x/tools/gopls: Completion: SEGV in deepSearch (cand slice has invalid data pointer)

[adonovan]

#!stacks
"sigpanic" && "completion.(*completer).deepSearch:+65"

Issue created by stacks.

This stack korW0A was reported by telemetry:

			// When searching deep, make sure we don't have a cycle in our chain.
			// We don't dedupe by object because we want to allow both "foo.Baz"
			// and "bar.Baz" even though "Baz" is represented the same types.Object
			// in both.
			for _, seenObj := range cand.path { <--- SEGV
				if seenObj == obj {
					continue outer
				}
			}

• crash/crash
• runtime.throw:+9
• runtime.sigpanic:+33
• golang.org/x/tools/gopls/internal/golang/completion.(*completer).deepSearch:+65
• golang.org/x/tools/gopls/internal/golang/completion.Completion:+169
• golang.org/x/tools/gopls/internal/server.(*server).Completion:+19
• golang.org/x/tools/gopls/internal/protocol.serverDispatch:+193
• golang.org/x/tools/gopls/internal/lsprpc.(*streamServer).ServeStream.ServerHandler.func3:+5
• golang.org/x/tools/gopls/internal/lsprpc.(*streamServer).ServeStream.handshaker.func4:+52
• golang.org/x/tools/gopls/internal/protocol.Handlers.MustReplyHandler.func1:+2
• golang.org/x/tools/gopls/internal/protocol.Handlers.AsyncHandler.func2.2:+3
• runtime.goexit:+0

golang.org/x/tools/gopls@v0.18.1 go1.23.6 linux/amd64 vscode (1)

Related:

• x/tools/gopls: Completion: SIGSEGV in scorePenalty #71355, which was closed as spurious/unreproducible. Different gopls (and thus go) version.