LambdaCall for statements

[quasilyte]

Function listed below is inlined, but it leads to compiler panic.

func utf8CharWidth(ch rune) int {
	switch {
	case ch <= 127:
		return 1
	case ch <= 2048:
		return 2
	case ch <= 65536:
		return 3
	default:
		return 4
	}
}

Debug info

[]           %empty 0 ''=> []
[]           %xlambda-enter 0 ''=> []
[]           constant 0 ''=> [<0> ]
[<0> ]       constant 1 ''=> [<0> <1> ]
[<0> <1> ]   num<= 0 ''=> [? ]
[? ]         goto-if-nil 2 'else'=> []
[]           %scope-enter 0 ''=> []
[]           constant 2 ''=> [<2> ]
[<2> ]       %xlambda-ret 0 'lambda-ret'=> [<2> ]
[<2> ]       %scope-leave 0 ''=> []
[]           goto 1 'endif'=> []
[]           label 2 'else'=> []
[]           constant 0 ''=> [<0> ]
[<0> ]       constant 3 ''=> [<0> <3> ]
[<0> <3> ]   num<= 0 ''=> [? ]
[? ]         goto-if-nil 4 'else'=> []
[]           %scope-enter 0 ''=> []
[]           constant 4 ''=> [<4> ]
[<4> ]       %xlambda-ret 0 'lambda-ret'=> [<4> ]
[<4> ]       %scope-leave 0 ''=> []
[]           goto 3 'endif'=> []
[]           label 4 'else'=> []
[]           constant 0 ''=> [<0> ]
[<0> ]       constant 5 ''=> [<0> <5> ]
[<0> <5> ]   num<= 0 ''=> [? ]
[? ]         goto-if-nil 6 'else'=> []
[]           %scope-enter 0 ''=> []
[]           constant 6 ''=> [<6> ]
[<6> ]       %xlambda-ret 0 'lambda-ret'=> [<6> ]
[<6> ]       %scope-leave 0 ''=> []
[]           goto 5 'endif'=> []
[]           label 6 'else'=> []
[]           %scope-enter 0 ''=> []
[]           constant 7 ''=> [<7> ]
[<7> ]       %xlambda-ret 0 'lambda-ret'=> [<7> ]
[<7> ]       %scope-leave 0 ''=> []
[]           label 5 'endif'=> []
[]           label 3 'endif'=> []
[]           label 1 'endif'=> []
[]           %xlambda-ret-label 0 'lambda-ret'
st.len=0 depth=0
panic: runtime error: slice bounds out of range [recovered]
	panic: runtime error: slice bounds out of range

goroutine 1 [running]:
exn.Catch(0x69f140, 0x829f30, 0x829f30, 0x82a160)
	/home/quasilyte/CODE/elgo/src/exn/exn.go:75 +0xb1
main.main.func1()
	/home/quasilyte/CODE/elgo/src/main/translate_package/translate_package.go:44 +0x43
panic(0x69f140, 0x829f30)
	/usr/local/go/src/runtime/panic.go:489 +0x2cf
backends/lapc/asm.(*converterY).convertInstr(0xc4201d9b70, 0xc42022cf30)
	/home/quasilyte/CODE/elgo/src/backends/lapc/asm/y_make.go:82 +0x93d
backends/lapc/asm.(*converterY).convert(0xc4201d9b70, 0xc42022c7b0)
	/home/quasilyte/CODE/elgo/src/backends/lapc/asm/y_make.go:54 +0x218
backends/lapc/asm.(*converterY).Convert(0xc4201d9b70, 0xc42022c7b0, 0xc42022cfc0, 0x0)
	/home/quasilyte/CODE/elgo/src/backends/lapc/asm/y_make.go:42 +0x35
backends/lapc/asm.makeY(0x8501c0, 0x0, 0x0, 0xc42022c7b0, 0xc42023a0e0, 0x6c6780)
	/home/quasilyte/CODE/elgo/src/backends/lapc/asm/y_make.go:38 +0x243
backends/lapc/asm.(*Assembler).Assemble(0xc42023c080, 0x8501c0, 0x0, 0x0, 0xc42023a0e0, 0xc4201d9cb0, 0x4b3a8b, 0xc4200ac0c0, 0xc4201d9c98)
	/home/quasilyte/CODE/elgo/src/backends/lapc/asm/assembler.go:32 +0x86

Bug reason: switch statement leaves no data on the stack.
lambda-ret expects there to be at least 1 value after evaluation.

This particular line triggers a panic:

cy.st.Discard(uint16(cy.st.Len() - depth - 1))
// cy.st.Len() => 0
// depth => 0

In correct code, cy.st.Len() should return 1.

Possible solution:
introduce temporary stack variable for cases like these.
Return statements should assign to that temporary.

[quasilyte]

Will make sexp.Switch.Cost() return -1 to forbid inlining of switch containing functions
until this issue is resolved.

[quasilyte]

After the bug is fixed, regression test must be added.

[quasilyte]

Worth mentioning that any statement inlined with LambdaCall leads to compilation error.

I will remove LambdaCall inlining from master branch temporary.