Negative numbers are not correctly identified as literals inside macros

[connorskees]

Negative numbers are no longer parsed as literals when used as a parameter to a macro inside a macro. This appears to be a regression from stable to nightly.

According to cargo-bisect-rustc, #69023 is responsible (thanks to @jebrosen for helping with this on the Rust discord)
Specifically, the changes to this function look suspicious.

The below fails to compile:

macro_rules! foo {
    ($a:literal) => {
        bar!($a)
    }
}

macro_rules! bar {
    ($b:literal) => {}
}

fn main() {
    // fails on nightly, but not stable
    foo!(-2);
    // doesn't fail
    bar!(-2);
}

with the following error on nightly-2020-02-14,

~ >cargo b
error: no rules expected the token `-2`
  --> src\main.rs:3:14
   |
3  |         bar!($a)
   |              ^^ no rules expected this token in macro call
...
7  | macro_rules! bar {
   | ---------------- when calling this macro
...
12 |     foo!(-2);
   |     --------- in this macro invocation
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

[jebrosen]

rustc 1.42.0-nightly (8417d68de 2020-02-03) (compiles)
rustc 1.43.0-nightly (58b834344 2020-02-05) (does not compile)

Strange, with 58b834344 2020-02-05 I can compile this successfully. Here's my cargo-bisect-rustc report, after originally narrowing it down from 2020-01-25 to 2020-02-14:

searched nightlies: from nightly-2020-02-13 to nightly-2020-02-14
regressed nightly: nightly-2020-02-14
searched commits: from https://github.com/rust-lang/rust/commit/a1912f2e89b77cfe2a0e64b96f444848fe4e2d49 to https://github.com/rust-lang/rust/commit/5d04ce67fd14538d03fa47a2598f80d49fd564c6
regressed commit: https://github.com/rust-lang/rust/commit/be493fe8cc40c3d3f6030a1313c1ff747fce770d

Specifically, the changes to this function look suspicious.

My reason for interest in that particular line is that it appears to be the only change that isn't related to async or const, but it is related to literals.

[jonas-schievink]

cc @Centril

[Centril]

Note to self: "this function" refers to pub fn can_begin_literal_or_bool(&self) -> bool.