&str.slice is bloated

[huonw]

#![crate_type = "lib"]
pub fn slice(x: &str, a: uint, b: uint) -> &str {
    x.slice(a, b)
}

with -O becomes

; Function Attrs: uwtable
define void @_ZN5slice20ha62b7cd23f98de21daaE(%str_slice* noalias nocapture sret dereferenceable(16), %str_slice* noalias nocapture dereferenceable(16), i64, i64) unnamed_addr #0 {
entry-block:
  %4 = alloca %str_slice, align 8
  %5 = alloca %"struct.core::fmt::Argument<[]>[#3]", align 8
  %__args.i.i = alloca %"struct.core::fmt::Arguments<[]>[#3]", align 8
  %6 = alloca %str_slice, align 8
  %7 = alloca %"struct.core::fmt::Argument<[]>[#3]", align 8
  %__args7.i.i = alloca %"struct.core::fmt::Arguments<[]>[#3]", align 8
  %begin.i = alloca i64, align 8
  %end.i = alloca i64, align 8
  %8 = alloca [3 x %"struct.core::fmt::Argument<[]>[#3]"], align 8
  %__args.i = alloca %"struct.core::fmt::Arguments<[]>[#3]", align 8
  %.sub.i = getelementptr inbounds [3 x %"struct.core::fmt::Argument<[]>[#3]"]* %8, i64 0, i64 0
  %9 = bitcast i64* %begin.i to i8*
  call void @llvm.lifetime.start(i64 8, i8* %9)
  store i64 %2, i64* %begin.i, align 8
  %10 = bitcast i64* %end.i to i8*
  call void @llvm.lifetime.start(i64 8, i8* %10)
  store i64 %3, i64* %end.i, align 8
  %.sroa.3.0..sroa_idx4.i.i.i = getelementptr inbounds %str_slice* %1, i64 0, i32 1
  %.sroa.3.0.copyload.i.i.i = load i64* %.sroa.3.0..sroa_idx4.i.i.i, align 8
  %11 = icmp eq i64 %.sroa.3.0.copyload.i.i.i, %2
  br i1 %11, label %before_rhs.i, label %next-block.i.i

next-block.i.i:                                   ; preds = %entry-block
  %12 = icmp ult i64 %.sroa.3.0.copyload.i.i.i, %2
  br i1 %12, label %then-block-54-.i, label %next-block1.i.i

next-block1.i.i:                                  ; preds = %next-block.i.i
  %13 = icmp ugt i64 %.sroa.3.0.copyload.i.i.i, %2
  br i1 %13, label %"_ZN3str39_$BP$$x27a$x20str.StrSlice$LT$$x27a$GT$16is_char_boundary20h77e6472c98e21760edaE.exit.i", label %cond.i.i, !prof !0

cond.i.i:                                         ; preds = %next-block1.i.i
  tail call void @_ZN7failure17fail_bounds_check20hb6567e7af36a3c08KykE({ %str_slice, i64 }* noalias nocapture readonly dereferenceable(24) @const, i64 %2, i64 %.sroa.3.0.copyload.i.i.i)
  unreachable

"_ZN3str39_$BP$$x27a$x20str.StrSlice$LT$$x27a$GT$16is_char_boundary20h77e6472c98e21760edaE.exit.i": ; preds = %next-block1.i.i
  %.sroa.0.0..sroa_idx.i.i = getelementptr inbounds %str_slice* %1, i64 0, i32 0
  %.sroa.0.0.copyload.i.i = load i8** %.sroa.0.0..sroa_idx.i.i, align 8
  %14 = getelementptr inbounds i8* %.sroa.0.0.copyload.i.i, i64 %2
  %15 = load i8* %14, align 1
  %16 = icmp sgt i8 %15, -1
  %17 = icmp ugt i8 %15, -65
  %..i.i = or i1 %16, %17
  br i1 %..i.i, label %before_rhs.i, label %then-block-54-.i

