Diagnose incorrect usages of the question mark operator

Author: Veykril

crates/hir-expand/src/mod_path.rs

@@ -259,6 +259,7 @@ macro_rules! __known_path {
     (core::future::Future) => {};
     (core::future::IntoFuture) => {};
     (core::ops::Try) => {};
+    (core::ops::FromResidual) => {};
     ($path:path) => {
         compile_error!("Please register your known path in the path module")
     };

crates/hir-expand/src/name.rs

@@ -279,6 +279,8 @@ pub mod known {
         RangeToInclusive,
         RangeTo,
         Range,
+        Residual,
+        FromResidual,
         Neg,
         Not,
         None,

crates/hir-ty/src/infer.rs

@@ -190,7 +190,9 @@ pub(crate) type InferResult<T> = Result<InferOk<T>, TypeError>;
 pub enum InferenceDiagnostic {
     NoSuchField { expr: ExprId },
     BreakOutsideOfLoop { expr: ExprId, is_break: bool },
+    IncorrectTryTarget { expr: ExprId },
     MismatchedArgCount { call_expr: ExprId, expected: usize, found: usize },
+    DoesNotImplement { expr: ExprId, trait_: TraitId, ty: Ty },
 }
 
 /// A mismatch between an expected and an inferred type.
@@ -905,17 +907,6 @@ impl<'a> InferenceContext<'a> {
         self.db.trait_data(trait_).associated_type_by_name(&name![Item])
     }
 
-    fn resolve_ops_try_ok(&self) -> Option<TypeAliasId> {
-        // FIXME resolve via lang_item once try v2 is stable
-        let path = path![core::ops::Try];
-        let trait_ = self.resolver.resolve_known_trait(self.db.upcast(), &path)?;
-        let trait_data = self.db.trait_data(trait_);
-        trait_data
-            // FIXME remove once try v2 is stable
-            .associated_type_by_name(&name![Ok])
-            .or_else(|| trait_data.associated_type_by_name(&name![Output]))
-    }
-
     fn resolve_ops_neg_output(&self) -> Option<TypeAliasId> {
         let trait_ = self.resolve_lang_item(name![neg])?.as_trait()?;
         self.db.trait_data(trait_).associated_type_by_name(&name![Output])

crates/hir-ty/src/infer/expr.rs

@@ -19,24 +19,24 @@ use hir_def::{
     resolver::resolver_for_expr,
     ConstParamId, FieldId, ItemContainerId, Lookup,
 };
-use hir_expand::name::Name;
+use hir_expand::{name, name::Name};
 use stdx::always;
 use syntax::ast::RangeOp;
 
 use crate::{
     autoderef::{self, Autoderef},
     consteval,
-    infer::{coerce::CoerceMany, find_continuable, BreakableKind},
+    infer::{coerce::CoerceMany, find_continuable, path, BreakableKind},
     lower::{
         const_or_path_to_chalk, generic_arg_to_chalk, lower_to_chalk_mutability, ParamLoweringMode,
     },
     mapping::{from_chalk, ToChalk},
     method_resolution::{self, lang_names_for_bin_op, VisibleFromModule},
     primitive::{self, UintTy},
-    static_lifetime, to_chalk_trait_id,
+    static_lifetime, to_assoc_type_id, to_chalk_trait_id,
     utils::{generics, Generics},
-    AdtId, Binders, CallableDefId, FnPointer, FnSig, FnSubst, Interner, Rawness, Scalar,
-    Substitution, TraitRef, Ty, TyBuilder, TyExt, TyKind,
+    AdtId, AliasEq, AliasTy, Binders, CallableDefId, FnPointer, FnSig, FnSubst, Interner,
+    ProjectionTy, Rawness, Scalar, Substitution, TraitRef, Ty, TyBuilder, TyExt, TyKind,
 };
 
