Suggest giving return type to closures on E0282

13 files changed, 199 insertions, 90 deletions

diff --git a/src/librustc/hir/mod.rs b/src/librustc/hir/mod.rs
index 3d049fe4ccd..e8d44ee1310 100644
--- a/src/librustc/hir/mod.rs
+++ b/src/librustc/hir/mod.rs
@@ -1541,7 +1541,7 @@ pub enum ExprKind {
     Match(P<Expr>, HirVec<Arm>, MatchSource),
     /// A closure (e.g., `move |a, b, c| {a + b + c}`).
     ///
-    /// The final span is the span of the argument block `|...|`.
+    /// The `Span` is the argument block `|...|`.
     ///
     /// This may also be a generator literal or an `async block` as indicated by the
     /// `Option<GeneratorMovability>`.
diff --git a/src/librustc/infer/error_reporting/need_type_info.rs b/src/librustc/infer/error_reporting/need_type_info.rs
index b5d78a80bf6..16fed3d42d6 100644
--- a/src/librustc/infer/error_reporting/need_type_info.rs
+++ b/src/librustc/infer/error_reporting/need_type_info.rs
@@ -1,5 +1,5 @@
 use crate::hir::def::Namespace;
-use crate::hir::{self, Local, Pat, Body, HirId};
+use crate::hir::{self, Body, FunctionRetTy, Expr, ExprKind, HirId, Local, Pat};
 use crate::hir::intravisit::{self, Visitor, NestedVisitorMap};
 use crate::infer::InferCtxt;
 use crate::infer::type_variable::TypeVariableOriginKind;
@@ -7,7 +7,7 @@ use crate::ty::{self, Ty, Infer, TyVar};
 use crate::ty::print::Print;
 use syntax::source_map::DesugaringKind;
 use syntax_pos::Span;
-use errors::DiagnosticBuilder;
+use errors::{Applicability, DiagnosticBuilder};
 
 struct FindLocalByTypeVisitor<'a, 'tcx> {
     infcx: &'a InferCtxt<'a, 'tcx>,
@@ -16,9 +16,26 @@ struct FindLocalByTypeVisitor<'a, 'tcx> {
     found_local_pattern: Option<&'tcx Pat>,
     found_arg_pattern: Option<&'tcx Pat>,
     found_ty: Option<Ty<'tcx>>,
+    found_closure: Option<&'tcx ExprKind>,
 }
 
 impl<'a, 'tcx> FindLocalByTypeVisitor<'a, 'tcx> {
+    fn new(
+        infcx: &'a InferCtxt<'a, 'tcx>,
+        target_ty: Ty<'tcx>,
+        hir_map: &'a hir::map::Map<'tcx>,
+    ) -> FindLocalByTypeVisitor<'a, 'tcx> {
+        FindLocalByTypeVisitor {
+            infcx,
+            target_ty,
+            hir_map,
+            found_local_pattern: None,
+            found_arg_pattern: None,
+            found_ty: None,
+            found_closure: None,
+        }
+    }
+
     fn node_matches_type(&mut self, hir_id: HirId) -> Option<Ty<'tcx>> {
         let ty_opt = self.infcx.in_progress_tables.and_then(|tables| {
             tables.borrow().node_type_opt(hir_id)
@@ -72,6 +89,16 @@ impl<'a, 'tcx> Visitor<'tcx> for FindLocalByTypeVisitor<'a, 'tcx> {
         }
         intravisit::walk_body(self, body);
     }
+
+    fn visit_expr(&mut self, expr: &'tcx Expr) {
+        if let (ExprKind::Closure(_, _fn_decl, _id, _sp, _), Some(_)) = (
+            &expr.node,
+            self.node_matches_type(expr.hir_id),
+        ) {
+            self.found_closure = Some(&expr.node);
+        }
+        intravisit::walk_expr(self, expr);
+    }
 }
 
 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
@@ -106,16 +133,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         let ty = self.resolve_vars_if_possible(&ty);
         let name = self.extract_type_name(&ty, None);
 
