Create Callables for generic types implementing FnOnce

crates/hir-ty/src/infer.rs

@@ -1020,7 +1020,7 @@ fn from_option(ty: Option<Ty>) -> Self {
     /// The primary use case is where the expected type is a fat pointer,
     /// like `&[isize]`. For example, consider the following statement:
     ///
-    ///    let x: &[isize] = &[1, 2, 3];
+    ///     let x: &[isize] = &[1, 2, 3];
     ///
     /// In this case, the expected type for the `&[1, 2, 3]` expression is
     /// `&[isize]`. If however we were to say that `[1, 2, 3]` has the

crates/hir-ty/src/lib.rs

@@ -38,10 +38,12 @@ macro_rules! eprintln {
 use chalk_ir::{
     fold::{Shift, TypeFoldable},
     interner::HasInterner,
-    NoSolution,
+    NoSolution, UniverseIndex,
 };
 use hir_def::{expr::ExprId, type_ref::Rawness, TypeOrConstParamId};
+use hir_expand::name;
 use itertools::Either;
+use traits::FnTrait;
 use utils::Generics;
 
 use crate::{consteval::unknown_const, db::HirDatabase, utils::generics};
@@ -508,3 +510,68 @@ fn try_fold_free_var_lifetime(
     });
     Canonical { value, binders: chalk_ir::CanonicalVarKinds::from_iter(Interner, kinds) }
 }
+
+pub fn callable_sig_from_fnonce(
+    self_ty: &Canonical<Ty>,
+    env: Arc<TraitEnvironment>,
+    db: &dyn HirDatabase,
+) -> Option<CallableSig> {
+    let krate = env.krate;
+    let fn_once_trait = FnTrait::FnOnce.get_id(db, krate)?;
+    let output_assoc_type = db.trait_data(fn_once_trait).associated_type_by_name(&name![Output])?;
+
+    let mut kinds = self_ty.binders.interned().to_vec();
+    let b = TyBuilder::trait_ref(db, fn_once_trait);
+    if b.remaining() != 2 {
+        return None;
+    }
+    let fn_once = b
+        .push(self_ty.value.clone())
+        .fill_with_bound_vars(DebruijnIndex::INNERMOST, kinds.len())
+        .build();
+    kinds.extend(fn_once.substitution.iter(Interner).skip(1).map(|x| {
+        let vk = match x.data(Interner) {
+            chalk_ir::GenericArgData::Ty(_) => {
+                chalk_ir::VariableKind::Ty(chalk_ir::TyVariableKind::General)
+            }
+            chalk_ir::GenericArgData::Lifetime(_) => chalk_ir::VariableKind::Lifetime,
+            chalk_ir::GenericArgData::Const(c) => {
+                chalk_ir::VariableKind::Const(c.data(Interner).ty.clone())
+            }
+        };
+        chalk_ir::WithKind::new(vk, UniverseIndex::ROOT)
+    }));
+
+    // FIXME: chalk refuses to solve `<Self as FnOnce<^0.0>>::Output == ^0.1`, so we first solve
+    // `<Self as FnOnce<^0.0>>` and then replace `^0.0` with the concrete argument tuple.
+    let trait_env = env.env.clone();
+    let obligation = InEnvironment { goal: fn_once.cast(Interner), environment: trait_env };
+    let canonical =
+        Canonical { binders: CanonicalVarKinds::from_iter(Interner, kinds), value: obligation };
+    let subst = match db.trait_solve(krate, canonical) {
+        Some(Solution::Unique(vars)) => vars.value.subst,
+        _ => return None,
+    };
+    let args = subst.at(Interner, self_ty.binders.interned().len()).ty(Interner)?;
+    let params = match args.kind(Interner) {
+        chalk_ir::TyKind::Tuple(_, subst) => {
+            subst.iter(Interner).filter_map(|arg| arg.ty(Interner).cloned()).collect::<Vec<_>>()
+        }
+        _ => return None,
+    };
+    if params.iter().any(|ty| ty.is_unknown()) {
+        return None;
+    }
+
+    let fn_once = TyBuilder::trait_ref(db, fn_once_trait)
+        .push(self_ty.value.clone())
+        .push(args.clone())
+        .build();
+    let projection =
+        TyBuilder::assoc_type_projection(db, output_assoc_type, Some(fn_once.substitution.clone()))
+            .build();
+
+    let ret_ty = db.normalize_projection(projection, env);
+
+    Some(CallableSig::from_params_and_return(params, ret_ty.clone(), false))
+}

crates/hir/src/lib.rs

@@ -2995,7 +2995,17 @@ pub fn as_callable(&self, db: &dyn HirDatabase) -> Option<Callable> {
         let callee = match self.ty.kind(Interner) {
             TyKind::Closure(id, _) => Callee::Closure(*id),
             TyKind::Function(_) => Callee::FnPtr,
-            _ => Callee::Def(self.ty.callable_def(db)?),
+            TyKind::FnDef(..) => Callee::Def(self.ty.callable_def(db)?),
+            _ => {
+                let ty = hir_ty::replace_errors_with_variables(&self.ty);
+                let sig = hir_ty::callable_sig_from_fnonce(&ty, self.env.clone(), db)?;
+                return Some(Callable {
+                    ty: self.clone(),
+                    sig,
+                    callee: Callee::Other,
+                    is_bound_method: false,
+                });
+            }
         };
 
         let sig = self.ty.callable_sig(db)?;
@@ -3464,6 +3474,7 @@ enum Callee {
     Def(CallableDefId),
     Closure(ClosureId),
     FnPtr,
+    Other,
 }
 
 pub enum CallableKind {
@@ -3472,6 +3483,8 @@ pub enum CallableKind {
     TupleEnumVariant(Variant),
     Closure,
     FnPtr,
+    /// Some other type that implements `FnOnce`.
+    Other,
 }
 
 impl Callable {
@@ -3483,6 +3496,7 @@ pub fn kind(&self) -> CallableKind {
             Def(CallableDefId::EnumVariantId(it)) => CallableKind::TupleEnumVariant(it.into()),
             Closure(_) => CallableKind::Closure,
             FnPtr => CallableKind::FnPtr,
+            Other => CallableKind::Other,
         }
     }
     pub fn receiver_param(&self, db: &dyn HirDatabase) -> Option<ast::SelfParam> {

crates/ide/src/signature_help.rs

@@ -149,7 +149,7 @@ fn signature_help_for_call(
                 variant.name(db)
             );
         }
-        hir::CallableKind::Closure | hir::CallableKind::FnPtr => (),
+        hir::CallableKind::Closure | hir::CallableKind::FnPtr | hir::CallableKind::Other => (),
     }
 
     res.signature.push('(');
@@ -189,9 +189,10 @@ fn signature_help_for_call(
         hir::CallableKind::Function(func) if callable.return_type().contains_unknown() => {
             render(func.ret_type(db))
         }
-        hir::CallableKind::Function(_) | hir::CallableKind::Closure | hir::CallableKind::FnPtr => {
-            render(callable.return_type())
-        }
+        hir::CallableKind::Function(_)
+        | hir::CallableKind::Closure
+        | hir::CallableKind::FnPtr
+        | hir::CallableKind::Other => render(callable.return_type()),
         hir::CallableKind::TupleStruct(_) | hir::CallableKind::TupleEnumVariant(_) => {}
     }
     Some(res)
@@ -387,10 +388,9 @@ pub(crate) fn position(ra_fixture: &str) -> (RootDatabase, FilePosition) {
     }
 
     fn check(ra_fixture: &str, expect: Expect) {
-        // Implicitly add `Sized` to avoid noisy `T: ?Sized` in the results.
         let fixture = format!(
             r#"
-#[lang = "sized"] trait Sized {{}}
+//- minicore: sized, fn
 {ra_fixture}
             "#
         );
@@ -1331,4 +1331,19 @@ fn foo(self: &Self, other: Self)
             "#]],
         );
     }
+
+    #[test]
+    fn help_for_generic_call() {
+        check(
+            r#"
+fn f<F: FnOnce(u8, u16) -> i32>(f: F) {
+    f($0)
+}
+"#,
+            expect![[r#"
+                (u8, u16) -> i32
+                 ^^  ---
+            "#]],
+        );
+    }
 }