cmd/compile: gracefully fallback when inline bodies are missing

Currently, we rely on a "crawling" step during export to identify
function and method bodies that need to be exported or re-exported so
we can trim out unnecessary ones and reduce build artifact sizes. To
catch cases where we expect a function to be inlinable but we failed
to export its body, we made this condition a fatal compiler error.

However, with generics, it's much harder to perfectly identify all
function bodies that need to be exported; and several attempts at
tweaking the algorithm have resulted in still having failure cases.
So for now, this CL changes a missing inline body into a graceful
failure instead.

Change-Id: I04b0872d0dcaae9c3de473e92ce584e4ec6fd782
Reviewed-on: https://go-review.googlesource.com/c/go/+/361403
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dan Scales <danscales@google.com>

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

@@ -309,7 +309,7 @@ func (v *hairyVisitor) doNode(n ir.Node) bool {
 			break
 		}
 
-		if fn := inlCallee(n.X); fn != nil && fn.Inl != nil {
+		if fn := inlCallee(n.X); fn != nil && typecheck.HaveInlineBody(fn) {
 			v.budget -= fn.Inl.Cost
 			break
 		}
@@ -585,7 +585,7 @@ func inlnode(n ir.Node, maxCost int32, inlMap map[*ir.Func]bool, edit func(ir.No
 		if ir.IsIntrinsicCall(call) {
 			break
 		}
-		if fn := inlCallee(call.X); fn != nil && fn.Inl != nil {
+		if fn := inlCallee(call.X); fn != nil && typecheck.HaveInlineBody(fn) {
 			n = mkinlcall(call, fn, maxCost, inlMap, edit)
 		}
 	}

src/cmd/compile/internal/typecheck/crawler.go

@@ -207,7 +207,7 @@ func (p *crawler) markInlBody(n *ir.Name) {
 	if fn == nil {
 		base.Fatalf("markInlBody: missing Func on %v", n)
 	}
-	if fn.Inl == nil {
+	if !HaveInlineBody(fn) {
 		return
 	}
 

src/cmd/compile/internal/typecheck/iimport.go

@@ -81,6 +81,27 @@ func ImportBody(fn *ir.Func) {
 	inimport = false
 }
 
+// HaveInlineBody reports whether we have fn's inline body available
+// for inlining.
+func HaveInlineBody(fn *ir.Func) bool {
+	if fn.Inl == nil {
+		return false
+	}
+
+	// Unified IR is much more conservative about pruning unreachable
+	// methods (at the cost of increased build artifact size).
+	if base.Debug.Unified != 0 {
+		return true
+	}
+
+	if fn.Inl.Body != nil {
+		return true
+	}
+
+	_, ok := inlineImporter[fn.Nname.Sym()]
+	return ok
+}
+
 func importReaderFor(sym *types.Sym, importers map[*types.Sym]iimporterAndOffset) *importReader {
 	x, ok := importers[sym]
 	if !ok {