The debug-step node bracket (MarkNodeStart/try/finally/MarkNodeEnd) was copied
into every hand-written WriteTo override and re-emitted by the T4 generator, so
a newly added instruction could silently omit it and lose step highlighting with
no compile error or test failure. Seal WriteTo on ILInstruction to apply the
bracket once and delegate to a new abstract WriteToCore; move every override
(hand-written and generated) to WriteToCore without the wrapper. Rendered output
is unchanged -- the marks are no-ops unless the output tracks nodes.
Assisted-by: Claude:claude-opus-4-8:Claude Code
The C# debug-steps view highlights and centers the exact AST node a
transform changed; the ILAst view already had the step tree and
replay-at-step but produced no highlight. Bring it to parity.
IL rendering has no token-writer seam like the C# output visitor, so
per-instruction text spans are recorded by bracketing
ILInstruction.WriteTo via a new INodeTrackingOutput. The dominant
inst.ReplaceWith(newInst) transform pattern detaches the instruction
passed to Step, so ILTransformContext gains EndStep to record the
produced instruction; Stepper additionally records the position's
ancestor chain as fallback candidates before the step-limit throw, so
the "show state before" view -- which halts at the selected step --
still resolves to a surviving ancestor (ultimately the ILFunction).
The highlight-range resolver is shared with the C# language.
Assisted-by: Claude:claude-opus-4-8:Claude Code
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.
This removes the need for "goto case" in the ILAst as we can just branch to the appropriate block.
It also means we can support "break;" within switch using the "leave" instruction (otherwise we'd have to introduce yet another special kind of jump).