-        let mut err_span = span;
-
-        let mut local_visitor = FindLocalByTypeVisitor {
-            infcx: &self,
-            target_ty: ty,
-            hir_map: &self.tcx.hir(),
-            found_local_pattern: None,
-            found_arg_pattern: None,
-            found_ty: None,
-        };
+        let mut local_visitor = FindLocalByTypeVisitor::new(&self, ty, &self.tcx.hir());
         let ty_to_string = |ty: Ty<'tcx>| -> String {
             let mut s = String::new();
             let mut printer = ty::print::FmtPrinter::new(self.tcx, &mut s, Namespace::TypeNS);
@@ -136,6 +154,35 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             let expr = self.tcx.hir().expect_expr(body_id.hir_id);
             local_visitor.visit_expr(expr);
         }
+        let err_span = if let Some(pattern) = local_visitor.found_arg_pattern {
+            pattern.span
+        } else {
+            span
+        };
+
+        let ty_msg = match local_visitor.found_ty {
+            Some(ty::TyS { sty: ty::Closure(def_id, substs), .. }) => {
+                let fn_sig = substs.closure_sig(*def_id, self.tcx);
+                let args = fn_sig.inputs()
+                    .skip_binder()
+                    .iter()
+                    .next()
+                    .map(|args| args.tuple_fields()
+                        .map(|arg| arg.to_string())
+                        .collect::<Vec<_>>().join(", "))
+                    .unwrap_or_default();
+                let ret = fn_sig.output().skip_binder().to_string();
+                format!(" for the closure `fn({}) -> {}`", args, ret)
+            }
+            Some(ty) if &ty.to_string() != "_" &&
+                // FIXME: Remove this check after `impl_trait_in_bindings` is stabilized. #63527
+                (!ty.is_impl_trait() || self.tcx.features().impl_trait_in_bindings) =>
+            {
+                let ty = ty_to_string(ty);
+                format!(" for `{}`", ty)
+            }
+            _ => String::new(),
+        };
 
