missing_fragment_specifier hard error

[pietroalbini]

In the beta run for Rust 1.47 the release team discovered that #75516 is breaking more than 475 crates or git repositories currently compiling, across at least the following crates:

• js-sys
• clap
• time-macros-impl

cc @petrochenkov @matklad @rust-lang/release

[pietroalbini]

Personally I think that's too much breakage to land, even though it was a deprecation warning since 2017.

[petrochenkov]

I agree that it's not an acceptable breakage, although I suspect that most of it happens due to committed Cargo.locks.
I think we can add a hack accepting a missing fragment specifier for the 2-3 specific crates responsible for majority of the breakage, like it was done in #73345.
(In a more local way than before #75516, without Option<NonterminalKind> in MetaVarDecl, and without missing_fragment_specifiers in session.)

[Mark-Simulacrum]

We could also make it a hard warning rather than error for all crates, but I'm not sure if that's better. It also doesn't need session state.

[tmandry]

To expand a bit: cargo hides warnings generated by your dependencies, so crate owners don't feel the pain of depending on these old versions. I believe it passes -Awarnings to rustc, but otherwise passes through the output unchanged. If we had a warning that printed in spite of -Awarnings it would encourage people to update their deps.

I assume that is what is meant by "hard warning"?

[Mark-Simulacrum]

I've posted a revert to current beta (#77456) but we still need to fix this on master. Moving milestone to 1.48, though.