join.i:                                           ; preds = %next-block1.i15.i
  %.sroa.0.0..sroa_idx.i16.i = getelementptr inbounds %str_slice* %1, i64 0, i32 0
  %.sroa.0.0.copyload.i17.i = load i8** %.sroa.0.0..sroa_idx.i16.i, align 8
  %18 = getelementptr inbounds i8* %.sroa.0.0.copyload.i17.i, i64 %3
  %19 = load i8* %18, align 1
  %20 = icmp sgt i8 %19, -1
  %21 = icmp ugt i8 %19, -65
  %..i18.i = or i1 %20, %21
  br i1 %..i18.i, label %next-block.i, label %then-block-54-.i

before_rhs.i:                                     ; preds = %"_ZN3str39_$BP$$x27a$x20str.StrSlice$LT$$x27a$GT$16is_char_boundary20h77e6472c98e21760edaE.exit.i", %entry-block
  %22 = icmp eq i64 %.sroa.3.0.copyload.i.i.i, %3
  br i1 %22, label %before_rhs.next-block_crit_edge.i, label %next-block.i14.i

before_rhs.next-block_crit_edge.i:                ; preds = %before_rhs.i
  %arg.sroa.0.0..sroa_idx.phi.trans.insert.i = getelementptr inbounds %str_slice* %1, i64 0, i32 0
  %arg.sroa.0.0.copyload.pre.i = load i8** %arg.sroa.0.0..sroa_idx.phi.trans.insert.i, align 8
  br label %next-block.i

next-block.i14.i:                                 ; preds = %before_rhs.i
  %23 = icmp ult i64 %.sroa.3.0.copyload.i.i.i, %3
  br i1 %23, label %then-block-54-.i, label %next-block1.i15.i

next-block1.i15.i:                                ; preds = %next-block.i14.i
  %24 = icmp ugt i64 %.sroa.3.0.copyload.i.i.i, %3
  br i1 %24, label %join.i, label %cond.i20.i, !prof !0

cond.i20.i:                                       ; preds = %next-block1.i15.i
  tail call void @_ZN7failure17fail_bounds_check20hb6567e7af36a3c08KykE({ %str_slice, i64 }* noalias nocapture readonly dereferenceable(24) @const, i64 %3, i64 %.sroa.3.0.copyload.i.i.i)
  unreachable

