Fix nimsuggest/#117

[Clyybber]

Fixes nim-lang/nimsuggest#117

[alaviss]

Should add [backport:1.4] if this is accepted.

[alaviss]

Looks like the --backend switch triggers some magic like defining the symbols corresponding to the backend:

Nim/compiler/main.nim

Lines 178 to 201 in d66e274

	proc customizeForBackend(backend: TBackend) =
	## Sets backend specific options but don't compile to backend yet in
	## case command doesn't require it. This must be called by all commands.
	if conf.backend == backendInvalid:
	# only set if wasn't already set, to allow override via `nim c -b:cpp`
	conf.backend = backend
	defineSymbol(graph.config.symbols, $conf.backend)
	case conf.backend
	of backendC:
	if conf.exc == excNone: conf.exc = excSetjmp
	of backendCpp:
	if conf.exc == excNone: conf.exc = excCpp
	of backendObjc: discard
	of backendJs:
	if conf.hcrOn:
	# XXX: At the moment, system.nim cannot be compiled in JS mode
	# with "-d:useNimRtl". The HCR option has been processed earlier
	# and it has added this define implictly, so we must undo that here.
	# A better solution might be to fix system.nim
	undefSymbol(conf.symbols, "useNimRtl")
	of backendInvalid: doAssert false
	if conf.selectedGC in {gcArc, gcOrc} and conf.backend != backendCpp:
	conf.exc = excGoto

And only the main compiler has it... We should move some of this over to the compiler library.

[alaviss]

/cc @timotheecour

[timotheecour]

hard to pinpoint where error started without git bisect but this commit also seems relevant: #15549

[Clyybber]

Error disappeared after clearing nimcache, going to close this for now then.

[Clyybber]

@timotheecour Its caused by this change 9502e39#diff-2179cfe66f0e3b26b6475c540e7af193fcaef3ccad352b45f43ecb488e99cc8bL629, although arguably we could/should? prevent the call to externalFileChanged without a backend specified in the first place, but that is preexisting behaviour, improving that counts as an optimization.

[Clyybber]

@narimiran Should be backported to 1.4, LMK if I should make a PR for the 1.4 branch