command/format: Fix multi-line diagnostic output

Previously, if a diagnostic context spanned multiple lines, any lines
which did not overlap with the highlight range would be displayed as
blank. This commit fixes the bug.

The problem was caused by the unconditional use of `PartitionAround` to
split the line into before/highlighted/after ranges. When two ranges
don't overlap, this method returns empty ranges, which results in a
blank line. Instead, we first check if the ranges do overlap, and if not
we print the entire line from the context.

command/format/diagnostic.go

@@ -100,6 +100,7 @@ func Diagnostic(diag tfdiags.Diagnostic, sources map[string][]byte, color *color
 				if !lineRange.Overlaps(snippetRange) {
 					continue
 				}
+				if lineRange.Overlaps(highlightRange) {
 					beforeRange, highlightedRange, afterRange := lineRange.PartitionAround(highlightRange)
 					before := beforeRange.SliceBytes(src)
 					highlighted := highlightedRange.SliceBytes(src)
@@ -109,6 +110,13 @@ func Diagnostic(diag tfdiags.Diagnostic, sources map[string][]byte, color *color
 						lineRange.Start.Line,
 						before, highlighted, after,
 					)
+				} else {
+					fmt.Fprintf(
+						&buf, "%4d: %s\n",
+						lineRange.Start.Line,
+						lineRange.SliceBytes(src),
+					)
+				}
 			}
 
 		}

command/format/diagnostic_test.go

@@ -71,3 +71,52 @@ func TestDiagnosticWarningsCompact(t *testing.T) {
 		)
 	}
 }
+
+// Test case via https://github.com/hashicorp/terraform/issues/21359
+func TestDiagnostic_nonOverlappingHighlightContext(t *testing.T) {
+	var diags tfdiags.Diagnostics
+
+	diags = diags.Append(&hcl.Diagnostic{
+		Severity: hcl.DiagError,
+		Summary:  "Some error",
+		Detail:   "...",
+		Subject: &hcl.Range{
+			Filename: "source.tf",
+			Start:    hcl.Pos{Line: 1, Column: 5, Byte: 5},
+			End:      hcl.Pos{Line: 1, Column: 5, Byte: 5},
+		},
+		Context: &hcl.Range{
+			Filename: "source.tf",
+			Start:    hcl.Pos{Line: 1, Column: 5, Byte: 5},
+			End:      hcl.Pos{Line: 4, Column: 2, Byte: 60},
+		},
+	})
+	sources := map[string][]byte{
+		"source.tf": []byte(`x = somefunc("testing", {
+  alpha = "foo"
+  beta  = "bar"
+})
+`),
+	}
+	color := &colorstring.Colorize{
+		Colors:  colorstring.DefaultColors,
+		Reset:   true,
+		Disable: true,
+	}
+	expected := `
+Error: Some error
+
+  on source.tf line 1:
+   1: x = somefunc("testing", {
+   2:   alpha = "foo"
+   3:   beta  = "bar"
+   4: })
+
+...
+`
+	output := Diagnostic(diags[0], sources, color, 80)
+
+	if output != expected {
+		t.Fatalf("unexpected output: got:\n%s\nwant\n%s\n", output, expected)
+	}
+}