Since scalar-alignment is set in inlinable code, switch the alignment
bit to one of the previously-reserved bits rather than a grapheme
cache bit. Setting a grapheme cache bit in inlinable would break
backward deployment, as older versions would interpret it as a cached
value.
Also adjust the name to "scalar-aligned", which is clearer, and
removed assertion (which should be a real precondition).
There are situations where you want to build against a libc that is out
of tree or that is not the system libc (Or for cross build scenarios).
This is a change for passing the -sdk and include paths for things like
this.
Rather than hardcoding the paths to /usr/include, allow the user to set
the path to their libc headers. This is particularly important for
environments which may not use the traditional layout (e.g. exherbo) or
for builds which wish to build against an out-of-tree, non-system
installed libc for a Unix target which does not match the host system
(i.e. foreign cross-compilation).
Fixes a general category (pun intended) of scalar-alignment bugs
surrounding exchanging non-scalar-aligned indices between views and
for slicing.
SE-0180 unifies the Index type of String and all its views and allows
non-scalar-aligned indices to be used across views. In order to
guarantee behavior, we often have to check and perform scalar
alignment. To speed up these checks, we allocate a bit denoting
known-to-be-aligned, so that the alignment check can skip the
load. The below shows what views need to check for alignment before
they can operate, and whether the indices they produce are aligned.
┌───────────────╥────────────────────┬──────────────────────────┐
│ View ║ Requires Alignment │ Produces Aligned Indices │
╞═══════════════╬════════════════════╪══════════════════════════╡
│ Native UTF8 ║ no │ no │
├───────────────╫────────────────────┼──────────────────────────┤
│ Native UTF16 ║ yes │ no │
╞═══════════════╬════════════════════╪══════════════════════════╡
│ Foreign UTF8 ║ yes │ no │
├───────────────╫────────────────────┼──────────────────────────┤
│ Foreign UTF16 ║ no │ no │
╞═══════════════╬════════════════════╪══════════════════════════╡
│ UnicodeScalar ║ yes │ yes │
├───────────────╫────────────────────┼──────────────────────────┤
│ Character ║ yes │ yes │
└───────────────╨────────────────────┴──────────────────────────┘
The "requires alignment" applies to any operation taking a
String.Index that's not defined entirely in terms of other operations
taking a String.Index. These include:
* index(after:)
* index(before:)
* subscript
* distance(from:to:) (since `to` is compared against directly)
* UTF16View._nativeGetOffset(for:)
ICU will return different results if we call with an offset into a
code unit buffer vs if we slice the buffer first and provide an offset
of zero. Slicing more closely models the semantics of SE-0180, so use
that.
Test case coming in subsequent commit enforcing index
scalar-alignment.
