Obsolete APIs

Fallout deprecates public APIs with System.ObsoleteAttribute and gives each deprecation a stable DiagnosticId so consumers can suppress that one deprecation without silencing every CS0618. This page is the canonical registry of allocated FALLOUTOBS0xx diagnostic IDs.

This is the counterpart to the [Experimental] registry: [Experimental] gates not-yet-stable surface you opt into (error-by-default); [Obsolete] marks on-the-way-out surface that still works (warning-by-default). The two use separate ID sequences — FALLOUTOBS0xx here, FALLOUT0xx there — so a suppression can never cross the two.

See the agent conventions for the contributor rules.

How it works

A deprecated API carries a message, a diagnostic ID, and a help URL:

using System;

[Obsolete(
    "Use [GitHubActionsInputAttribute] instead. Removed in 2027.x.x.",
    DiagnosticId = "FALLOUTOBS001",
    UrlFormat = "https://github.com/Fallout-build/Fallout/blob/main/docs/obsolete_apis.md")]
public string[] OnWorkflowDispatchOptionalInputs { get; set; } = new string[0];

ObsoleteAttribute is warning-by-default: consumers keep compiling, but see the deprecation. Without a DiagnosticId the compiler reports the generic CS0618; setting one makes it report FALLOUTOBS001 instead, which is what lets a consumer suppress a single deprecation:

<PropertyGroup>
  <NoWarn>$(NoWarn);FALLOUTOBS001</NoWarn>
</PropertyGroup>

This matters for consumers building with TreatWarningsAsErrors: without a per-deprecation ID their only options are to fix every usage at once or blanket-disable all of CS0618. DiagnosticId gives them a targeted, acknowledge-and-move-on escape hatch while they migrate. UrlFormat (optional; {0} is replaced with the diagnostic ID when present) points at this registry so the warning links to the migration path.

Diagnostic-ID scheme

• IDs use the form FALLOUTOBS0xx and are allocated sequentially.
• The FALLOUTOBS0xx sequence is independent of the FALLOUT0xx sequence used by [Experimental] — allocate from this registry only.
• An ID is never reused — once retired it stays retired, so a consumer's NoWarn can never silently re-bind to a different deprecation.
• Every allocation is recorded in the registry table below, in the same PR that introduces the attribute.
• Adding [Obsolete] is not a breaking change — a warning-level deprecation keeps existing code compiling. The break is removing the API, which is batched to the next yearly major (see AGENTS.md rule #2 and the release/versioning policy). Record the removal target in the message and the registry so consumers can plan.
• When the API is finally removed, its row moves to Removed status and the ID is retired, not recycled.

Registry

Status values: Deprecated (live, warns on use), Removed (API deleted — ID retired).

ID	Surface	Deprecated	Status	Notes
FALLOUTOBS001	GitHubActionsAttribute.OnWorkflowDispatch{Optional,Required}Inputs, GitHubActionsWorkflowDispatchTrigger.{Optional,Required}Inputs	2026.x	Deprecated	Untyped workflow_dispatch input arrays. Use [GitHubActionsInputAttribute] / GitHubActionsWorkflowDispatchTrigger.Inputs instead. Removal target: 2027.0.0.