Runtime async is a compiler feature that emits ordinary async/await (a
C# 5 construct), so reconstructing it should not require selecting C# 15.
The dedicated RuntimeAsync setting was also redundant: AsyncAwaitDecompiler
already runs the runtime-async transforms only when AsyncAwait is enabled.
Fold the behavior into the AsyncAwait setting and drop the separate toggle.
Assisted-by: Claude:claude-opus-4-8:Claude Code
Convert `call System.Runtime.CompilerServices.AsyncHelpers.Await(value)`
to the IL Await instruction whenever DecompilerSettings.RuntimeAsync is
enabled. The state-machine async pipeline (AsyncAwaitDecompiler) already
produces the IL Await for downstream transforms (UsingTransform's
MatchDisposeBlock pattern-matches on it via UnwrapAwait); doing the
conversion in EarlyExpressionTransforms gives the runtime-async output
the same canonical shape before any consumer runs.
CopyPropagation will replace `ref StructWithStringField reference = ref array[0];` with:
```
var x = array;
var y = 0;
```
and then every use of `reference` is replaced with `x[y]`.
This lets us avoid rough locals while preserving the semantics in every case except that we re-order when a NullReferenceException/IndexOutOfRangeException occurs.
This way we avoid having to extract later, as we will never inline if the `isinst` argument if this could result in it being unrepresentable in C#.
This commit also refactors inlining restrictions to avoid requiring special cases in ILInlining itself.
But when making this change, I discovered that this broke our pattern-matching tests, and that the weird IL with double `isinst` is indeed generated by the C# compiler for `if (genericParam is StringComparison.Ordinal)` style code. So instead we also allow `isinst` with a `box(expr-without-side-effects)` argument to be represented with the `expr is T ? (T)expr : null` emulation.
- Updated `Issue3421.cs`.
- Updated `MatchLegacySwitchOnStringWithDict` to check for `leaveContainer` and handle null sections accordingly.
- Introduced an overload for `AddNullSection` to accept `ILInstruction` as the body, improving flexibility.
- Modified existing `AddNullSection` to utilize the new overload, allowing for varied body types in `SwitchSection`.