         // When `name` corresponds to a type argument, show the path of the full type we're
         // trying to infer. In the following example, `ty_msg` contains
@@ -150,34 +197,47 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         //   |         consider giving `b` the explicit type `std::result::Result<i32, E>`, where
         //   |         the type parameter `E` is specified
         // ```
-        let (ty_msg, suffix) = match &local_visitor.found_ty {
-            Some(ty) if &ty.to_string() != "_" &&
-                name == "_" &&
-                // FIXME: Remove this check after `impl_trait_in_bindings` is stabilized. #63527
-                (!ty.is_impl_trait() || self.tcx.features().impl_trait_in_bindings) &&
-                !ty.is_closure() => // The suggestion doesn't make sense for closures.
-            {
-                let ty = ty_to_string(ty);
-                (format!(" for `{}`", ty),
-                 format!("the explicit type `{}`, with the type parameters specified", ty))
-            }
-            Some(ty) if &ty.to_string() != "_" &&
-                ty.to_string() != name &&
-                // FIXME: Remove this check after `impl_trait_in_bindings` is stabilized. #63527
-                (!ty.is_impl_trait() || self.tcx.features().impl_trait_in_bindings) &&
-                !ty.is_closure() => // The suggestion doesn't make sense for closures.
-            {
-                let ty = ty_to_string(ty);
-                (format!(" for `{}`", ty),
-                 format!(
-                     "the explicit type `{}`, where the type parameter `{}` is specified",
-                    ty,
-                    name,
-                 ))
-            }
+        let mut err = struct_span_err!(
+            self.tcx.sess,
+            err_span,
+            E0282,
+            "type annotations needed{}",
+            ty_msg,
+        );
+
+        let suffix = match local_visitor.found_ty {
             Some(ty::TyS { sty: ty::Closure(def_id, substs), .. }) => {
-                let msg = " for the closure".to_string();
                 let fn_sig = substs.closure_sig(*def_id, self.tcx);
+                let ret = fn_sig.output().skip_binder().to_string();
+
+                if let Some(ExprKind::Closure(_, decl, body_id, ..)) = local_visitor.found_closure {
+                    let (arrow, post) = match decl.output {
+                        FunctionRetTy::DefaultReturn(_) => ("-> ", " "),
+                        _ => ("", ""),
+                    };
+                    if let Some(body) = self.tcx.hir().krate().bodies.get(body_id) {
+                        let suggestion = match body.value.node {
+                            ExprKind::Block(..) => {
+                                vec![(decl.output.span(), format!("{}{}{}", arrow, ret, post))]
+                            }
+                            _ => {
+                                vec![
+                                    (decl.output.span(), format!("{}{}{}{{ ", arrow, ret, post)),
+                                    (body.value.span.shrink_to_hi(), " }".to_string()),
+                                ]
+                            }
+                        };
+                        err.multipart_suggestion(
+                            "give this closure an explicit return type without `_` placeholders",
+                            suggestion,
+                            Applicability::HasPlaceholders,
+                        );
+                        err.span_label(span, InferCtxt::missing_type_msg(&name));
+                        return err;
+                    }
+                }
+
+                // This shouldn't be reachable, but just in case we leave a reasonable fallback.
                 let args = fn_sig.inputs()
                     .skip_binder()
                     .iter()
@@ -189,18 +249,32 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 // This suggestion is incomplete, as the user will get further type inference
                 // errors due to the `_` placeholders and the introduction of `Box`, but it does
                 // nudge them in the right direction.
-                (msg, format!(
