while with a comptime false condition should work like static if

[andrewrk]

export fn entry() void {
    const c = false;
    const x: error!i32 = error.Failure;
    var z = while (x) |a| {
        if (c) break a;
    } else |_| i32(3);
}

This generates analyzed Zig IR:

fn entry() { // (analyzed)
Entry_0:
    #3  | void        | - | const c = false // comptime = false
    #14 | void        | - | const x: error!i32 = error!i32(error.Failure) // comptime = false
    #18 | noreturn    | - | goto $WhileCond_17
WhileCond_17:
    #23 | noreturn    | - | goto $WhileElse_22
WhileElse_22:
    #25 | void        | - | var _anon = error.Failure // comptime = false
    #30 | noreturn    | - | goto $WhileEnd_29
WhileEnd_29:
    #31 | void        | - | var z = 3 // comptime = false
    #34 | noreturn    | - | return {}
}

which generates the llvm ir:

define void @entry() #2 !dbg !42 {
Entry:
  %c = alloca i1, align 1
  %x = alloca { i16, i32 }, align 4
  %_anon = alloca i16, align 2
  %z = alloca i32, align 4
  store i1 false, i1* %c, align 1, !dbg !59
  call void @llvm.dbg.declare(metadata i1* %c, metadata !46, metadata !DIExpression()), !dbg !59
  %0 = bitcast { i16, i32 }* %x to i8*, !dbg !60
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %0, i8* align 4 bitcast ({ i16, i32 }* @0 to i8*), i64 8, i1 false), !dbg !60
  call void @llvm.dbg.declare(metadata { i16, i32 }* %x, metadata !49, metadata !DIExpression()), !dbg !60
  br label %WhileCond, !dbg !61

WhileCond:                                        ; preds = %Entry
  br label %WhileElse, !dbg !62

WhileElse:                                        ; preds = %WhileCond
  store i16 1, i16* %_anon, align 2, !dbg !63
  call void @llvm.dbg.declare(metadata i16* %_anon, metadata !56, metadata !DIExpression()), !dbg !65
  br label %WhileEnd, !dbg !61

WhileEnd:                                         ; preds = %WhileElse
  store i32 3, i32* %z, align 4, !dbg !66
  call void @llvm.dbg.declare(metadata i32* %z, metadata !58, metadata !DIExpression()), !dbg !66
  ret void, !dbg !67
}

Instead this should be equivalent to:

export fn entry() void {
    const c = false;
    const x: error!i32 = error.Failure;
    var z = if (x) |a| blk: {
        if (c) break :blk a;
    } else |_| i32(3);
}

which generates this zig ir:

fn entry() { // (analyzed)
Entry_0:
    #3  | void        | - | const c = false // comptime = false
    #14 | void        | - | const x: error!i32 = error!i32(error.Failure) // comptime = false
    #26 | void        | - | var z = 3 // comptime = false
    #29 | noreturn    | - | return {}
}

and this LLVM IR:

define void @entry() #2 !dbg !42 {
Entry:
  %c = alloca i1, align 1
  %x = alloca { i16, i32 }, align 4
  %z = alloca i32, align 4
  store i1 false, i1* %c, align 1, !dbg !58
  call void @llvm.dbg.declare(metadata i1* %c, metadata !46, metadata !DIExpression()), !dbg !58
  %0 = bitcast { i16, i32 }* %x to i8*, !dbg !59
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %0, i8* align 4 bitcast ({ i16, i32 }* @0 to i8*), i64 8, i1 false), !dbg !59
  call void @llvm.dbg.declare(metadata { i16, i32 }* %x, metadata !49, metadata !DIExpression()), !dbg !59
  store i32 3, i32* %z, align 4, !dbg !60
  call void @llvm.dbg.declare(metadata i32* %z, metadata !56, metadata !DIExpression()), !dbg !60
  ret void, !dbg !61
}

Note that with #287 the expected LLVM IR is even slimmer:

define void @entry() #2 !dbg !42 {
Entry:
  %z = alloca i32, align 4
  call void @llvm.dbg.declare(metadata i1* @1, metadata !46, metadata !DIExpression()), !dbg !58
  call void @llvm.dbg.declare(metadata { i16, i32 }* @3, metadata !49, metadata !DIExpression()), !dbg !59
  store i32 3, i32* %z, align 4, !dbg !60
  call void @llvm.dbg.declare(metadata i32* %z, metadata !56, metadata !DIExpression()), !dbg !60
  ret void, !dbg !61
}