then-block-54-.i:                                 ; preds = %next-block.i14.i, %join.i, %"_ZN3str39_$BP$$x27a$x20str.StrSlice$LT$$x27a$GT$16is_char_boundary20h77e6472c98e21760edaE.exit.i", %next-block.i.i
  %25 = bitcast [3 x %"struct.core::fmt::Argument<[]>[#3]"]* %8 to i8*
  call void @llvm.lifetime.start(i64 16, i8* %25)
  %26 = getelementptr inbounds [3 x %"struct.core::fmt::Argument<[]>[#3]"]* %8, i64 0, i64 0, i32 0
  store %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)* bitcast (%"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (i64*, %"struct.core::fmt::Formatter<[]>[#3]"*)* @_ZN3fmt11secret_show20h2484714462526658409E to %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)*), %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)** %26, align 8
  %27 = getelementptr inbounds [3 x %"struct.core::fmt::Argument<[]>[#3]"]* %8, i64 0, i64 0, i32 1
  %.c.i.i = bitcast i64* %begin.i to %"enum.core::fmt::Void<[]>[#3]"*
  store %"enum.core::fmt::Void<[]>[#3]"* %.c.i.i, %"enum.core::fmt::Void<[]>[#3]"** %27, align 8
  %28 = getelementptr inbounds [3 x %"struct.core::fmt::Argument<[]>[#3]"]* %8, i64 0, i64 1, i32 0
  store %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)* bitcast (%"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (i64*, %"struct.core::fmt::Formatter<[]>[#3]"*)* @_ZN3fmt11secret_show20h2484714462526658409E to %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)*), %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)** %28, align 8
  %29 = getelementptr inbounds [3 x %"struct.core::fmt::Argument<[]>[#3]"]* %8, i64 0, i64 1, i32 1
  %.c.i25.i = bitcast i64* %end.i to %"enum.core::fmt::Void<[]>[#3]"*
  store %"enum.core::fmt::Void<[]>[#3]"* %.c.i25.i, %"enum.core::fmt::Void<[]>[#3]"** %29, align 8
  %30 = getelementptr inbounds [3 x %"struct.core::fmt::Argument<[]>[#3]"]* %8, i64 0, i64 2, i32 0
  store %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)* bitcast (%"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%str_slice*, %"struct.core::fmt::Formatter<[]>[#3]"*)* @_ZN3fmt11secret_show20h1520870893265926329E to %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)*), %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)** %30, align 8
  %31 = getelementptr inbounds [3 x %"struct.core::fmt::Argument<[]>[#3]"]* %8, i64 0, i64 2, i32 1
  %.c.i26.i = bitcast %str_slice* %1 to %"enum.core::fmt::Void<[]>[#3]"*
  store %"enum.core::fmt::Void<[]>[#3]"* %.c.i26.i, %"enum.core::fmt::Void<[]>[#3]"** %31, align 8
  %32 = bitcast %"struct.core::fmt::Arguments<[]>[#3]"* %__args.i to i8*
  call void @llvm.lifetime.start(i64 32, i8* %32)
  %__adjust.sroa.0.0..sroa_idx.i = getelementptr inbounds %"struct.core::fmt::Arguments<[]>[#3]"* %__args.i, i64 0, i32 0, i32 0
  store %"enum.core::fmt::rt::Piece<[]>[#3]"* bitcast ({ { i8, %str_slice, [48 x i8] }, { i8, { { i8, [15 x i8] }, { i32, i8, i64, { i8, [15 x i8] }, { i8, [15 x i8] } } }, [0 x i8] }, { i8, %str_slice, [48 x i8] }, { i8, { { i8, [15 x i8] }, { i32, i8, i64, { i8, [15 x i8] }, { i8, [15 x i8] } } }, [0 x i8] }, { i8, %str_slice, [48 x i8] }, { i8, { { i8, [15 x i8] }, { i32, i8, i64, { i8, [15 x i8] }, { i8, [15 x i8] } } }, [0 x i8] }, { i8, %str_slice, [48 x i8] } }* @"_ZN3str39_$BP$$x27a$x20str.StrSlice$LT$$x27a$GT$5slice15__STATIC_FMTSTR20hecbbb0a2a6789145mDrE" to %"enum.core::fmt::rt::Piece<[]>[#3]"*), %"enum.core::fmt::rt::Piece<[]>[#3]"** %__adjust.sroa.0.0..sroa_idx.i, align 8
  %__adjust.sroa.4.0..sroa_idx34.i = getelementptr inbounds %"struct.core::fmt::Arguments<[]>[#3]"* %__args.i, i64 0, i32 0, i32 1
  store i64 7, i64* %__adjust.sroa.4.0..sroa_idx34.i, align 8
  %__adjust.sroa.0.0..sroa_idx.i27.i = getelementptr inbounds %"struct.core::fmt::Arguments<[]>[#3]"* %__args.i, i64 0, i32 1, i32 0
  store %"struct.core::fmt::Argument<[]>[#3]"* %.sub.i, %"struct.core::fmt::Argument<[]>[#3]"** %__adjust.sroa.0.0..sroa_idx.i27.i, align 8
  %__adjust.sroa.3.0..sroa_idx1.i.i = getelementptr inbounds %"struct.core::fmt::Arguments<[]>[#3]"* %__args.i, i64 0, i32 1, i32 1
  store i64 3, i64* %__adjust.sroa.3.0..sroa_idx1.i.i, align 8
  call void @_ZN7failure12begin_unwind20hd0af3f4657afff41yAkE(%"struct.core::fmt::Arguments<[]>[#3]"* noalias nocapture readonly dereferenceable(32) %__args.i, { %str_slice, i64 }* noalias nocapture readonly dereferenceable(24) @"_ZN3str39_$BP$$x27a$x20str.StrSlice$LT$$x27a$GT$5slice8_run_fmt10_FILE_LINE20h4965d27d95ea2255QCrE")
  unreachable

