Use param.name.indent().span if trait bounds not exists.

To fix the following issue.
Diagnostic suggests adding : ?Sized in an incorrect place if a type parameter default is present

Author: OdenShirataki

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -2993,7 +2993,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         {
             (s, " +")
         } else {
-            (span.shrink_to_hi(), ":")
+            (param.name.ident().span.shrink_to_hi(), ":")
         };
         err.span_suggestion_verbose(
             span,

tests/ui/traits/issue-120878.rs

@@ -0,0 +1,10 @@
+fn main() {
+    struct StructA<A, B = A> {
+        _marker: std::marker::PhantomData<fn() -> (A, B)>,
+    }
+
+    struct StructB {
+        //~^ ERROR: the size for values of type `[u8]` cannot be known
+        a: StructA<isize, [u8]>,
+    }
+}

tests/ui/traits/issue-120878.stderr

@@ -0,0 +1,20 @@
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/issue-120878.rs:8:12
+   |
+LL |         a: StructA<isize, [u8]>,
+   |            ^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `[u8]`
+note: required by an implicit `Sized` bound in `StructA`
+  --> $DIR/issue-120878.rs:2:23
+   |
+LL |     struct StructA<A, B = A> {
+   |                       ^^^^^ required by the implicit `Sized` requirement on this type parameter in `StructA`
+help: consider relaxing the implicit `Sized` restriction
+   |
+LL |     struct StructA<A, B: ?Sized = A> {
+   |                        ++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/traits/issue-28576.stderr

@@ -36,8 +36,8 @@ LL | pub trait Bar: Foo<Assoc=()> + Sized {
    |                              +++++++
 help: consider relaxing the implicit `Sized` restriction
    |
-LL | pub trait Foo<RHS=Self: ?Sized> {
-   |                       ++++++++
+LL | pub trait Foo<RHS: ?Sized=Self> {
+   |                  ++++++++
 
 error[E0277]: the size for values of type `Self` cannot be known at compilation time
   --> $DIR/issue-28576.rs:5:16
@@ -56,8 +56,8 @@ LL |     ) where Self: Sized;
    |       +++++++++++++++++
 help: consider relaxing the implicit `Sized` restriction
    |
-LL | pub trait Foo<RHS=Self: ?Sized> {
-   |                       ++++++++
+LL | pub trait Foo<RHS: ?Sized=Self> {
+   |                  ++++++++
 
 error: aborting due to 3 previous errors