Using `while` in `const fn` results in a misleading error message on stable

[not-an-aardvark]

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When while is used in a const fn on stable, it produces an error message: `if`, `match`, `&&` and `||` are not stable in const fn. This is a bit confusing because the function does not explicitly contain if, match, &&, or ||.

I would expect the error message to also mention that while is disallowed, or say that "loops are not allowed in const fn" (which it does when loop {} construct is encountered).

(It seems like it would be even better to have a specific error message for the disallowed construct that the compiler encounters rather than just providing a list of disallowed constructs, if this is possible architecturally.)

rustc version

rustc 1.35.0 (3c235d560 2019-05-20)
binary: rustc
commit-hash: 3c235d5600393dfe6c36eeed34042efad8d4f26e
commit-date: 2019-05-20
host: x86_64-apple-darwin
release: 1.35.0
LLVM version: 8.0