[dev.unified] cmd/compile: allow inlining to fail gracefully

Change-Id: I20c7df52d110fb88eb22d57bdad9264d0c5e22fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/419674
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/inline/inl.go

@@ -685,9 +685,8 @@ var inlgen int
 var SSADumpInline = func(*ir.Func) {}
 
 // NewInline allows the inliner implementation to be overridden.
-// If it returns nil, the legacy inliner will handle this call
-// instead.
-var NewInline = func(call *ir.CallExpr, fn *ir.Func, inlIndex int) *ir.InlinedCallExpr { return nil }
+// If it returns nil, the function will not be inlined.
+var NewInline = oldInline
 
 // If n is a OCALLFUNC node, and fn is an ONAME node for a
 // function with an inlinable body, return an OINLCALL node that can replace n.
@@ -807,7 +806,7 @@ func mkinlcall(n *ir.CallExpr, fn *ir.Func, maxCost int32, inlMap map[*ir.Func]b
 
 	res := NewInline(n, fn, inlIndex)
 	if res == nil {
-		res = oldInline(n, fn, inlIndex)
+		return n
 	}
 
 	// transitive inlining

src/cmd/compile/internal/noder/reader.go

@@ -2227,7 +2227,12 @@ func InlineCall(call *ir.CallExpr, fn *ir.Func, inlIndex int) *ir.InlinedCallExp
 
 	pri, ok := bodyReader[fn]
 	if !ok {
-		base.FatalfAt(call.Pos(), "missing function body for call to %v", fn)
+		// TODO(mdempsky): Reconsider this diagnostic's wording, if it's
+		// to be included in Go 1.20.
+		if base.Flag.LowerM != 0 {
+			base.WarnfAt(call.Pos(), "cannot inline call to %v: missing inline body", fn)
+		}
+		return nil
 	}
 
 	if fn.Inl.Body == nil {