Enforce that enum values are monotonically increasing

[RikkiGibson]

Describe the problem you are trying to solve

In Roslyn, we often add new entries to ErrorCode in feature branches. When the feature branches merge, there are issues with duplicates, which are caught already by CA1069.

Even when there aren't duplicates, though, we can have issues with misordering of the error codes:

namespace Microsoft.CodeAnalysis.CSharp;

public enum ErrorCode
{
    // ...
    ERR_CannotUseRefInUnmanagedCallersOnly = 8977,

    ERR_IncorrectNullCheckSyntax = 8990,
    ERR_MustNullCheckInImplementation = 8991,
    ERR_NonNullableValueTypeIsNullChecked = 8992,
    WRN_NullCheckedHasDefaultNull = 8993,
    ERR_NullCheckingOnByRefParameter = 8994,
    WRN_NullCheckingOnNullableType = 8995,

    #endregion

    #region diagnostics introduced for C# 11.0

    ERR_CannotBeMadeNullable = 8978,
    // ...
}

https://github.com/dotnet/roslyn/blob/d47bc385061323c9d1350e1a4de1cb7a256759e1/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs#L2008-L2021

Here, a feature branch was merged and included some new entries in the "middle" of the sequence. This makes navigating and interpreting the error code list more difficult, so we'd like to avoid it.

I think this analyzer would have to be opt-in, similar to CA1069, since many enums intentionally do not adhere to such ordering.

Describe suggestions on how to achieve the rule

• If an enum field is smaller than its immediately preceding field (if another field precedes it), report a diagnostic.
• Code fix would move the violating field to appear after another field in the enum which is the closest possible in value without being larger. In the above example, ERR_CannotBeMadeNullable would move to appear directly after ERR_CannotUseRefInUnmanagedCallersOnly.

For flags enums, it's not clear when/if this rule should be enforced. Perhaps you could identify when a field initializer consists only of references to other enum fields and exclude it from the rule.

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Issue Details

---

### Describe the problem you are trying to solve In Roslyn, we often add new entries to ErrorCode in feature branches. When the feature branches merge, there are issues with duplicates, which are caught already by [CA1069](https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1069). Even when there aren't duplicates, though, we can have issues with misordering of the error codes: ```cs namespace Microsoft.CodeAnalysis.CSharp; public enum ErrorCode { // ... ERR_CannotUseRefInUnmanagedCallersOnly = 8977, ERR_IncorrectNullCheckSyntax = 8990, ERR_MustNullCheckInImplementation = 8991, ERR_NonNullableValueTypeIsNullChecked = 8992, WRN_NullCheckedHasDefaultNull = 8993, ERR_NullCheckingOnByRefParameter = 8994, WRN_NullCheckingOnNullableType = 8995, #endregion #region diagnostics introduced for C# 11.0 ERR_CannotBeMadeNullable = 8978, // ... } ``` https://github.com/dotnet/roslyn/blob/d47bc385061323c9d1350e1a4de1cb7a256759e1/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs#L2008-L2021 Here, a feature branch was merged and included some new entries in the "middle" of the sequence. This makes navigating and interpreting the error code list more difficult, so we'd like to avoid it. I think this analyzer would have to be opt-in, similar to CA1069, since many enums intentionally do not adhere to such ordering. ### Describe suggestions on how to achieve the rule - If an enum field is smaller than its immediately preceding field (if another field precedes it), report a diagnostic. - Code fix would move the violating field to appear after another field in the enum which is the closest possible in value without being larger. In the above example, `ERR_CannotBeMadeNullable` would move to appear directly after `ERR_CannotUseRefInUnmanagedCallersOnly`. For flags enums, it's not clear when/if this rule should be enforced. Perhaps you could identify when a field initializer consists only of references to other enum fields and exclude it from the rule.

Author:	RikkiGibson
Assignees:	-
Labels:	`area-Meta`, `untriaged`, `code-analyzer`
Milestone:	-

[ghost]

Tagging subscribers to this area: @dotnet/area-system-runtime See info in area-owners.md if you want to be subscribed.

Issue Details

---

### Describe the problem you are trying to solve In Roslyn, we often add new entries to ErrorCode in feature branches. When the feature branches merge, there are issues with duplicates, which are caught already by [CA1069](https://docs.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1069). Even when there aren't duplicates, though, we can have issues with misordering of the error codes: ```cs namespace Microsoft.CodeAnalysis.CSharp; public enum ErrorCode { // ... ERR_CannotUseRefInUnmanagedCallersOnly = 8977, ERR_IncorrectNullCheckSyntax = 8990, ERR_MustNullCheckInImplementation = 8991, ERR_NonNullableValueTypeIsNullChecked = 8992, WRN_NullCheckedHasDefaultNull = 8993, ERR_NullCheckingOnByRefParameter = 8994, WRN_NullCheckingOnNullableType = 8995, #endregion #region diagnostics introduced for C# 11.0 ERR_CannotBeMadeNullable = 8978, // ... } ``` https://github.com/dotnet/roslyn/blob/d47bc385061323c9d1350e1a4de1cb7a256759e1/src/Compilers/CSharp/Portable/Errors/ErrorCode.cs#L2008-L2021 Here, a feature branch was merged and included some new entries in the "middle" of the sequence. This makes navigating and interpreting the error code list more difficult, so we'd like to avoid it. I think this analyzer would have to be opt-in, similar to CA1069, since many enums intentionally do not adhere to such ordering. ### Describe suggestions on how to achieve the rule - If an enum field is smaller than its immediately preceding field (if another field precedes it), report a diagnostic. - Code fix would move the violating field to appear after another field in the enum which is the closest possible in value without being larger. In the above example, `ERR_CannotBeMadeNullable` would move to appear directly after `ERR_CannotUseRefInUnmanagedCallersOnly`. For flags enums, it's not clear when/if this rule should be enforced. Perhaps you could identify when a field initializer consists only of references to other enum fields and exclude it from the rule.

Author:	RikkiGibson
Assignees:	-
Labels:	`area-System.Runtime`, `untriaged`, `code-analyzer`
Milestone:	-