Improve suggestions for closure parameters

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -617,6 +617,32 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     });
 
                 if let Some((decl, body_id)) = closure_decl_and_body_id {
+                    for arg in fn_sig.inputs().skip_binder().iter() {
+                        if let Some((ty, ty_span)) = self.unresolved_type_vars(arg) {
+                            let ty_span = match ty_span {
+                                Some(ty_span) => ty_span,
+                                None => match ty.ty_vid() {
+                                    Some(ty_vid) => {
+                                        self.inner
+                                            .borrow_mut()
+                                            .type_variables()
+                                            .var_origin(ty_vid)
+                                            .span
+                                    }
+                                    None => continue,
+                                },
+                            };
+                            err.span_label(
+                                ty_span,
+                                "give this closure parameter an explicit type instead of `_`",
+                            );
+                            // We don't want to give the other suggestions when the problem is
+                            // a closure parameter type.
+                            return err;
+                        }
+                    }
+
+                    // Parameters were all fine, assume there was an issue with the return type
                     closure_return_type_suggestion(
                         &mut err,
                         &decl.output,

src/test/ui/closures/closure-signature/unspecified-output.rs

@@ -0,0 +1,3 @@
+fn main() {
+    let a = |a: i32, b: &i32| Vec::new(); //~ ERROR E0282
+}

src/test/ui/closures/closure-signature/unspecified-output.stderr

@@ -0,0 +1,14 @@
+error[E0282]: type annotations needed for the closure `fn(i32, &i32) -> Vec<_>`
+  --> $DIR/unspecified-output.rs:2:31
+   |
+LL |     let a = |a: i32, b: &i32| Vec::new();
+   |                               ^^^^^^^^ cannot infer type for type parameter `T`
+   |
+help: give this closure an explicit return type without `_` placeholders
+   |
+LL |     let a = |a: i32, b: &i32| -> Vec<_> { Vec::new() };
+   |                               +++++++++++            +
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.

src/test/ui/closures/closure-signature/unspecified-parameter-fn.rs

@@ -0,0 +1,3 @@
+fn main() {
+    let a = |a: i32, b: fn(i32) -> _| -> Vec<i32> { Vec::new() }; //~ ERROR E0282
+}

src/test/ui/closures/closure-signature/unspecified-parameter-fn.stderr

@@ -0,0 +1,11 @@
+error[E0282]: type annotations needed for the closure `fn(i32, fn(i32) -> _) -> Vec<i32>`
+  --> $DIR/unspecified-parameter-fn.rs:2:13
+   |
+LL |     let a = |a: i32, b: fn(i32) -> _| -> Vec<i32> { Vec::new() };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^-^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                                    |
+   |                                    give this closure parameter an explicit type instead of `_`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.

src/test/ui/closures/closure-signature/unspecified-parameter-ref.rs

@@ -0,0 +1,3 @@
+fn main() {
+    let a = |a: i32, b: &_| -> Vec<i32> { Vec::new() }; //~ ERROR E0282
+}

src/test/ui/closures/closure-signature/unspecified-parameter-ref.stderr

@@ -0,0 +1,11 @@
+error[E0282]: type annotations needed for the closure `fn(i32, &_) -> Vec<i32>`
+  --> $DIR/unspecified-parameter-ref.rs:2:13
+   |
+LL |     let a = |a: i32, b: &_| -> Vec<i32> { Vec::new() };
+   |             ^^^^^^^^^^^^^-^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                          |
+   |                          give this closure parameter an explicit type instead of `_`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.

src/test/ui/closures/closure-signature/unspecified-parameter-type-param.rs

@@ -0,0 +1,3 @@
+fn main() {
+    let a = |a: i32, b: Vec<_>| -> Vec<i32> { Vec::new() }; //~ ERROR E0282
+}

src/test/ui/closures/closure-signature/unspecified-parameter-type-param.stderr

@@ -0,0 +1,11 @@
+error[E0282]: type annotations needed for the closure `fn(i32, Vec<_>) -> Vec<i32>`
+  --> $DIR/unspecified-parameter-type-param.rs:2:13
+   |
+LL |     let a = |a: i32, b: Vec<_>| -> Vec<i32> { Vec::new() };
+   |             ^^^^^^^^^^^^^^^^-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                             |
+   |                             give this closure parameter an explicit type instead of `_`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.

src/test/ui/closures/closure-signature/unspecified-parameter.rs

@@ -0,0 +1,3 @@
+fn main() {
+    let a = |a: _, b: &i32| -> Vec<i32> { Vec::new() }; //~ ERROR E0282
+}

src/test/ui/closures/closure-signature/unspecified-parameter.stderr

@@ -0,0 +1,9 @@
+error[E0282]: type annotations needed
+  --> $DIR/unspecified-parameter.rs:2:14
+   |
+LL |     let a = |a: _, b: &i32| -> Vec<i32> { Vec::new() };
+   |              ^ consider giving this closure parameter a type
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.