-                    "a boxed closure type like `Box<dyn Fn({}) -> {}>`",
-                    args,
-                    fn_sig.output().skip_binder().to_string(),
-                ))
+                format!("a boxed closure type like `Box<dyn Fn({}) -> {}>`", args, ret)
+            }
+            Some(ty) if &ty.to_string() != "_" &&
+                name == "_" &&
+                // FIXME: Remove this check after `impl_trait_in_bindings` is stabilized. #63527
+                (!ty.is_impl_trait() || self.tcx.features().impl_trait_in_bindings) =>
+            {
+                let ty = ty_to_string(ty);
+                format!("the explicit type `{}`, with the type parameters specified", ty)
             }
-            _ => (String::new(), "a type".to_owned()),
+            Some(ty) if &ty.to_string() != "_" &&
+                ty.to_string() != name &&
+                // FIXME: Remove this check after `impl_trait_in_bindings` is stabilized. #63527
+                (!ty.is_impl_trait() || self.tcx.features().impl_trait_in_bindings) =>
+            {
+                let ty = ty_to_string(ty);
+                format!(
+                    "the explicit type `{}`, where the type parameter `{}` is specified",
+                    ty,
+                    name,
+                )
+            }
+            _ => "a type".to_string(),
         };
-        let mut labels = vec![(span, InferCtxt::missing_type_msg(&name))];
 
         if let Some(pattern) = local_visitor.found_arg_pattern {
-            err_span = pattern.span;
             // We don't want to show the default label for closures.
             //
             // So, before clearing, the output would look something like this:
@@ -217,39 +291,36 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             //          ^ consider giving this closure parameter the type `[_; 0]`
             //            with the type parameter `_` specified
             // ```
-            labels.clear();
-            labels.push((
+            err.span_label(
                 pattern.span,
                 format!("consider giving this closure parameter {}", suffix),
-            ));
+            );
         } else if let Some(pattern) = local_visitor.found_local_pattern {
             if let Some(simple_ident) = pattern.simple_ident() {
                 match pattern.span.desugaring_kind() {
-                    None => labels.push((
-                        pattern.span,
-                        format!("consider giving `{}` {}", simple_ident, suffix),
-                    )),
-                    Some(DesugaringKind::ForLoop) => labels.push((
-                        pattern.span,
-                        "the element type for this iterator is not specified".to_owned(),
-                    )),
+                    None => {
+                        err.span_label(
+                            pattern.span,
+                            format!("consider giving `{}` {}", simple_ident, suffix),
+                        );
+                    }
+                    Some(DesugaringKind::ForLoop) => {
+                        err.span_label(
+                            pattern.span,
+                            "the element type for this iterator is not specified".to_string(),
+                        );
+                    }
                     _ => {}
                 }
             } else {
-                labels.push((pattern.span, format!("consider giving this pattern {}", suffix)));
+                err.span_label(pattern.span, format!("consider giving this pattern {}", suffix));
             }
-        };
-
-        let mut err = struct_span_err!(
-            self.tcx.sess,
-            err_span,
-            E0282,
-            "type annotations needed{}",
-            ty_msg,
-        );
-
-        for (target_span, label_message) in labels {
-            err.span_label(target_span, label_message);
+        }
+        if !err.span.span_labels().iter().any(|span_label| {
+                span_label.label.is_some() && span_label.span == span
+            }) && local_visitor.found_arg_pattern.is_none()
+        { // Avoid multiple labels pointing at `span`.
+            err.span_label(span, InferCtxt::missing_type_msg(&name));
         }
 
