Helper functions require standard library substitutes.

                      require the extreme slice edge-case: slice of
                      comptime-known pointer with runtime-known end.

…gate of

      type `ty` permits sentinels.

             'convenience' declaration of `runtime_safety`.

* Added convenience declaration `runtime_safety`.
* This value modified directly from the command line in `main.zig`.
* This value is used to initialize `Builtin` options field `runtime_safety`.

* Updated command line parser to parse controls for each panic cause compile command:
     `-funwrapped-error`,
     `-funwrapped-null`,
     `-freturned-noreturn`,
     `-freached-unreachable`,
     `-faccessed-out-of-bounds`,
     `-faccessed-out-of-order`,
     `-faccessed-inactive-field`,
     `-fdivided-by-zero`,
     `-fmemcpy-argument-aliasing`,
     `-fmismatched-memcpy-argument-lengths`,
     `-fmismatched-for-loop-capture-lengths`,
     `-fmismatched-sentinel`,
     `-fshift-amt-overflowed`,
     `-farith-overflowed`,
     `-fcast-truncated-data`,
     `-fcast-to-enum-from-invalid`,
     `-fcast-to-error-from-invalid`,
     `-fcast-to-pointer-from-invalid`,
     `-fcast-to-int-from-invalid`,
     `-fcast-to-unsigned-from-negative`.
* Updated command line parser to parse convenience/test flags:
    `-fruntime-safety`,
    `-f[no-]analyze-slice2`.

…Safety`.

      When testing is complete this namespace will be removed and all
      analogous functions in the file scope will be replaced.

* Added helper functions:
    `preparePanicCause`,
    `failBlock`,
    `preparePanicExtraType`,
    `callBuiltinHelper`,
    `wantPanicCause`,
    `bestGenericIntType`,
    `resolveArithOverflowedPanicImpl`,
    `resolveArithOverflowedPanicCause`,
    `abiSizeOfContainingDecl`.

* Added panic preparation functions:
    `panicWithMsg`,
    `panicReachedUnreachable`,
    `panicCastToEnumFromInvalid`,
    `panicUnwrappedError`,
    `checkAccessNullValue`,
    `checkUnwrappedError`,
    `checkDivisionByZero`,
    `checkAccessOutOfBounds`,
    `checkAccessOutOfOrder`,
    `checkAccessOutOfOrderExtra`,
    `checkAliasingMemcpyArguments`,
    `checkMismatchedMemcpyArgumentLengths`,
    `checkMismatchedForLoopCaptureLengths`,
    `checkAccessInactiveUnionField`,
    `checkMismatchedSentinel`,
    `checkMismatchedNullTerminator`,
    `checkShiftAmountOverflow`,
    `checkArithmeticOverflow`,
    `checkCastToEnumFromInvalid`,
    `checkCastToEnumFromInvalidHelper`,
    `checkCastToErrorFromInvalid`,
    `checkCastToErrorFromInvalidHelper`,
    `checkCastToPointerFromInvalid`,
    `checkCastToIntFromInvalid`,
    `checkCastTruncatedData`,
    `checkCastToUnsignedFromNegative`.

* Added primary functions:
    `zirSliceStart`,
    `zirSliceEnd`,
    `zirSliceSentinel`,
    `zirSliceLength`,
    `analyzeSlice2`.

      `zirReify`, `zirArrayTypeSentinel`,
      `zirPtrType`, `analyzeSlice`.

      `zirSliceStart`, `zirSliceEnd`,
      `zirSliceSentinel`, and `zirSliceLength`.

      `Block.addUnreachable`.

      by `zirForLen`.

…ll`.

      by `zirErrorFromInt`.

      `zirEnumFromInt`.

      `analyzeOptionalPayloadPtr`.

      `zirOptionalPayload`.

      `analyzeErrUnionPayload`.

      `analyzeErrUnionPayloadPtr`.

      `checkCastToUnsignedFromNegative` by `intCast`.

      `zirSwitchBlock`. This is potentially the wrong panic cause, but
      aligns more closely with the backend requirement.

      `panicReachedUnreachable` by `analyzeSwitchRuntimeBlock`.

…rap`.

      `checkArithmeticOverflow` by `zirShl`.

      `checkArithmeticOverflow` by `zirShr`.

      `zirDivExact`.

      `addDivIntOverflowSafety`.

      `addDivByZeroSafety`.

      `analyzeArithmetic`.

      `zirTagName`.

      `zirIntFromFloat`.

      `zirPtrFromInt`.

      `zirErrorCast`.

      `ptrCastFull`.

      `checkMismatchedMemcpyArgumentLengths`, and `analyzeSlice2` by
      `zirMemcpy`.

      `unionFieldPtr`.

      `unionFieldVal`.

      `elemValArray`.

      `elemPtrArray`.

      `elemValSlice`.

      `elemPtrSlice`.

      `coerceCompatiblePtrs`.

* Added optional usage of `checkAccessNullValue`,
  `checkAccessOutOfBounds`, `checkAccessOutOfOrder`, and
  `checkMismatchedSentinel` by `analyzeSlice`.
* Updated fail block initializer.

