This is rocket_codegen minimized down to 14 exported macros that just loop { }, and the plugin_registrar function that registers them.
$ git clone https://github.com/ExpHP/Rocket-issue-48387
$ cd Rocket-issue-48387
$ cargo build --example=issue-48387
Compiling rocket_codegen v0.4.0-dev (file:///home/lampam/asd/clone/Rocket)
error: internal compiler error: Error constructed but not emitted
thread 'rustc' panicked at 'explicit panic', librustc_errors/diagnostic_builder.rs:242:13
A function that just loops surely cannot be responsible for dropping a DiagnosticBuilder, so the Error constructed but not emitted must be a bug in rustc.