         err
diff --git a/src/test/ui/error-codes/E0282.stderr b/src/test/ui/error-codes/E0282.stderr
index 3a5040eb6da..0f610a5e42f 100644
--- a/src/test/ui/error-codes/E0282.stderr
+++ b/src/test/ui/error-codes/E0282.stderr
@@ -2,10 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/E0282.rs:2:9
    |
 LL |     let x = "hello".chars().rev().collect();
-   |         ^
-   |         |
-   |         cannot infer type
-   |         consider giving `x` a type
+   |         ^ consider giving `x` a type
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/for/for-loop-unconstrained-element-type.stderr b/src/test/ui/for/for-loop-unconstrained-element-type.stderr
index 02fdb808da4..0672014a929 100644
--- a/src/test/ui/for/for-loop-unconstrained-element-type.stderr
+++ b/src/test/ui/for/for-loop-unconstrained-element-type.stderr
@@ -2,10 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/for-loop-unconstrained-element-type.rs:8:14
    |
 LL |     for i in Vec::new() { }
-   |              ^^^^^^^^^^
-   |              |
-   |              cannot infer type
-   |              the element type for this iterator is not specified
+   |              ^^^^^^^^^^ the element type for this iterator is not specified
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/inference/cannot-infer-closure.stderr b/src/test/ui/inference/cannot-infer-closure.stderr
index fb78dedc887..5f30b5d993c 100644
--- a/src/test/ui/inference/cannot-infer-closure.stderr
+++ b/src/test/ui/inference/cannot-infer-closure.stderr
@@ -1,10 +1,12 @@
-error[E0282]: type annotations needed for the closure
+error[E0282]: type annotations needed for the closure `fn((), ()) -> std::result::Result<(), _>`
   --> $DIR/cannot-infer-closure.rs:3:9
    |
-LL |     let x = |a: (), b: ()| {
-   |         - consider giving `x` a boxed closure type like `Box<dyn Fn((), ()) -> std::result::Result<(), _>>`
 LL |         Err(a)?;
    |         ^^^^^^^ cannot infer type
+help: give this closure an explicit return type without `_` placeholders
+   |
+LL |     let x = |a: (), b: ()| -> std::result::Result<(), _> {
+   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/issues/issue-18159.stderr b/src/test/ui/issues/issue-18159.stderr
index 6048344125c..9b890be3c78 100644
--- a/src/test/ui/issues/issue-18159.stderr
+++ b/src/test/ui/issues/issue-18159.stderr
@@ -2,10 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/issue-18159.rs:2:9
    |
 LL |     let x;
-   |         ^
-   |         |
-   |         cannot infer type
-   |         consider giving `x` a type
+   |         ^ consider giving `x` a type
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/match/match-unresolved-one-arm.stderr b/src/test/ui/match/match-unresolved-one-arm.stderr
index ad8569b4802..77df9921b75 100644
--- a/src/test/ui/match/match-unresolved-one-arm.stderr
+++ b/src/test/ui/match/match-unresolved-one-arm.stderr
@@ -2,10 +2,7 @@ error[E0282]: type annotations needed
   --> $DIR/match-unresolved-one-arm.rs:4:9
    |
 LL |     let x = match () {
-   |         ^
-   |         |
-   |         cannot infer type
-   |         consider giving `x` a type
+   |         ^ consider giving `x` a type
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/suggestions/suggest-closure-return-type-1.rs b/src/test/ui/suggestions/suggest-closure-return-type-1.rs
new file mode 100644
index 00000000000..910f273b972
--- /dev/null
+++ b/src/test/ui/suggestions/suggest-closure-return-type-1.rs
@@ -0,0 +1,3 @@
+fn main() {
+    let _v = || -> _ { [] }; //~ ERROR type annotations needed for the closure
+}
diff --git a/src/test/ui/suggestions/suggest-closure-return-type-1.stderr b/src/test/ui/suggestions/suggest-closure-return-type-1.stderr
new file mode 100644
index 00000000000..de2d29f1270
--- /dev/null
+++ b/src/test/ui/suggestions/suggest-closure-return-type-1.stderr
@@ -0,0 +1,13 @@
+error[E0282]: type annotations needed for the closure `fn() -> [_; 0]`
+  --> $DIR/suggest-closure-return-type-1.rs:2:24
+   |
+LL |     let _v = || -> _ { [] };
+   |                        ^^ cannot infer type
+help: give this closure an explicit return type without `_` placeholders
+   |
+LL |     let _v = || -> [_; 0] { [] };
+   |                    ^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.
diff --git a/src/test/ui/suggestions/suggest-closure-return-type-2.rs b/src/test/ui/suggestions/suggest-closure-return-type-2.rs
new file mode 100644
index 00000000000..6955b37ad97
--- /dev/null
+++ b/src/test/ui/suggestions/suggest-closure-return-type-2.rs
@@ -0,0 +1,3 @@
+fn main() {
+    let _v = || { [] }; //~ ERROR type annotations needed for the closure
+}
diff --git a/src/test/ui/suggestions/suggest-closure-return-type-2.stderr b/src/test/ui/suggestions/suggest-closure-return-type-2.stderr
new file mode 100644
index 00000000000..9dbd822fbb5
--- /dev/null
+++ b/src/test/ui/suggestions/suggest-closure-return-type-2.stderr
@@ -0,0 +1,13 @@
+error[E0282]: type annotations needed for the closure `fn() -> [_; 0]`
+  --> $DIR/suggest-closure-return-type-2.rs:2:19
+   |
+LL |     let _v = || { [] };
+   |                   ^^ cannot infer type
+help: give this closure an explicit return type without `_` placeholders
+   |
+LL |     let _v = || -> [_; 0] { [] };
+   |                 ^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.
diff --git a/src/test/ui/suggestions/suggest-closure-return-type-3.rs b/src/test/ui/suggestions/suggest-closure-return-type-3.rs
new file mode 100644
index 00000000000..ec6c094027e
--- /dev/null
+++ b/src/test/ui/suggestions/suggest-closure-return-type-3.rs
@@ -0,0 +1,3 @@
+fn main() {
+    let _v = || []; //~ ERROR type annotations needed for the closure
+}
diff --git a/src/test/ui/suggestions/suggest-closure-return-type-3.stderr b/src/test/ui/suggestions/suggest-closure-return-type-3.stderr
new file mode 100644
index 00000000000..ad0d4e41f78
--- /dev/null
+++ b/src/test/ui/suggestions/suggest-closure-return-type-3.stderr
@@ -0,0 +1,13 @@
+error[E0282]: type annotations needed for the closure `fn() -> [_; 0]`
+  --> $DIR/suggest-closure-return-type-3.rs:2:17
+   |
+LL |     let _v = || [];
+   |                 ^^ cannot infer type
+help: give this closure an explicit return type without `_` placeholders
+   |
+LL |     let _v = || -> [_; 0] { [] };
+   |                 ^^^^^^^^^^^    ^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.