 use super::{
@@ -564,9 +564,29 @@ impl<'a> InferenceContext<'a> {
                 let inner_ty = self.infer_expr_inner(*expr, &Expectation::none());
                 self.resolve_associated_type(inner_ty, self.resolve_future_future_output())
             }
-            Expr::Try { expr } => {
-                let inner_ty = self.infer_expr_inner(*expr, &Expectation::none());
-                self.resolve_associated_type(inner_ty, self.resolve_ops_try_ok())
+            &Expr::Try { expr } => {
+                let inner_ty = self.infer_expr_inner(expr, &Expectation::none());
+                match self.resolve_try_impl_for(inner_ty.clone()) {
+                    Some((_, Some((output, residual)))) => {
+                        if let Some((_trait, false)) =
+                            self.implements_from_residual(self.return_ty.clone(), residual)
+                        {
+                            self.push_diagnostic(InferenceDiagnostic::IncorrectTryTarget {
+                                expr: tgt_expr,
+                            });
+                        }
+                        output
+                    }
+                    Some((trait_, None)) => {
+                        self.push_diagnostic(InferenceDiagnostic::DoesNotImplement {
+                            expr,
+                            trait_,
+                            ty: inner_ty,
+                        });
+                        self.err_ty()
+                    }
+                    None => self.err_ty(),
+                }
             }
             Expr::Cast { expr, type_ref } => {
                 // FIXME: propagate the "castable to" expectation (and find a test case that shows this is necessary)
@@ -1530,4 +1550,67 @@ impl<'a> InferenceContext<'a> {
         let ctx = self.breakables.pop().expect("breakable stack broken");
         (ctx.may_break.then(|| ctx.coerce.complete()), res)
     }
+
+    /// Check whether `ty` implements `FromResidual<r>`
+    fn implements_from_residual(&mut self, ty: Ty, r: Ty) -> Option<(hir_def::TraitId, bool)> {
+        let from_residual_trait = self
+            .resolver
+            .resolve_known_trait(self.db.upcast(), &(super::path![core::ops::FromResidual]))?;
+        let r = GenericArgData::Ty(r).intern(Interner);
+        let b = TyBuilder::trait_ref(self.db, from_residual_trait);
+        if b.remaining() != 2 {
+            return Some((from_residual_trait, false));
+        }
+        let trait_ref = b.push(ty).push(r).build();
+        Some((from_residual_trait, self.table.try_obligation(trait_ref.cast(Interner)).is_some()))
+    }
+
+    fn resolve_try_impl_for(&mut self, ty: Ty) -> Option<(hir_def::TraitId, Option<(Ty, Ty)>)> {
+        let path = path![core::ops::Try];
+        let trait_ = self.resolver.resolve_known_trait(self.db.upcast(), &path)?;
+
+        let trait_ref = TyBuilder::trait_ref(self.db, trait_).push(ty).build();
+        let substitution = trait_ref.substitution.clone();
+        self.push_obligation(trait_ref.clone().cast(Interner));
+
+        let trait_data = self.db.trait_data(trait_);
+        let output = trait_data.associated_type_by_name(&name![Output]);
+        let residual = trait_data.associated_type_by_name(&name![Residual]);
+
+        let output_ty = match output {
+            Some(output) => {
+                let output_ty = self.table.new_type_var();
+                let alias_eq = AliasEq {
+                    alias: AliasTy::Projection(ProjectionTy {
+                        associated_ty_id: to_assoc_type_id(output),
+                        substitution: substitution.clone(),
+                    }),
+                    ty: output_ty.clone(),
+                };
+                self.push_obligation(alias_eq.cast(Interner));
+                output_ty
+            }
+            None => self.err_ty(),
+        };
+        let residual_ty = match residual {
+            Some(residual) => {
+                let residual_ty = self.table.new_type_var();
+                let alias_eq = AliasEq {
+                    alias: AliasTy::Projection(ProjectionTy {
+                        associated_ty_id: to_assoc_type_id(residual),
+                        substitution,
+                    }),
+                    ty: residual_ty.clone(),
+                };
+                self.push_obligation(alias_eq.cast(Interner));
+                residual_ty
+            }
+            None => self.err_ty(),
+        };
+        // FIXME: We are doing the work twice here I think?
+        Some((
+            trait_,
+            self.table.try_obligation(trait_ref.cast(Interner)).map(|_| (output_ty, residual_ty)),
+        ))
+    }
 }

crates/hir-ty/src/method_resolution.rs

@@ -1111,6 +1111,24 @@ pub fn resolve_indexing_op(
     }
     None
 }
+/// Returns the receiver type for the try branch trait call.
+pub fn resolve_branch_op(
+    db: &dyn HirDatabase,
+    env: Arc<TraitEnvironment>,
+    ty: Canonical<Ty>,
+    try_trait: TraitId,
+) -> Option<ReceiverAdjustments> {
+    let mut table = InferenceTable::new(db, env.clone());
+    let ty = table.instantiate_canonical(ty);
+    let (deref_chain, adj) = autoderef_method_receiver(&mut table, ty);
+    for (ty, adj) in deref_chain.into_iter().zip(adj) {
+        let goal = generic_implements_goal(db, env.clone(), try_trait, &ty);
+        if db.trait_solve(env.krate, goal.cast(Interner)).is_some() {
+            return Some(adj);
+        }
+    }
+    None
+}
 
