That seems slightly pedantic. While it's true that there's no cross-platform C ABI, most platforms have a C ABI, and it's well-defined (and defined in terms of C data types) on that platform.
It's sort of like saying that there's no such thing as "rules of the road", because the road makers don't specify how you're supposed to drive on them, and the rules differ by jurisdiction and sometimes you're in a wilderness with no roads. The term is commonly understood to mean "rules for using roads, in the road-containing jurisdiction which you're in". I think the same thing is true of the phrase "C ABI", although if people are coming away thinking there's a cross-platform C ABI, then yes, we should be more precise.
> That seems slightly pedantic. While it's true that there's no cross-platform C ABI, most platforms have a C ABI, and it's well-defined (and defined in terms of C data types) on that platform.
Every platform has an ABI, obviously, and yes it's generally formed in view of C.
This doesn't change the fact that the phrase "The C ABI" is, in a vacuum, meaningless and that "The C ABI on Platform X" is just a convoluted, cart-before-the-horse way of saying "Platform X's ABI"
But what an ABI does is specify a binding between language-level concepts and binary representations. That's why mentioning the language matters. If "Platform X's ABI" says that structs are passed on the stack, or ints are a certain size, or pointers have a certain alignment, or whatever, it (usually) means that C structures are passed on the C stack, C ints are a certain size, C pointers have a certain alignment, etc. Another language can use its own data types and even its own concept of a stack. (For instance, Go uses a C-incompatible stack.) And the reason that we care about "the C ABI" is whether the implementation of a language on platform X has data types, stack usage, etc. that matches the implementation of C on platform X.
Put another way, I'm arguing that "C" is a more useful descriptor than "platform X's", because the sentence "Rust is compatible with the C ABI" is short for "forall X, Rust on platform X is compatible with the C ABI on platform X", and means something different from "Rust is compatible with the SysV ABI" (which implies that it follows the SysV ABI on Windows and OS X, too). And since the typical way of doing that is by interfacing with a C compiler or the C library (... and you could make this exact argument about the phrase "C library", I think), it's worth mentioning C. For instance, Vala and Nim, which both compile to C, are always compatible with the C ABI. If you port them to a mainframe they remain compatible with the C ABI, but not with the native ABI.
C is merely flexible, not in some position of authority where it dictates what platforms must do. C would be ABI-compatible on a platform that wasn't designed for it, too (which, historically, was the case on many platforms it was ported to).
It's sort of like saying that there's no such thing as "rules of the road", because the road makers don't specify how you're supposed to drive on them, and the rules differ by jurisdiction and sometimes you're in a wilderness with no roads. The term is commonly understood to mean "rules for using roads, in the road-containing jurisdiction which you're in". I think the same thing is true of the phrase "C ABI", although if people are coming away thinking there's a cross-platform C ABI, then yes, we should be more precise.