internal/lsp/source: fix completion crash in append()

We were crashing in cases like:

    var _ []byte = append([]byte{}, ""...<>)

We were type asserting the type of append's second param
to *types.Slice, but in this case it is a string (*types.Basic). Fix
by checking the type assert was successful.

Note that we still don't attempt to give string completions when
appending to a byte slice. We can add that special case later once
everyone is clamoring for it.

Change-Id: I1d2fbd7f538e580d33c2dab4ef127a88e16d7ced
Reviewed-on: https://go-review.googlesource.com/c/tools/+/219144
Run-TryBot: Muir Manders <muir@mnd.rs>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/source/completion.go

@@ -1424,7 +1424,9 @@ Nodes:
 							// param w/ further expressions, we expect a single
 							// variadic item.
 							if beyondLastParam || isLastParam && len(node.Args) > numParams {
-								inf.objType = sig.Params().At(numParams - 1).Type().(*types.Slice).Elem()
+								if slice, ok := sig.Params().At(numParams - 1).Type().(*types.Slice); ok {
+									inf.objType = slice.Elem()
+								}
 								break Nodes
 							}
 

internal/lsp/testdata/lsp/primarymod/builtins/builtin_args.go

@@ -22,6 +22,8 @@ func _() {
 
 	append() //@rank(")", builtinSlice, builtinChan)
 
+	var _ []byte = append([]byte(nil), ""...) //@rank(") //")
+
 	copy()           //@rank(")", builtinSlice, builtinChan)
 	copy(aSlice, aS) //@rank(")", builtinSlice, builtinString)
 	copy(aS, aSlice) //@rank(",", builtinSlice, builtinString)

internal/lsp/testdata/lsp/summary.txt.golden

@@ -4,7 +4,7 @@ CompletionSnippetCount = 67
 UnimportedCompletionsCount = 11
 DeepCompletionsCount = 5
 FuzzyCompletionsCount = 8
-RankedCompletionsCount = 85
+RankedCompletionsCount = 86
 CaseSensitiveCompletionsCount = 4
 DiagnosticsCount = 38
 FoldingRangesCount = 2