Add notes on unsized argument errors.

src/librustc/traits/error_reporting.rs

@@ -1455,6 +1455,9 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             ObligationCauseCode::VariableType(_) => {
                 err.note("all local variables must have a statically known size");
             }
+            ObligationCauseCode::SizedArgumentType => {
+                err.note("all function arguments must have a statically known size");
+            }
             ObligationCauseCode::SizedReturnType => {
                 err.note("the return type of a function must have a \
                           statically known size");

src/librustc/traits/mod.rs

@@ -185,6 +185,8 @@ pub enum ObligationCauseCode<'tcx> {
     StructInitializerSized,
     /// Type of each variable must be Sized
     VariableType(ast::NodeId),
+    /// Argument type must be Sized
+    SizedArgumentType,
     /// Return type must be Sized
     SizedReturnType,
     /// Yield type must be Sized

src/librustc/traits/structural_impls.rs

@@ -203,6 +203,7 @@ impl<'a, 'tcx> Lift<'tcx> for traits::ObligationCauseCode<'a> {
             super::StructInitializerSized => Some(super::StructInitializerSized),
             super::VariableType(id) => Some(super::VariableType(id)),
             super::ReturnType(id) => Some(super::ReturnType(id)),
+            super::SizedArgumentType => Some(super::SizedArgumentType),
             super::SizedReturnType => Some(super::SizedReturnType),
             super::SizedYieldType => Some(super::SizedYieldType),
             super::RepeatVec => Some(super::RepeatVec),

src/librustc_typeck/check/mod.rs

@@ -1049,7 +1049,7 @@ fn check_fn<'a, 'gcx, 'tcx>(inherited: &'a Inherited<'a, 'gcx, 'tcx>,
         // for simple cases like `fn foo(x: Trait)`,
         // where we would error once on the parameter as a whole, and once on the binding `x`.
         if arg.pat.simple_ident().is_none() {
-            fcx.require_type_is_sized(arg_ty, decl.output.span(), traits::MiscObligation);
+            fcx.require_type_is_sized(arg_ty, decl.output.span(), traits::SizedArgumentType);
         }
 
         fcx.write_ty(arg.hir_id, arg_ty);

src/test/ui/issues/issue-38954.stderr

@@ -6,6 +6,7 @@ LL | fn _test(ref _p: str) {}
    |
    = help: the trait `std::marker::Sized` is not implemented for `str`
    = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+   = note: all function arguments must have a statically known size
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-41229-ref-str.stderr

@@ -6,6 +6,7 @@ LL | pub fn example(ref s: str) {}
    |
    = help: the trait `std::marker::Sized` is not implemented for `str`
    = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+   = note: all function arguments must have a statically known size
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-42312.stderr

@@ -7,6 +7,7 @@ LL |     fn baz(_: Self::Target) where Self: Deref {}
    = help: the trait `std::marker::Sized` is not implemented for `<Self as std::ops::Deref>::Target`
    = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
    = help: consider adding a `where <Self as std::ops::Deref>::Target: std::marker::Sized` bound
+   = note: all function arguments must have a statically known size
 
 error[E0277]: the size for values of type `(dyn std::string::ToString + 'static)` cannot be known at compilation time
   --> $DIR/issue-42312.rs:18:23
@@ -16,6 +17,7 @@ LL | pub fn f(_: ToString) {}
    |
    = help: the trait `std::marker::Sized` is not implemented for `(dyn std::string::ToString + 'static)`
    = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
+   = note: all function arguments must have a statically known size
 
 error: aborting due to 2 previous errors