[MBCook]

Why is one capitalized and one lowercase?

I kind of agree with the sibling that the naming is odd. Why not call them String and Slice or String and MutableString? Using different ‘spellings” of the same word seems like it only encourage some confusion.

[masklinn]

> Why is one capitalized and one lowercase?

Because one is a pretty bog-standard struct: https://doc.rust-lang.org/src/alloc/string.rs.html#294-296 while the other is a primitive type: https://doc.rust-lang.org/std/#primitives

Lowercase (i8, u64) or anonymity (`[]`, &, *) means you're dealing with something fundamental to the language. Not just with special compiler/language support (like Result) but way below that, something the language doesn't itself express but has intrinsic knowledge of.

[steveklabnik]

The lowercase one is a language primitive, and so is lowercased like all language primitives.

The uppercase one is a standard library type, and so is uppercased like all library types.

[juliangoldsmith]

std::String is a struct, and str is a type defined by the language.

There's a summary of the differences at [0].

[0]: http://www.ameyalokare.com/rust/2017/10/12/rust-str-vs-Strin...