lsp/completion: fix postfix completions preceding assignments

In cases like:

  foo.<>
  bar = 123

We weren't detecting that the selector preceding <> was a statement.
The above is parsed as "foo.<>bar = 123", so it looks like <> is
contained in an AssignStmt. This matters because we give different
postfix completions depending on whether it is valid to replace the
surrounding selector with a statement or not.

Updates golang/go#45718.

Change-Id: I8f74505b2c8c7060f1e94433904ff0a987d0cc57
Reviewed-on: https://go-review.googlesource.com/c/tools/+/313269
Run-TryBot: Muir Manders <muir@mnd.rs>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Trust: Rebecca Stambler <rstambler@golang.org>

internal/lsp/source/completion/postfix_snippets.go

@@ -303,11 +303,25 @@ func (c *completer) addPostfixSnippetCandidates(ctx context.Context, sel *ast.Se
 		return
 	}
 
+	tokFile := c.snapshot.FileSet().File(c.pos)
+
 	// Only replace sel with a statement if sel is already a statement.
 	var stmtOK bool
 	for i, n := range c.path {
 		if n == sel && i < len(c.path)-1 {
-			_, stmtOK = c.path[i+1].(*ast.ExprStmt)
+			switch p := c.path[i+1].(type) {
+			case *ast.ExprStmt:
+				stmtOK = true
+			case *ast.AssignStmt:
+				// In cases like:
+				//
+				//   foo.<>
+				//   bar = 123
+				//
+				// detect that "foo." makes up the entire statement since the
+				// apparent selector spans lines.
+				stmtOK = tokFile.Line(c.pos) < tokFile.Line(p.TokPos)
+			}
 			break
 		}
 	}
@@ -328,7 +342,6 @@ func (c *completer) addPostfixSnippetCandidates(ctx context.Context, sel *ast.Se
 	//
 	// and adjust afterDot so that we don't mistakenly delete the
 	// newline thinking "bar" is part of our selector.
-	tokFile := c.snapshot.FileSet().File(c.pos)
 	if startLine := tokFile.Line(sel.Pos()); startLine != tokFile.Line(afterDot) {
 		if tokFile.Line(c.pos) != startLine {
 			return

internal/lsp/testdata/snippets/postfix.go

@@ -25,3 +25,18 @@ func _() {
 	var blah func() []int
 	blah().append //@complete(" //")
 }
+
+func _() {
+	/* append! */ //@item(postfixAppend, "append!", "append and re-assign slice", "snippet")
+	/* last! */ //@item(postfixLast, "last!", "s[len(s)-1]", "snippet")
+	/* print! */ //@item(postfixPrint, "print!", "print to stdout", "snippet")
+	/* range! */ //@item(postfixRange, "range!", "range over slice", "snippet")
+	/* reverse! */ //@item(postfixReverse, "reverse!", "reverse slice", "snippet")
+	/* sort! */ //@item(postfixSort, "sort!", "sort.Slice()", "snippet")
+	/* var! */ //@item(postfixVar, "var!", "assign to variable", "snippet")
+
+	var foo []int
+	foo. //@complete(" //", postfixAppend, postfixCopy, postfixLast, postfixPrint, postfixRange, postfixReverse, postfixSort, postfixVar)
+
+		foo = nil
+}

internal/lsp/testdata/summary.txt.golden

@@ -1,7 +1,7 @@
 -- summary --
 CallHierarchyCount = 2
 CodeLensCount = 5
-CompletionsCount = 262
+CompletionsCount = 263
 CompletionSnippetCount = 95
 UnimportedCompletionsCount = 5
 DeepCompletionsCount = 5