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I didn’t see that error. It seems cbindgen failed to generate .toml file probably rust copmpiler use.
cbindgen is a rust binding generator for C(/limited C++) library. Mozilla’s build system has build phases including configure, export, compile, misc and so on. In export phase it generates source files from data or other type files. cbindgen failed to generate correct output here, but not catched.
I don’t know what failed in detail, but want to see clean generated files and regenerate it.
Run ./mach clobber which cleans build output, then ./mach build again.
See also: