The style guide may or may not prescribe formatting behavior for nightly syntax in the "nightly" chapter in preparation for a feature's eventual stabilization or to reflect actual the behavior implemented by rustfmt around nightly syntax. This behavior is not binding until the feature is stabilized and the text is moved into the appropriate section of the style guide.
Initial PR(s) implementing new syntax filed against rust-lang/rust should generally leave the rustfmt subtree untouched. In cases that that need to modify rustfmt (for example, to fix compiler errors that come from adding new variants to AST representation), they should modify rustfmt in such a way to keep existing formatting verbatim.
Rustfmt is allowed to implement nightly-only formatting behavior without that syntax being specified by the style guide. The initial authors of PRs implementing new features in rust-lang/rust are encouraged, but not required, to open a PR against rustfmt suggesting an initial formatting behavior, or formatting may later be implemented as a PR by anyone, pending approval of the implementation from T-rustfmt. T-style should be notified to approve the interim style proposed by these PRs, but this decision is not binding and may be revisited until the feature is stabilized and the formatting is codified in the style guide.
Much like breaking nightly feature changes in the Rust compiler, any changes to formatting behavior for nightly syntax should be made cautiously and with thorough consideration to avoid churn. Changes should not be done unnecessarily, and should take into account the feature's adoption and readiness for stabilization. However, changes may be done until the feature is stabilized for various reasons: new understanding of the feature's usage in the language, recommendation from T-style, or changes in the implementation of the feature.
Feature stabilization should be blocked on confirmation and codification of formatting behavior. At this point, T-style may also propose alternative formatting at the time of stabilization, with any breaking changes weighted according to the breaking changes principle stated above.