Issue 18948 - std.uni.toLower and std.uni.toUpper should work with random access ranges

std/uni.d

@@ -8997,20 +8997,24 @@ private alias UpperTriple = AliasSeq!(toUpperIndex, MAX_SIMPLE_UPPER, toUpperTab
 private alias LowerTriple = AliasSeq!(toLowerIndex, MAX_SIMPLE_LOWER, toLowerTab);
 
 // generic toUpper/toLower on whole string, creates new or returns as is
-private S toCase(alias indexFn, uint maxIdx, alias tableFn, alias asciiConvert, S)(S s) @trusted pure
-if (isSomeString!S)
+private ElementEncodingType!S[] toCase(alias indexFn, uint maxIdx, alias tableFn, alias asciiConvert, S)(S s)
+if (isSomeString!S || (isRandomAccessRange!S && hasLength!S && hasSlicing!S && isSomeChar!(ElementType!S)))
 {
-    import std.array : appender;
+    import std.array : appender, array;
     import std.ascii : isASCII;
+    import std.utf : byDchar;
 
-    foreach (i, dchar cOuter; s)
+    auto r = s.byDchar;
+    for (size_t i; !r.empty; ++i, r.popFront())
     {
+        auto cOuter = r.front;
         ushort idx = indexFn(cOuter);
         if (idx == ushort.max)
             continue;
-        auto result = appender!S(s[0 .. i]);
+        auto result = appender!(ElementEncodingType!S[])();
+        result.put(s[0 .. i]);
         result.reserve(s.length);
-        foreach (dchar c; s[i .. $])
+        foreach (dchar c; s[i .. $].byDchar)
         {
             if (c.isASCII)
             {
@@ -9039,7 +9043,11 @@ if (isSomeString!S)
         }
         return result.data;
     }
-    return s;
+
+    static if (isSomeString!S)
+        return s;
+    else
+        return s.array;
 }
 
 @safe unittest //12428
@@ -9783,16 +9791,28 @@ dchar toLower(dchar c)
 }
 
 /++
-    Returns a string which is identical to `s` except that all of its
+    Creates a new array which is identical to `s` except that all of its
     characters are converted to lowercase (by preforming Unicode lowercase mapping).
-    If none of `s` characters were affected, then `s` itself is returned.
+    If none of `s` characters were affected, then `s` itself is returned if `s` is a
+    `string`-like type.
+
+    Params:
+        s = A $(REF_ALTTEXT random access range, isRandomAccessRange, std,range,primitives)
+        of characters
+    Returns:
+        An array with the same element type as `s`.
 +/
-S toLower(S)(S s) @trusted pure
-if (isSomeString!S)
+ElementEncodingType!S[] toLower(S)(S s)
+if (isSomeString!S || (isRandomAccessRange!S && hasLength!S && hasSlicing!S && isSomeChar!(ElementType!S)))
 {
     static import std.ascii;
-    return toCase!(LowerTriple, std.ascii.toLower)(s);
+
+    static if (isSomeString!S)
+        return () @trusted { return toCase!(LowerTriple, std.ascii.toLower)(s); } ();
+    else
+        return toCase!(LowerTriple, std.ascii.toLower)(s);
 }
+
 // overloads for the most common cases to reduce compile time
 @safe pure /*TODO nothrow*/
 {
@@ -9900,6 +9920,15 @@ if (isSomeString!S)
     assert(toUpper(c) == '\u1F8F');
 }
 
+@safe pure unittest
+{
+    import std.algorithm.comparison : cmp, equal;
+    import std.utf : byCodeUnit;
+    auto r1 = "FoL".byCodeUnit;
+    assert(r1.toLower.cmp("fol") == 0);
+    auto r2 = "A\u0460B\u0461d".byCodeUnit;
+    assert(r2.toLower.cmp("a\u0461b\u0461d") == 0);
+}
 
 /++
     If `c` is a Unicode lowercase $(CHARACTER), then its uppercase equivalent
@@ -9965,16 +9994,28 @@ dchar toUpper(dchar c)
 }
 
 /++
-    Returns a string which is identical to `s` except that all of its
+    Allocates a new array which is identical to `s` except that all of its
     characters are converted to uppercase (by preforming Unicode uppercase mapping).
-    If none of `s` characters were affected, then `s` itself is returned.
+    If none of `s` characters were affected, then `s` itself is returned if `s`
+    is a `string`-like type.
+
+    Params:
+        s = A $(REF_ALTTEXT random access range, isRandomAccessRange, std,range,primitives)
+        of characters
+    Returns:
+        An new array with the same element type as `s`.
 +/
-S toUpper(S)(S s) @trusted pure
-if (isSomeString!S)
+ElementEncodingType!S[] toUpper(S)(S s)
+if (isSomeString!S || (isRandomAccessRange!S && hasLength!S && hasSlicing!S && isSomeChar!(ElementType!S)))
 {
     static import std.ascii;
-    return toCase!(UpperTriple, std.ascii.toUpper)(s);
+
+    static if (isSomeString!S)
+        return () @trusted { return toCase!(UpperTriple, std.ascii.toUpper)(s); } ();
+    else
+        return toCase!(UpperTriple, std.ascii.toUpper)(s);
 }
+
 // overloads for the most common cases to reduce compile time
 @safe pure /*TODO nothrow*/
 {
@@ -10088,6 +10129,16 @@ if (isSomeString!S)
     }}
 }
 
+// test random access ranges
+@safe pure unittest
+{
+    import std.algorithm.comparison : cmp;
+    import std.utf : byCodeUnit;
+    auto s1 = "FoL".byCodeUnit;
+    assert(s1.toUpper.cmp("FOL") == 0);
+    auto s2 = "a\u0460B\u0461d".byCodeUnit;
+    assert(s2.toUpper.cmp("A\u0460B\u0460D") == 0);
+}
 
 /++
     Returns whether `c` is a Unicode alphabetic $(CHARACTER)