next-block.i:                                     ; preds = %before_rhs.next-block_crit_edge.i, %join.i
  %arg.sroa.0.0.copyload.i = phi i8* [ %arg.sroa.0.0.copyload.pre.i, %before_rhs.next-block_crit_edge.i ], [ %.sroa.0.0.copyload.i17.i, %join.i ]
  %33 = icmp ult i64 %3, %2
  br i1 %33, label %then-block-583-.i.i, label %next-block.i24.i

then-block-583-.i.i:                              ; preds = %next-block.i
  %34 = bitcast %str_slice* %4 to i8*
  call void @llvm.lifetime.start(i64 16, i8* %34)
  %35 = getelementptr inbounds %str_slice* %4, i64 0, i32 0
  store i8* getelementptr inbounds ([30 x i8]* @str1159, i64 0, i64 0), i8** %35, align 8
  %36 = getelementptr inbounds %str_slice* %4, i64 0, i32 1
  store i64 30, i64* %36, align 8
  %37 = bitcast %"struct.core::fmt::Argument<[]>[#3]"* %5 to i8*
  call void @llvm.lifetime.start(i64 16, i8* %37)
  %38 = getelementptr inbounds %"struct.core::fmt::Argument<[]>[#3]"* %5, i64 0, i32 0
  store %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)* bitcast (%"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%str_slice*, %"struct.core::fmt::Formatter<[]>[#3]"*)* @_ZN3fmt11secret_show20h8865777717318847987E to %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)*), %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)** %38, align 8
  %39 = getelementptr inbounds %"struct.core::fmt::Argument<[]>[#3]"* %5, i64 0, i32 1
  %.c.i.i.i = bitcast %str_slice* %4 to %"enum.core::fmt::Void<[]>[#3]"*
  store %"enum.core::fmt::Void<[]>[#3]"* %.c.i.i.i, %"enum.core::fmt::Void<[]>[#3]"** %39, align 8
  %40 = bitcast %"struct.core::fmt::Arguments<[]>[#3]"* %__args.i.i to i8*
  call void @llvm.lifetime.start(i64 32, i8* %40)
  %__adjust.sroa.0.0..sroa_idx.i.i = getelementptr inbounds %"struct.core::fmt::Arguments<[]>[#3]"* %__args.i.i, i64 0, i32 0, i32 0
  store %"enum.core::fmt::rt::Piece<[]>[#3]"* bitcast ({ { i8, { { i8, [15 x i8] }, { i32, i8, i64, { i8, [15 x i8] }, { i8, [15 x i8] } } }, [0 x i8] } }* @_ZN3str3raw11slice_bytes15__STATIC_FMTSTR20hf82a49d95a7811fb4irE to %"enum.core::fmt::rt::Piece<[]>[#3]"*), %"enum.core::fmt::rt::Piece<[]>[#3]"** %__adjust.sroa.0.0..sroa_idx.i.i, align 8
  %__adjust.sroa.4.0..sroa_idx37.i.i = getelementptr inbounds %"struct.core::fmt::Arguments<[]>[#3]"* %__args.i.i, i64 0, i32 0, i32 1
  store i64 1, i64* %__adjust.sroa.4.0..sroa_idx37.i.i, align 8
  %__adjust.sroa.0.0..sroa_idx.i.i.i = getelementptr inbounds %"struct.core::fmt::Arguments<[]>[#3]"* %__args.i.i, i64 0, i32 1, i32 0
  store %"struct.core::fmt::Argument<[]>[#3]"* %5, %"struct.core::fmt::Argument<[]>[#3]"** %__adjust.sroa.0.0..sroa_idx.i.i.i, align 8
  %__adjust.sroa.3.0..sroa_idx1.i.i.i = getelementptr inbounds %"struct.core::fmt::Arguments<[]>[#3]"* %__args.i.i, i64 0, i32 1, i32 1
  store i64 1, i64* %__adjust.sroa.3.0..sroa_idx1.i.i.i, align 8
  call void @_ZN7failure12begin_unwind20hd0af3f4657afff41yAkE(%"struct.core::fmt::Arguments<[]>[#3]"* noalias nocapture readonly dereferenceable(32) %__args.i.i, { %str_slice, i64 }* noalias nocapture readonly dereferenceable(24) @_ZN3str3raw11slice_bytes8_run_fmt10_FILE_LINE20h4965d27d95ea2255ghrE)
  unreachable