* Effectively renamed `panicImpl` to `panicImplDefault`, and replaced
  `panicImpl` definition of former `builtin.default_panic`.
* Replaced usage of `builtin.panic` by `panicExtra` with call to
  `panicImpl`.

* Removed sanity check (for unreachable causes).
* Other minor fixes and improvements.

…vious.

      confusing `uncasted_*_opt` references by `analyzeSlice2`.

…e_safety

…safety

…`fmtValue`.

         `accessed_inactive_field`.

…safety

         error cases. An ideal solution would distinguish between index
         out-of-bounds and slice operand out-of-bounds.

…safety

…safety

…safety

* Added functions to enable slicing reinterpreted compile-time
  memory when the element type has no runtime bits.
* Reworded documentation comment for `abiSizeOfContainingDecl`.

* Updated `RuntimeSafety.analyzeSlice2` to support slicing
  pointers-to-many for element types with zero runtime bits.
* Simplified compile error message for `slice_start` when sentinel index
  is out of bounds.

…safety

      can be null if `usize` is larger than `u64`.

* Generally updated panic message writers to move closer to a final version.
* Added sums of bytes written to panic message buffers for some code blocks.
* Added comment reminding to treat indexing bounds and slicing bounds separately
  for clarity and consistency of error messages.
* Updated panic message writers writing arbitrarily long strings to use
  `cpyEquTrunc`. This limits the number of bytes taken by any single
  component of the error message. The primary motivation for this is to
  prevent very large type names from consuming the entire buffer before
  any other panic data can be rendered.

  Truncated strings are indicated like 'looooooong_stri[..]', so the
  minimum budget for any copy is 4.

  Example for `panicAccessedInactiveField` with a purposefully low tag name
  budget of 16:

  "access of union field 'b_long_inact[..]' while field 'b_long_activ[..]' is active"

…safety

* Added standard documentation about compiler dependency to
  `RuntimeSafety` as well as note on how it might be used in the
  future.
* Renamed `panicNew_default` to `panicImpl`.
* Relocated (removed) writer functions.
* Relocated or deleted helper functions.

debug:
* Relocated (added) writer functions.

* Added behavior test for slice success variants.
* Removed redundant slice runtime safety test. This and other tests will
  be restored when feedback is given on the allowed number of slice runtime
  safety tests.

      tests which will be redundant if the `Sema.analyzeSlice2`
      runtime safety tests are added. However if that addition is not
      approved these tests will provide basic coverage until a more concise
      testsuite can be generated.

      Between 60 and 70 of these 82 files will pass for the x86-64
      backend. This difference is caused by missing features, such
      as vector operations and checking enumeration values and error codes.

      version (0.13.0-dev.46+3648d7df1) of the compiler will crash when
      attempting `zig test test/behavior/slice2.zig`.

      so that it does not require the creation of an invalid slice.
      This change does not impact the test result.

…inel)"

      so that the type of local slice `target` includes the necessary sentinel.

      `slice_end` slicing compile-time-known undefined pointer using
      runtime-known end operand.

* Updated test `foo_cvector_ConstPtrSpecialRef` to include required
  sentinel in type definition.
* Updated various compile errors.

      This selection could be improved by adding variants for 0-bit and
      `comptime`-only types.

…safety

* Updated definitions of `panicNew` to account for the loss of
  `runtime_safety` builtin information. `builtin.PanicData` can no
  longer predict `void` values. This may be restored in future updates.
* Changed some cases from `reached_unreachable` to `unwrapped_error`.
* Changed some cases from `corrupt_switch` to `cast_to_enum_from_invalid`.

      this is no longer usable.

      possible that the current implementation only calls this function
      after having already established backend support, however it might
      not be the case that backend support for `error_return_trace` and
      `unwrap_error` are the same. In this case case a lot of the `unwrap_error`
      panic data may be reported without the `builtin.StackTrace` pointer.

* Improved general usability of panic preparation functions by
  preventing usage by backends which do not support the feature.

  Note: The x86-64 backend actually does support all but three of the
  current panic causes. It does not support `checkCastToEnumFromInvalid`
  and `checkCastToErrorFromInvalid`, nor does it support the former
  `corrupt_switch` panic cause (except in one unconditional case)--but
  this might be unreachable for other reasons.

* Updated `reinterpretLength` to allow recursively reinterpreting
  arrays of arrays, arrays of vectors, and vectors of arrays of the
  child type.

* Relocated helper functions `reinterpretLength`,
  `reinterpretLengthOfContainingDecl`, and `abiSizeOfContainingDecl`, which may
  be suitable for general use.

* Removed extra definition of `Sema.analyzeSlice2` from the sub-namespace.

* Added experimental `addSubSafeOptimized`. This function has been
  tested to produce a small optimisation to arithmetic overflow checks,
  which are currently very expensive when generated by LLVM. The
  soundness of this function has not been totally verified, so it
  remains unused.