Fix custom numeric format parsing with long digit runs

src/libraries/Common/src/System/Number.Formatting.Common.cs

@@ -147,20 +147,29 @@ internal static char ParseFormatSpecifier(ReadOnlySpan<char> format, out int dig
                     // digits.  Further, for compat, we need to stop when we hit a null char.
                     int n = 0;
                     int i = 1;
+                    bool precisionTooLarge = false;
                     while ((uint)i < (uint)format.Length && char.IsAsciiDigit(format[i]))
                     {
-                        // Check if we are about to overflow past our limit of 9 digits
                         if (n >= 100_000_000)
                         {
-                            ThrowHelper.ThrowFormatException_BadFormatSpecifier();
+                            precisionTooLarge = true;
+                        }
+                        else
+                        {
+                            n = (n * 10) + format[i] - '0';
                         }
-                        n = (n * 10) + format[i++] - '0';
+                        i++;
                     }
 
                     // If we're at the end of the digits rather than having stopped because we hit something
-                    // other than a digit or overflowed, return the standard format info.
+                    // other than a digit, this is a standard format.
                     if ((uint)i >= (uint)format.Length || format[i] == '\0')
                     {
+                        if (precisionTooLarge)
+                        {
+                            ThrowHelper.ThrowFormatException_BadFormatSpecifier();
+                        }
+
                         digits = n;
                         return c;
                     }

src/libraries/System.Runtime.Numerics/tests/BigInteger/BigIntegerToStringTests.cs

@@ -464,6 +464,8 @@ public static void RunCustomFormatLiterals()
             RunCustomFormatToStringTests(s_random, "0\"\\\"", CultureInfo.CurrentCulture.NumberFormat.NegativeSign, 1, ExtraFormatter(ZeroFormatter, "\\"));
             RunCustomFormatToStringTests(s_random, "#\',\'%", CultureInfo.CurrentCulture.NumberFormat.NegativeSign, 0, ExtraFormatter(PercentSymbolFormatter, ",", CultureInfo.CurrentCulture.NumberFormat.PercentSymbol.Length));
             RunCustomFormatToStringTests(s_random, "000\",\".000", CultureInfo.CurrentCulture.NumberFormat.NegativeSign, 3, ExtraFormatter(DecimalPointFormatter, ",", CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator.Length + 3));
+            VerifyToString("7", "A1111111111-0", "A1111111111-7");
+            VerifyToString("7", "A1234567890Z", "A1234567897Z");
         }
 
         [Fact]

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/Int32Tests.cs

@@ -131,6 +131,8 @@ public static IEnumerable<object[]> ToString_TestData()
             }
 
             NumberFormatInfo invariantFormat = NumberFormatInfo.InvariantInfo;
+            yield return new object[] { 7, "A1111111111-0", invariantFormat, "A1111111111-7" };
+            yield return new object[] { 7, "A1234567890Z", invariantFormat, "A1234567897Z" };
             yield return new object[] { 32, "C100", invariantFormat, "\u00A432.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000" };
             yield return new object[] { 32, "P100", invariantFormat, "3,200.0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 %" };
             yield return new object[] { 32, "D100", invariantFormat, "0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000032" };
@@ -197,6 +199,8 @@ public static void ToString_InvalidFormat_ThrowsFormatException()
             int i = 123;
             Assert.Throws<FormatException>(() => i.ToString("Y")); // Invalid format
             Assert.Throws<FormatException>(() => i.ToString("Y", null)); // Invalid format
+            Assert.Throws<FormatException>(() => i.ToString("D1000000000")); // Precision too large
+            Assert.Throws<FormatException>(() => i.ToString("A1234567890")); // Standard-shaped format
         }
 
         public static IEnumerable<object[]> Parse_Valid_TestData()