Symbol::intern("cfg_if") changes to "//!" during rustfmt lifetime

[topecongiro]

In rustfmt, we define rustc_span::Symbol like the following:

lazy_static! {
    static ref CFG_IF: Symbol = Symbol::intern("cfg_if");
}

Recently we found out that the value of CFG_IF may change to other value (cc rust-lang/rustfmt#4266 (comment)).

I was assuming that the value defined inside Symbol::intern is immutable within the same process. Is this assumption incorrect?

[ehuss]

I believe that the interned symbols are only scoped per thread for the duration of the Globals object. It looks like rustfmt re-initializes the globals for each input it processes. If the first input triggers the CFG_IF lazy static, it will initialize the Symbol in the first Globals. Then during the next input, there is a fresh Globals, but the CFG_IF lazy static still has a value from the first one.

[topecongiro]

@ehuss Thank you for the reply! It seems like we are using the API in a way it wasn't intended.

I will be closing this as it is a rustfmt-side issue; sorry for the noise.