 macro_rules! check_that {
     ($cond:expr) => {

crates/hir-ty/src/tests/traits.rs

@@ -162,98 +162,16 @@ fn test() {
     );
 }
 
-#[test]
-fn infer_try() {
-    check_types(
-        r#"
-//- /main.rs crate:main deps:core
-fn test() {
-    let r: Result<i32, u64> = Result::Ok(1);
-    let v = r?;
-    v;
-} //^ i32
-
-//- /core.rs crate:core
-pub mod ops {
-    pub trait Try {
-        type Ok;
-        type Error;
-    }
-}
-
-pub mod result {
-    pub enum Result<O, E> {
-        Ok(O),
-        Err(E)
-    }
-
-    impl<O, E> crate::ops::Try for Result<O, E> {
-        type Ok = O;
-        type Error = E;
-    }
-}
-
-pub mod prelude {
-    pub mod rust_2018 {
-        pub use crate::{result::*, ops::*};
-    }
-}
-"#,
-    );
-}
-
 #[test]
 fn infer_try_trait_v2() {
     check_types(
         r#"
-//- /main.rs crate:main deps:core
-fn test() {
-    let r: Result<i32, u64> = Result::Ok(1);
+//- minicore: try
+fn test() -> core::ops::ControlFlow<u32, f32> {
+    let r: core::ops::ControlFlow<u32, f32> = core::ops::ControlFlow::Continue(1.0);
     let v = r?;
-    v;
-} //^ i32
-
-//- /core.rs crate:core
-mod ops {
-    mod try_trait {
-        pub trait Try: FromResidual {
-            type Output;
-            type Residual;
-        }
-        pub trait FromResidual<R = <Self as Try>::Residual> {}
-    }
-
-    pub use self::try_trait::FromResidual;
-    pub use self::try_trait::Try;
-}
-
-mod convert {
-    pub trait From<T> {}
-    impl<T> From<T> for T {}
-}
-
-pub mod result {
-    use crate::convert::From;
-    use crate::ops::{Try, FromResidual};
-
-    pub enum Infallible {}
-    pub enum Result<O, E> {
-        Ok(O),
-        Err(E)
-    }
-
-    impl<O, E> Try for Result<O, E> {
-        type Output = O;
-        type Error = Result<Infallible, E>;
-    }
-
-    impl<T, E, F: From<E>> FromResidual<Result<Infallible, E>> for Result<T, F> {}
-}
-
-pub mod prelude {
-    pub mod rust_2018 {
-        pub use crate::result::*;
-    }
+      //^ f32
+    r
 }
 "#,
     );

crates/hir/src/diagnostics.rs

@@ -6,7 +6,7 @@
 use base_db::CrateId;
 use cfg::{CfgExpr, CfgOptions};
 use either::Either;
-use hir_def::path::ModPath;
+use hir_def::{path::ModPath, TraitId};
 use hir_expand::{name::Name, HirFileId, InFile};
 use syntax::{ast, AstPtr, SyntaxNodePtr, TextRange};
 
@@ -33,13 +33,15 @@ diagnostics![
     BreakOutsideOfLoop,
     InactiveCode,
     IncorrectCase,
+    IncorrectTryExpr,
     InvalidDeriveTarget,
     MacroError,
     MalformedDerive,
     MismatchedArgCount,
     MissingFields,
     MissingMatchArms,
     MissingUnsafe,
+    NotImplemented,
     NoSuchField,
     ReplaceFilterMapNextWithFindMap,
     TypeMismatch,
@@ -153,6 +155,16 @@ pub struct MismatchedArgCount {
     pub expected: usize,
     pub found: usize,
 }
+#[derive(Debug)]
+pub struct IncorrectTryExpr {
+    pub expr: InFile<AstPtr<ast::Expr>>,
+}
+#[derive(Debug)]
+pub struct NotImplemented {
+    pub expr: InFile<AstPtr<ast::Expr>>,
+    pub trait_: TraitId,
+    pub ty: Type,
+}
 
 #[derive(Debug)]
 pub struct MissingMatchArms {