error: casts cannot be followed by a function call
  --> $DIR/issue-54516.rs:4:20
   |
LL |     println!("{}", std::mem:size_of::<BTreeMap<u32, u32>>());
   |                    ^^^^^^^^-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |                            |
   |                            help: maybe write a path separator here: `::`
   |
   = note: `#![feature(type_ascription)]` lets you annotate an expression with a type: `<expr>: <type>`
   = note: see issue #23416 <https://github.com/rust-lang/rust/issues/23416> for more information

error[E0423]: expected value, found module `std::mem`
  --> $DIR/issue-54516.rs:4:20
   |
LL |     println!("{}", std::mem:size_of::<BTreeMap<u32, u32>>());
   |                    ^^^^^^^^- help: maybe you meant to write a path separator here: `::`
   |                    |
   |                    not a value

error[E0412]: cannot find type `size_of` in this scope
  --> $DIR/issue-54516.rs:4:29
   |
LL |     println!("{}", std::mem:size_of::<BTreeMap<u32, u32>>());
   |                            -^^^^^^^ not found in this scope
   |                            |
   |                            help: maybe you meant to write a path separator here: `::`

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0412, E0423.
For more information about an error, try `rustc --explain E0412`.