A "mutable" variable is contrasted to an "immutable" variable, not to a "constant".
You may not like the name "variable" for something that cannot be changed within a given scope, but it's still something that can take multiple values during the execution of a program.
This thing and Rust: "constant" -> something unable to change "mutable constant" -> a changeable constant...what? Even "mutable variable" -> a changeable thing that is a thing that is subject change doesn't make much more sense.
It is fine for "things" to be immutable by default, and in fact I think they should be. I just think "mutable" is keyword smell similar to "decltype" because type wasn't keyworded from the start.
"What?" indeed. Rust doesn't have "mutable constant". Rust's "const" is actually a constant, unlike in C where "const" means "Sort of immutable, although maybe not".
I guess maybe you've been told something about Rust like "let is kinda like C++ const" and so you've come to the erroneous conclusion that somehow "let mut" means "mutable constant" but that's just because you didn't really understand, blame either your attention or the poor explanation, it's surely nothing to do with Rust which has never said this is "mutable constant" since that's nonsense.