Add MSBuildTask0012: diagnose tasks constructed without TaskEnvironment propagation

[Copilot]

Context

MSBuild assigns TaskEnvironment only to the task instances it creates itself (TaskExecutionHost.cs:545, guarded by TaskInstance is IMultiThreadableTask). A task that constructs another ITask directly — the arcade ExecWithRetries → Exec pattern — leaves the inner task on TaskEnvironment.Fallback, resolving paths and environment variables against the shared node state.

No existing rule catches this: the outer task uses TaskEnvironment correctly throughout its own body and passes the analyzer cleanly, while handing the real work to a task with no environment.

Changes Made

New rule MSBuildTask0012 (Warning) — TaskEnvironmentPropagationAnalyzer reports an object creation when:

• the containing type is multithreadable (IMultiThreadableTask or [MSBuildMultiThreadableTask]) and holds a readable TaskEnvironment member of its own — a task with nothing to propagate is not reported;
• the created type implements ITask;
• the created type can actually receive an environment — a publicly settable TaskEnvironment property (as on ToolTask, where it is public virtual) or a constructor parameter — keeping the diagnostic actionable;
• the environment is not already supplied via constructor argument, object initializer, or a later assignment on the instance.

Suppression via later assignment spans the whole type: candidates are collected in a symbol-start action and reported in the symbol-end action, so a field configured in a helper method is recognized. Members are matched by type rather than by name, so any TaskEnvironment-typed field or property counts as the source.

Code fix — TaskEnvironmentPropagationCodeFixProvider adds the missing entry, creating the object initializer when absent, and is skipped in static contexts where no instance member is reachable:

_runningExec = new Exec
{
    BuildEngine = BuildEngine,
    TaskEnvironment = TaskEnvironment,   // added by the fix
    Command = Command,
};

Docs — rule section plus scope/severity/code-fix table rows in src/TaskAnalyzer/README.md, a "Tasks That Construct Other Tasks" note in documentation/specs/multithreading/thread-safe-tasks.md, and a row in AnalyzerReleases.Unshipped.md.

Testing

20 new tests covering the arcade ToolTask shape, constructor injection, cross-method configuration, field initializers, implicit new(), and static contexts; the TaskAnalyzer.Tests suite is at 268 passing.

Verified end-to-end against the real built Microsoft.Build.Framework/Microsoft.Build.Utilities.Core: the rule fires on a reproduction of the arcade pattern, dotnet format analyzers --diagnostics MSBuildTask0012 applies the fix, and the rebuilt project is warning-free.

Notes

src/Tasks built with -p:BuildAnalyzer=true produces zero MSBuildTask0012 occurrences, so the rule was deliberately left out of that project's WarningsNotAsErrors list — if it ever fires there, the build should fail rather than warn.

Known trade-off: a factory-style method returning new SomeTask() is flagged, consistent with the issue's near-zero false-positive expectation. Diagnostics inside static members are reported but offer no fix, since there is no instance environment to reference.

• Fixes TaskAnalyzer: no diagnostic when a task constructs another ITask without propagating TaskEnvironment #14792

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[Copilot]

Pull request overview

Adds a new TaskAnalyzer diagnostic (MSBuildTask0012) to catch multithreadable tasks that construct inner ITask instances without propagating TaskEnvironment, addressing #14792 and preventing inner tasks from silently using TaskEnvironment.Fallback.

Changes:

• Introduces TaskEnvironmentPropagationAnalyzer (MSBuildTask0012) to detect missing propagation via ctor args, object initializers, or later assignments.
• Adds TaskEnvironmentPropagationCodeFixProvider to auto-insert TaskEnvironment = <source> into object initializers when an instance source is available.
• Updates analyzer documentation/spec text and adds new unit tests covering common construction/configuration patterns.

Show a summary per file

File	Description
src/TaskAnalyzer/TaskEnvironmentPropagationCodeFixProvider.cs	New code fix provider for MSBuildTask0012 that adds a TaskEnvironment initializer entry when possible.
src/TaskAnalyzer/TaskEnvironmentPropagationAnalyzer.cs	New analyzer implementing MSBuildTask0012 detection and suppression via later assignment tracking.
src/TaskAnalyzer/README.md	Documents MSBuildTask0012 and updates rule/scope/code-fix tables.
src/TaskAnalyzer/DiagnosticIds.cs	Adds the MSBuildTask0012 diagnostic ID constant.
src/TaskAnalyzer/DiagnosticDescriptors.cs	Adds the MSBuildTask0012 descriptor and includes it in the descriptor list.
src/TaskAnalyzer/AnalyzerReleases.Unshipped.md	Records MSBuildTask0012 in the unshipped analyzer releases list.
src/TaskAnalyzer.Tests/TestHelpers.cs	Updates framework stubs to model ToolTask as IMultiThreadableTask with TaskEnvironment.
src/TaskAnalyzer.Tests/TaskEnvironmentPropagationCodeFixProviderTests.cs	New tests validating the code fix behavior (including static-context no-fix).
src/TaskAnalyzer.Tests/TaskEnvironmentPropagationAnalyzerTests.cs	New tests validating detection/suppression scenarios for MSBuildTask0012.
documentation/specs/multithreading/thread-safe-tasks.md	Adds guidance on propagating TaskEnvironment when a task constructs other tasks.

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