size: fix compile error for a fixed-size convert shim (#446)

[youdie006]

Fixes #446.

Problem

A //msgp:shim T as:BASE using:to/from mode:convert directive makes the size generator emit a Msgsize() that declares a temporary for the converted base value and never assigns it. For a fixed-size base (int64, float64, ...) basesizeExpr returns a compile-time constant that ignores the temporary, so the generated code fails to compile:

_generated/convert_gen.go:273:6: declared and not used: zb0001

Fix

In (*sizeGen).gBase, split the mode:convert branch on fixedSize(b.Value). For a fixed-size base the wire size is a constant, so emit it directly (s.addConstant(basesizeExpr(b.Value, "", b.BaseName()))) and skip the temporary entirely.

Variable-size bases are intentionally left as-is (they compile and only under-report the size). Per the discussion on #446, computing their exact size in Msgsize would require calling the shim, which can be more expensive than the resulting under-allocation - so this PR fixes the compile error without adding shim calls to Msgsize.

Test

Added a fixed-size (int64) convert-shim fixture (ConvertIntVal / ConvertInt) to _generated/convert.go and a TestConvertInt round-trip test to _generated/convert_test.go. Red-green verified: go generate ./_generated && go test ./_generated fails to build (declared and not used) with the old generator and passes with the fixed one; ConvertInt.Msgsize() is now the constant 1 + 4 + msgp.Int64Size. gofmt and go vet ./gen are clean.

[klauspost]

lgtm

[klauspost]

No show-stoppers, but a few easy wins.

[klauspost]

nit: We can easily avoid the extra indent (and lets keep out the historic stuff from the comments).

Suggested change

	if fixedSize(b.Value) {
	// A fixed-size base has a constant wire size, so there is no need
	// for a temporary holding the converted value. Emitting an
	// (unassigned) temporary left it unused, producing a "declared and
	// not used" compile error in the generated Msgsize (#446).
	s.addConstant(basesizeExpr(b.Value, "", b.BaseName()))
	} else {
	s.state = add
	vname := randIdent()
	s.p.printf("\nvar %s %s", vname, b.BaseType())
	// ensure we don't get "unused variable" warnings from outer slice iterations
	s.p.printf("\n_ = %s", b.Varname())
	s.p.printf("\ns += %s", basesizeExpr(b.Value, vname, b.BaseName()))
	s.state = expr
	}
	if fixedSize(b.Value) {
	// A fixed-size base has a constant wire size, so there is no need
	// for a temporary holding the converted value.
	s.addConstant(basesizeExpr(b.Value, "", b.BaseName()))
	return
	}
	s.state = add
	vname := randIdent()
	s.p.printf("\nvar %s %s", vname, b.BaseType())
	// ensure we don't get "unused variable" warnings from outer slice iterations
	s.p.printf("\n_ = %s", b.Varname())
	s.p.printf("\ns += %s", basesizeExpr(b.Value, vname, b.BaseName()))
	s.state = expr

[klauspost]

Since this is free, lets just add a few more types to roundtrip tests:

Suggested change

	Int ConvertIntVal
	Int ConvertIntVal
	Ptr *ConvertIntVal
	Map map[string]ConvertIntVal
	MapP map[string]*ConvertIntVal
	Arr []ConvertIntVal
	ArrP []*ConvertIntVal

(needs gofmt)