next-block.i24.i:                                 ; preds = %next-block.i
  %41 = icmp ult i64 %.sroa.3.0.copyload.i.i.i, %3
  br i1 %41, label %then-block-670-.i.i, label %"_ZN3str39_$BP$$x27a$x20str.StrSlice$LT$$x27a$GT$5slice20he30e5671c97c5789AaaE.exit"

then-block-670-.i.i:                              ; preds = %next-block.i24.i
  %42 = bitcast %str_slice* %6 to i8*
  call void @llvm.lifetime.start(i64 16, i8* %42)
  %43 = getelementptr inbounds %str_slice* %6, i64 0, i32 0
  store i8* getelementptr inbounds ([32 x i8]* @str1160, i64 0, i64 0), i8** %43, align 8
  %44 = getelementptr inbounds %str_slice* %6, i64 0, i32 1
  store i64 32, i64* %44, align 8
  %45 = bitcast %"struct.core::fmt::Argument<[]>[#3]"* %7 to i8*
  call void @llvm.lifetime.start(i64 16, i8* %45)
  %46 = getelementptr inbounds %"struct.core::fmt::Argument<[]>[#3]"* %7, i64 0, i32 0
  store %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)* bitcast (%"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%str_slice*, %"struct.core::fmt::Formatter<[]>[#3]"*)* @_ZN3fmt11secret_show20h8865777717318847987E to %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)*), %"enum.core::result::Result<[(), core::fmt::FormatError]>[#3]" (%"enum.core::fmt::Void<[]>[#3]"*, %"struct.core::fmt::Formatter<[]>[#3]"*)** %46, align 8
  %47 = getelementptr inbounds %"struct.core::fmt::Argument<[]>[#3]"* %7, i64 0, i32 1
  %.c.i25.i.i = bitcast %str_slice* %6 to %"enum.core::fmt::Void<[]>[#3]"*
  store %"enum.core::fmt::Void<[]>[#3]"* %.c.i25.i.i, %"enum.core::fmt::Void<[]>[#3]"** %47, align 8
  %48 = bitcast %"struct.core::fmt::Arguments<[]>[#3]"* %__args7.i.i to i8*
  call void @llvm.lifetime.start(i64 32, i8* %48)
  %__adjust8.sroa.0.0..sroa_idx.i.i = getelementptr inbounds %"struct.core::fmt::Arguments<[]>[#3]"* %__args7.i.i, i64 0, i32 0, i32 0
  store %"enum.core::fmt::rt::Piece<[]>[#3]"* bitcast ({ { i8, { { i8, [15 x i8] }, { i32, i8, i64, { i8, [15 x i8] }, { i8, [15 x i8] } } }, [0 x i8] } }* @_ZN3str3raw11slice_bytes15__STATIC_FMTSTR20hf82a49d95a7811fb4irE to %"enum.core::fmt::rt::Piece<[]>[#3]"*), %"enum.core::fmt::rt::Piece<[]>[#3]"** %__adjust8.sroa.0.0..sroa_idx.i.i, align 8
  %__adjust8.sroa.4.0..sroa_idx32.i.i = getelementptr inbounds %"struct.core::fmt::Arguments<[]>[#3]"* %__args7.i.i, i64 0, i32 0, i32 1
  store i64 1, i64* %__adjust8.sroa.4.0..sroa_idx32.i.i, align 8
  %__adjust.sroa.0.0..sroa_idx.i23.i.i = getelementptr inbounds %"struct.core::fmt::Arguments<[]>[#3]"* %__args7.i.i, i64 0, i32 1, i32 0
  store %"struct.core::fmt::Argument<[]>[#3]"* %7, %"struct.core::fmt::Argument<[]>[#3]"** %__adjust.sroa.0.0..sroa_idx.i23.i.i, align 8
  %__adjust.sroa.3.0..sroa_idx1.i24.i.i = getelementptr inbounds %"struct.core::fmt::Arguments<[]>[#3]"* %__args7.i.i, i64 0, i32 1, i32 1
  store i64 1, i64* %__adjust.sroa.3.0..sroa_idx1.i24.i.i, align 8
  call void @_ZN7failure12begin_unwind20hd0af3f4657afff41yAkE(%"struct.core::fmt::Arguments<[]>[#3]"* noalias nocapture readonly dereferenceable(32) %__args7.i.i, { %str_slice, i64 }* noalias nocapture readonly dereferenceable(24) @_ZN3str3raw11slice_bytes8_run_fmt10_FILE_LINE20h4965d27d95ea2255FirE)
  unreachable

