codegen: a method call on a struct returned by another call takes the address of an rvalue

Hi,

General context of this work is in #188, including the AI-disclaimers.

No in-tree codec is affected by this issue; std works around it with a local variable, and 18 std functions carry a TODO saying that it should be able to chain the two calls. Claude judges the prospective risk of it as "low": the checker accepts the chained call, but the generated C does not compile.

The following is Sol 6.1's writeup of the issue. Includes a repro, generated C, MSVC 19.51, clang-cl 18, gcc 13.3 and clang 18 compile errors, and a control with a local that compiles. The cause is reasoned from source.

A method call on a struct value passes the receiver's address. When the receiver is itself a call result, such as args.pb.pixel_format(), cgen takes the address of that call expression, which is not an lvalue in C.

Minimal repro:

pub struct repro?(
        n : base.u32,
)

pub func repro.f!(pb: base.pixel_buffer) {
    this.n = args.pb.pixel_format().bits_per_pixel()
}

Actual: at b36a81d3, wuffs-c gen -package_name repro repro.wuffs exits 0 with empty stderr and emits:

  self->private_impl.f_n = wuffs_base__pixel_format__bits_per_pixel(&wuffs_base__pixel_buffer__pixel_format(&a_pb));

Compiling the amalgamation with WUFFS_IMPLEMENTATION fails with gcc 13.3 and clang 18 (-std=c99 -Wall -c, exit 1), MSVC (cl /c /W3, exit 2) and clang-cl 18 (exit 1):

repro-release.c:26163:69: error: lvalue required as unary ‘&’ operand
repro-release.c:26163:69: error: cannot take the address of an rvalue of type 'wuffs_base__pixel_format' (aka 'struct wuffs_base__pixel_format__struct')
repro-release.c(26163): error C2102: '&' requires l-value

Control: pf = args.pb.pixel_format() into a local pf : base.pixel_format, then this.n = pf.bits_per_pixel(), compiles with all four compilers. std's TODO, for example:

wuffs/std/bmp/decode_bmp.wuffs

Lines 540 to 543 in b36a81d

    
           // TODO: the dst_pixfmt variable shouldn't be necessary. We should be able 
        
           // to chain the two calls: "args.dst.pixel_format().bits_per_pixel()". 
        
           dst_pixfmt = args.dst.pixel_format() 
        
           dst_bits_per_pixel = dst_pixfmt.bits_per_pixel()

Expected: cgen lowers a method call on a call result, for example through a temporary, or the checker rejects it.

Cause: writeExprUserDefinedCall writes & before any receiver that is not a pointer:

wuffs/internal/cgen/expr.go

Lines 543 to 573 in b36a81d

    
           func (g *gen) writeExprUserDefinedCall(b *buffer, n *a.Expr, depth uint32) error { 
        
           	method := n.LHS().AsExpr() 
        
           	recv := method.LHS().AsExpr() 
        
           	recvTyp, addr := recv.MType(), "&" 
        
           	if p := recvTyp.Decorator(); p == t.IDNptr || p == t.IDPtr { 
        
           		recvTyp, addr = recvTyp.Inner(), "" 
        
           	} 
        
           	if recvTyp.Decorator() != 0 { 
        
           		return fmt.Errorf("cannot generate user-defined method call %q for receiver type %q", 
        
           			n.Str(g.tm), recv.MType().Str(g.tm)) 
        
           	} 
        
           	qid := recvTyp.QID() 
        
           	pkgStr := g.packagePrefix(qid) 
        
           	typStr := qid[1].Str(g.tm) 
        
           	methodStr := method.Ident().Str(g.tm) 
        
           	if (qid[0] == t.IDBase) && qid[1].IsRangeType() && strings.HasPrefix(methodStr, "get_") { 
        
           		pkgStr = "wuffs_private_impl__" 
        
           	} 
        
           	b.printf("%s%s__%s(", pkgStr, typStr, methodStr) 
        
           	if !recvTyp.IsEtcUtilityType() { 
        
           		b.writes(addr) 
        
           		if err := g.writeExpr(b, recv, false, depth); err != nil { 
        
           			return err 
        
           		} 
        
           		if len(n.Args()) > 0 { 
        
           			b.writes(", ") 
        
           		} 
        
           	} 
        
           	return g.writeArgs(b, n.Args(), depth)

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codegen: a method call on a struct returned by another call takes the address of an rvalue #276

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	// TODO: the dst_pixfmt variable shouldn't be necessary. We should be able
	// to chain the two calls: "args.dst.pixel_format().bits_per_pixel()".
	dst_pixfmt = args.dst.pixel_format()
	dst_bits_per_pixel = dst_pixfmt.bits_per_pixel()

	func (g gen) writeExprUserDefinedCall(b buffer, n *a.Expr, depth uint32) error {
	method := n.LHS().AsExpr()
	recv := method.LHS().AsExpr()
	recvTyp, addr := recv.MType(), "&"
	if p := recvTyp.Decorator(); p == t.IDNptr \|\| p == t.IDPtr {
	recvTyp, addr = recvTyp.Inner(), ""
	}
	if recvTyp.Decorator() != 0 {
	return fmt.Errorf("cannot generate user-defined method call %q for receiver type %q",
	n.Str(g.tm), recv.MType().Str(g.tm))
	}

	qid := recvTyp.QID()
	pkgStr := g.packagePrefix(qid)
	typStr := qid[1].Str(g.tm)
	methodStr := method.Ident().Str(g.tm)
	if (qid[0] == t.IDBase) && qid[1].IsRangeType() && strings.HasPrefix(methodStr, "get_") {
	pkgStr = "wuffs_private_impl__"
	}
	b.printf("%s%s__%s(", pkgStr, typStr, methodStr)

	if !recvTyp.IsEtcUtilityType() {
	b.writes(addr)
	if err := g.writeExpr(b, recv, false, depth); err != nil {
	return err
	}
	if len(n.Args()) > 0 {
	b.writes(", ")
	}
	}
	return g.writeArgs(b, n.Args(), depth)

codegen: a method call on a struct returned by another call takes the address of an rvalue #276

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