json: Add json.ParseWithStartPos function

This is like json.Parse but allows specifying a non-default start position,
in case the caller is parsing a fragment from a larger JSON document.

json/parser.go

@@ -8,15 +8,8 @@ import (
 	"github.com/zclconf/go-cty/cty"
 )
 
-func parseFileContent(buf []byte, filename string) (node, hcl.Diagnostics) {
-	tokens := scan(buf, pos{
-		Filename: filename,
-		Pos: hcl.Pos{
-			Byte:   0,
-			Line:   1,
-			Column: 1,
-		},
-	})
+func parseFileContent(buf []byte, filename string, start hcl.Pos) (node, hcl.Diagnostics) {
+	tokens := scan(buf, pos{Filename: filename, Pos: start})
 	p := newPeeker(tokens)
 	node, diags := parseValue(p)
 	if len(diags) == 0 && p.Peek().Type != tokenEOF {

json/parser_test.go

@@ -598,7 +598,49 @@ func TestParse(t *testing.T) {
 
 	for _, test := range tests {
 		t.Run(test.Input, func(t *testing.T) {
-			got, diag := parseFileContent([]byte(test.Input), "")
+			got, diag := parseFileContent([]byte(test.Input), "", hcl.Pos{Byte: 0, Line: 1, Column: 1})
+
+			if len(diag) != test.DiagCount {
+				t.Errorf("got %d diagnostics; want %d", len(diag), test.DiagCount)
+				for _, d := range diag {
+					t.Logf("  - %s", d.Error())
+				}
+			}
+
+			if diff := deep.Equal(got, test.Want); diff != nil {
+				for _, problem := range diff {
+					t.Error(problem)
+				}
+			}
+		})
+	}
+}
+
+func TestParseWithPos(t *testing.T) {
+	tests := []struct {
+		Input     string
+		StartPos  hcl.Pos
+		Want      node
+		DiagCount int
+	}{
+		// Simple, single-token constructs
+		{
+			`true`,
+			hcl.Pos{Byte: 0, Line: 3, Column: 10},
+			&booleanVal{
+				Value: true,
+				SrcRange: hcl.Range{
+					Start: hcl.Pos{Line: 3, Column: 10, Byte: 0},
+					End:   hcl.Pos{Line: 3, Column: 14, Byte: 4},
+				},
+			},
+			0,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.Input, func(t *testing.T) {
+			got, diag := parseFileContent([]byte(test.Input), "", test.StartPos)
 
 			if len(diag) != test.DiagCount {
 				t.Errorf("got %d diagnostics; want %d", len(diag), test.DiagCount)

json/public.go

@@ -18,7 +18,16 @@ import (
 // from its HasErrors method. If HasErrors returns true, the file represents
 // the subset of data that was able to be parsed, which may be none.
 func Parse(src []byte, filename string) (*hcl.File, hcl.Diagnostics) {
-	rootNode, diags := parseFileContent(src, filename)
+	return ParseWithStartPos(src, filename, hcl.Pos{Byte: 0, Line: 1, Column: 1})
+}
+
+// ParseWithStartPos attempts to parse like json.Parse, but unlike json.Parse
+// you can pass a start position of the given JSON as a hcl.Pos.
+//
+// In most cases json.Parse should be sufficient, but it can be useful for parsing
+// a part of JSON with correct positions.
+func ParseWithStartPos(src []byte, filename string, start hcl.Pos) (*hcl.File, hcl.Diagnostics) {
+	rootNode, diags := parseFileContent(src, filename, start)
 
 	switch rootNode.(type) {
 	case *objectVal, *arrayVal:

json/public_test.go

@@ -112,3 +112,73 @@ func TestParse_malformed(t *testing.T) {
 		t.Errorf("got nil File; want actual file")
 	}
 }
+
+func TestParseWithStartPos(t *testing.T) {
+	src := `{
+  "foo": {
+    "bar": "baz"
+  }
+}`
+	part := `{
+    "bar": "baz"
+  }`
+
+	file, diags := Parse([]byte(src), "")
+	partFile, partDiags := ParseWithStartPos([]byte(part), "", hcl.Pos{Byte: 0, Line: 2, Column: 10})
+	if len(diags) != 0 {
+		t.Errorf("got %d diagnostics on parse src; want 0", len(diags))
+		for _, diag := range diags {
+			t.Logf("- %s", diag.Error())
+		}
+	}
+	if len(partDiags) != 0 {
+		t.Errorf("got %d diagnostics on parse part src; want 0", len(partDiags))
+		for _, diag := range partDiags {
+			t.Logf("- %s", diag.Error())
+		}
+	}
+
+	if file == nil {
+		t.Errorf("got nil File; want actual file")
+	}
+	if file.Body == nil {
+		t.Fatalf("got nil Body; want actual body")
+	}
+	if partFile == nil {
+		t.Errorf("got nil part File; want actual file")
+	}
+	if partFile.Body == nil {
+		t.Fatalf("got nil part Body; want actual body")
+	}
+
+	content, diags := file.Body.Content(&hcl.BodySchema{
+		Blocks: []hcl.BlockHeaderSchema{{Type: "foo"}},
+	})
+	if len(diags) != 0 {
+		t.Errorf("got %d diagnostics on decode; want 0", len(diags))
+		for _, diag := range diags {
+			t.Logf("- %s", diag.Error())
+		}
+	}
+	attrs, diags := content.Blocks[0].Body.JustAttributes()
+	if len(diags) != 0 {
+		t.Errorf("got %d diagnostics on decode; want 0", len(diags))
+		for _, diag := range diags {
+			t.Logf("- %s", diag.Error())
+		}
+	}
+	srcRange := attrs["bar"].Expr.Range()
+
+	partAttrs, diags := partFile.Body.JustAttributes()
+	if len(diags) != 0 {
+		t.Errorf("got %d diagnostics on decode; want 0", len(diags))
+		for _, diag := range diags {
+			t.Logf("- %s", diag.Error())
+		}
+	}
+	partRange := partAttrs["bar"].Expr.Range()
+
+	if srcRange.String() != partRange.String() {
+		t.Errorf("The two ranges did not match: src=%s, part=%s", srcRange, partRange)
+	}
+}