"_ZN3str39_$BP$$x27a$x20str.StrSlice$LT$$x27a$GT$5slice20he30e5671c97c5789AaaE.exit": ; preds = %next-block.i24.i
  %49 = getelementptr inbounds %str_slice* %0, i64 0, i32 0
  %50 = getelementptr inbounds i8* %arg.sroa.0.0.copyload.i, i64 %2
  store i8* %50, i8** %49, align 8
  %51 = getelementptr inbounds %str_slice* %0, i64 0, i32 1
  %52 = sub i64 %3, %2
  store i64 %52, i64* %51, align 8
  call void @llvm.lifetime.end(i64 8, i8* %10)
  call void @llvm.lifetime.end(i64 8, i8* %9)
  %53 = bitcast %str_slice* %1 to i8*
  tail call void @llvm.lifetime.end(i64 16, i8* %53)
  ret void
}

The worst part is there's five separate sites that unwind (each unreachable is just after an unwinding function) due to various assertions and bounds-checks. (&[].slice is mildly better.)

[thestinger]

This is the cost of the fail!() macro. Instead, we should call a function (fail("foo")) and let backtraces handle reporting the location of the error.

[huonw]

It's also the cost of doing significantly more work than necessary:

• bounds checks, neither (there is two is_char_boundary calls) are eliminated, although this looks a bit like an LLVM bug, since the two length comparisons above it cover the OOB case, and it's even touching the same variables in the IR:

    %11 = icmp eq i64 %.sroa.3.0.copyload.i.i.i, %2
    br i1 %11, label %before_rhs.i, label %next-block.i.i
  
  next-block.i.i:                                   ; preds = %entry-block
    %12 = icmp ult i64 %.sroa.3.0.copyload.i.i.i, %2
    br i1 %12, label %then-block-54-.i, label %next-block1.i.i
  
  next-block1.i.i:                                  ; preds = %next-block.i.i
    %13 = icmp ugt i64 %.sroa.3.0.copyload.i.i.i, %2 ; impossible to fail!

• an assert! that is impossible to trigger due to bounds checking in is_char_boundary.

It should be possible to at least reduce it to 2 unwinding (one for the character boundaries, one for start <= end), or even 1 if we're willing to lump everything together.

[thestinger]

The intricate details of the error handling aren't really important, because there's no reason for it to be inlined everywhere. It should be a function call passing a string literal. Using macros to report error locations doesn't work anyway because you don't get any context beyond the fact that a very low-level function like slice failed. You already need backtraces and those don't bloat all of the code.

[pcwalton]

I think the problem is not the error location (that can be fixed if it's not already) but the fact that there's string formatting going on. Asserts should never have string formatting.

[bluss]

Do .slice_from and .slice_to optimize out the checks for their respective fixed endpoints? If not, that's a possible improvement in the same area.

[bluss]

fail!("assertion failed: {:s}", stringify!($cond)) should be easy to fix. Replace with
fail!(concat!("assertion failed: ", stringify!($cond)))

[bluss]

This issue has been resolved by resolving the example given, but what is the follow up, what is the more general issue? Would allowing a format-free assert! code path help (issue #16700)? Should we extend the pattern of using a "cold" function to perform elaborate error reporting?

[alexcrichton]

I would not like to propagate the #[inline(never)] + #[cold] pattern except where absolutely necessary as it is in general indicative of a more subtle underlying problem than a fix that should only happen locally.

This specific issue has been addressed, however, so I'm going to close this. More targeted follow up issues can be opened and addressed separately.