Code change:
LL | fn test_many_bounds_where<X>(x: X) where X: Sized, X: Sized, X: Debug { |                                                            ^^^^^^^^^^ <del> error: aborting due to 6 previous errors </del> <ins> error[E0277]: the size for values of type `Self` cannot be known at compilation time --> $DIR/bound-suggestions.rs:44:46 | LL |     const SIZE: usize = core::mem::size_of::<Self>(); |                                              ^^^^ doesn't have a size known at compile-time | ::: $SRC_DIR/core/src/mem/mod.rs:LL:COL | LL | pub const fn size_of<T>() -> usize { |                      - required by this bound in `std::mem::size_of` | help: consider further restricting `Self` | LL | trait Foo<T>: Sized { |             ^^^^^^^ error[E0277]: the size for values of type `Self` cannot be known at compilation time --> $DIR/bound-suggestions.rs:49:46 | LL |     const SIZE: usize = core::mem::size_of::<Self>(); |                                              ^^^^ doesn't have a size known at compile-time | ::: $SRC_DIR/core/src/mem/mod.rs:LL:COL | LL | pub const fn size_of<T>() -> usize { |                      - required by this bound in `std::mem::size_of` | help: consider further restricting `Self` | LL | trait Bar: std::fmt::Display + Sized { |                              ^^^^^^^ error[E0277]: the size for values of type `Self` cannot be known at compilation time --> $DIR/bound-suggestions.rs:54:46 | LL |     const SIZE: usize = core::mem::size_of::<Self>(); |                                              ^^^^ doesn't have a size known at compile-time | ::: $SRC_DIR/core/src/mem/mod.rs:LL:COL | LL | pub const fn size_of<T>() -> usize { |                      - required by this bound in `std::mem::size_of` | help: consider further restricting `Self` | LL | trait Baz: Sized where Self: std::fmt::Display { |          ^^^^^^^ error[E0277]: the size for values of type `Self` cannot be known at compilation time --> $DIR/bound-suggestions.rs:59:46 | LL |     const SIZE: usize = core::mem::size_of::<Self>(); |                                              ^^^^ doesn't have a size known at compile-time | ::: $SRC_DIR/core/src/mem/mod.rs:LL:COL | LL | pub const fn size_of<T>() -> usize { |                      - required by this bound in `std::mem::size_of` | help: consider further restricting `Self` | LL | trait Qux<T>: Sized where Self: std::fmt::Display { |             ^^^^^^^ error[E0277]: the size for values of type `Self` cannot be known at compilation time --> $DIR/bound-suggestions.rs:64:46 | LL |     const SIZE: usize = core::mem::size_of::<Self>(); |                                              ^^^^ doesn't have a size known at compile-time | ::: $SRC_DIR/core/src/mem/mod.rs:LL:COL | LL | pub const fn size_of<T>() -> usize { |                      - required by this bound in `std::mem::size_of` | help: consider further restricting `Self` | LL | trait Bat<T>: std::fmt::Display + Sized { |                                 ^^^^^^^ error: aborting due to 11 previous errors </ins> For more information about this error, try `rustc --explain E0277`.

Related discussion:
text: The following code errors, and the error contains a suggestion how to fix the error: The error message states: Obviously, writing is invalid syntax.
It seems the issue is that it's inserting the suggestion after the trait name and before the generics, rather than after the generics.
No-nightly-features code that exhibits the same problem: label -requires-nightly (should the F-labels go as well?)