trait_assoc_type(db, &[], into_future_type)?;
+                ty = future_trait.ty;
+            }
+        }
+
+        let poll_fn = db
             .lang_item(self.resolver.krate(), hir_expand::name![poll].to_smol_str())?
             .as_function()?;
-        let substs = hir_ty::TyBuilder::subst_for_def(db, op_fn).push(ty.clone()).build();
-
-        Some(self.resolve_impl_method_or_trait_def(db, op_fn, &substs))
+        let substs = hir_ty::TyBuilder::subst_for_def(db, poll_fn).push(ty.clone()).build();
+        Some(self.resolve_impl_method_or_trait_def(db, poll_fn, &substs))
     }
 
     pub(crate) fn resolve_prefix_expr(