internal/lsp/source: support completing "range" keyword

We now offer "range" keyword in the for loop init statement:

for i := r<> // offer "range" completion candidate

Updates golang/go#34009.

Change-Id: I2e4c1db11c37127406c78191681c39b9dd3439f7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/220504
Run-TryBot: Muir Manders <muir@mnd.rs>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/source/completion_keywords.go

@@ -86,6 +86,14 @@ func (c *completer) addKeywordCompletions() {
 		return
 	}
 
+	if len(c.path) > 2 {
+		// Offer "range" if we are in ast.ForStmt.Init. This is what the
+		// AST looks like before "range" is typed, e.g. "for i := r<>".
+		if loop, ok := c.path[2].(*ast.ForStmt); ok && nodeContains(loop.Init, c.pos) {
+			addKeywords(stdScore, RANGE)
+		}
+	}
+
 	// Only suggest keywords if we are beginning a statement.
 	switch c.path[1].(type) {
 	case *ast.BlockStmt, *ast.CommClause, *ast.CaseClause, *ast.ExprStmt:

internal/lsp/testdata/lsp/primarymod/keywords/keywords.go

@@ -61,6 +61,7 @@ func _() {
 	v //@complete(" //", var)
 	c //@complete(" //", const)
 
+	for i := r //@complete(" //", range)
 }
 
 /* package */ //@item(package, "package", "", "keyword")
@@ -90,3 +91,4 @@ func _() {
 /* map */ //@item(map, "map", "", "keyword")
 /* func */ //@item(func, "func", "", "keyword")
 /* chan */ //@item(chan, "chan", "", "keyword")
+/* range */ //@item(range, "range", "", "keyword")

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

@@ -1,6 +1,6 @@
 -- summary --
 CodeLensCount = 0
-CompletionsCount = 229
+CompletionsCount = 230
 CompletionSnippetCount = 68
 UnimportedCompletionsCount = 11
 DeepCompletionsCount = 5