fixes #20026; marks system procs which can raise defects (#20864)

* marks system procs which can raise defects

* add tests

* add more systemRaisesDefect

* add comment

1 files changed, 18 insertions, 15 deletions

diff --git a/lib/system/indices.nim b/lib/system/indices.nim
index 3f90d4399..fd6770e23 100644
--- a/lib/system/indices.nim
+++ b/lib/system/indices.nim
@@ -1,3 +1,6 @@
+when not defined(nimHasSystemRaisesDefect):
+  {.pragma: systemRaisesDefect.}
+
 type
   BackwardsIndex* = distinct int ## Type that is constructed by `^` for
                                  ## reversed array accesses.
@@ -16,24 +19,24 @@ template `^`*(x: int): BackwardsIndex = BackwardsIndex(x)
   ##   echo b[^2] # => g
   ##   ```
 
-proc `[]`*[T](s: openArray[T]; i: BackwardsIndex): T {.inline.} =
+proc `[]`*[T](s: openArray[T]; i: BackwardsIndex): T {.inline, systemRaisesDefect.} =
   system.`[]`(s, s.len - int(i))
 
-proc `[]`*[Idx, T](a: array[Idx, T]; i: BackwardsIndex): T {.inline.} =
+proc `[]`*[Idx, T](a: array[Idx, T]; i: BackwardsIndex): T {.inline, systemRaisesDefect.} =
   a[Idx(a.len - int(i) + int low(a))]
-proc `[]`*(s: string; i: BackwardsIndex): char {.inline.} = s[s.len - int(i)]
+proc `[]`*(s: string; i: BackwardsIndex): char {.inline, systemRaisesDefect.} = s[s.len - int(i)]
 
-proc `[]`*[T](s: var openArray[T]; i: BackwardsIndex): var T {.inline.} =
+proc `[]`*[T](s: var openArray[T]; i: BackwardsIndex): var T {.inline, systemRaisesDefect.} =
   system.`[]`(s, s.len - int(i))
-proc `[]`*[Idx, T](a: var array[Idx, T]; i: BackwardsIndex): var T {.inline.} =
+proc `[]`*[Idx, T](a: var array[Idx, T]; i: BackwardsIndex): var T {.inline, systemRaisesDefect.} =
   a[Idx(a.len - int(i) + int low(a))]
-proc `[]`*(s: var string; i: BackwardsIndex): var char {.inline.} = s[s.len - int(i)]
+proc `[]`*(s: var string; i: BackwardsIndex): var char {.inline, systemRaisesDefect.} = s[s.len - int(i)]
 
-proc `[]=`*[T](s: var openArray[T]; i: BackwardsIndex; x: T) {.inline.} =
+proc `[]=`*[T](s: var openArray[T]; i: BackwardsIndex; x: T) {.inline, systemRaisesDefect.} =
   system.`[]=`(s, s.len - int(i), x)
-proc `[]=`*[Idx, T](a: var array[Idx, T]; i: BackwardsIndex; x: T) {.inline.} =
+proc `[]=`*[Idx, T](a: var array[Idx, T]; i: BackwardsIndex; x: T) {.inline, systemRaisesDefect.} =
   a[Idx(a.len - int(i) + int low(a))] = x
-proc `[]=`*(s: var string; i: BackwardsIndex; x: char) {.inline.} =
+proc `[]=`*(s: var string; i: BackwardsIndex; x: char) {.inline, systemRaisesDefect.} =
   s[s.len - int(i)] = x
 
 template `..^`*(a, b: untyped): untyped =
@@ -70,7 +73,7 @@ template spliceImpl(s, a, L, b: typed): untyped =
   # fill the hole:
   for i in 0 ..< b.len: s[a+i] = b[i]
 
-proc `[]`*[T, U: Ordinal](s: string, x: HSlice[T, U]): string {.inline.} =
+proc `[]`*[T, U: Ordinal](s: string, x: HSlice[T, U]): string {.inline, systemRaisesDefect.} =
   ## Slice operation for strings.
   ## Returns the inclusive range `[s[x.a], s[x.b]]`:
   ##   ```
@@ -82,7 +85,7 @@ proc `[]`*[T, U: Ordinal](s: string, x: HSlice[T, U]): string {.inline.} =
   result = newString(L)
   for i in 0 ..< L: result[i] = s[i + a]
 
-proc `[]=`*[T, U: Ordinal](s: var string, x: HSlice[T, U], b: string) =
+proc `[]=`*[T, U: Ordinal](s: var string, x: HSlice[T, U], b: string) {.systemRaisesDefect.} =
   ## Slice assignment for strings.
   ##
   ## If `b.len` is not exactly the number of elements that are referred to
@@ -100,7 +103,7 @@ proc `[]=`*[T, U: Ordinal](s: var string, x: HSlice[T, U], b: string) =
   else:
     spliceImpl(s, a, L, b)
 
-proc `[]`*[Idx, T; U, V: Ordinal](a: array[Idx, T], x: HSlice[U, V]): seq[T] =
+proc `[]`*[Idx, T; U, V: Ordinal](a: array[Idx, T], x: HSlice[U, V]): seq[T] {.systemRaisesDefect.} =
   ## Slice operation for arrays.
   ## Returns the inclusive range `[a[x.a], a[x.b]]`:
   ##   ```
@@ -112,7 +115,7 @@ proc `[]`*[Idx, T; U, V: Ordinal](a: array[Idx, T], x: HSlice[U, V]): seq[T] =
   result = newSeq[T](L)
   for i in 0..<L: result[i] = a[Idx(i + xa)]
 
-proc `[]=`*[Idx, T; U, V: Ordinal](a: var array[Idx, T], x: HSlice[U, V], b: openArray[T]) =
+proc `[]=`*[Idx, T; U, V: Ordinal](a: var array[Idx, T], x: HSlice[U, V], b: openArray[T]) {.systemRaisesDefect.} =
   ## Slice assignment for arrays.
   ##   ```
   ##   var a = [10, 20, 30, 40, 50]
@@ -126,7 +129,7 @@ proc `[]=`*[Idx, T; U, V: Ordinal](a: var array[Idx, T], x: HSlice[U, V], b: ope
   else:
     sysFatal(RangeDefect, "different lengths for slice assignment")
 
-proc `[]`*[T; U, V: Ordinal](s: openArray[T], x: HSlice[U, V]): seq[T] =
+proc `[]`*[T; U, V: Ordinal](s: openArray[T], x: HSlice[U, V]): seq[T] {.systemRaisesDefect.} =
   ## Slice operation for sequences.
   ## Returns the inclusive range `[s[x.a], s[x.b]]`:
   ##   ```
@@ -138,7 +141,7 @@ proc `[]`*[T; U, V: Ordinal](s: openArray[T], x: HSlice[U, V]): seq[T] =
   newSeq(result, L)
   for i in 0 ..< L: result[i] = s[i + a]
 
-proc `[]=`*[T; U, V: Ordinal](s: var seq[T], x: HSlice[U, V], b: openArray[T]) =
+proc `[]=`*[T; U, V: Ordinal](s: var seq[T], x: HSlice[U, V], b: openArray[T]) {.systemRaisesDefect.} =
   ## Slice assignment for sequences.
   ##
   ## If `b.len` is not exactly the number of elements that are referred to