std.simd.iota: make it always called at comptime

There's no reason for this to ever run at runtime; it should always be
used to generate a constant.

Author: andrewrk

lib/std/simd.zig

@@ -86,16 +86,18 @@ pub fn VectorCount(comptime VectorType: type) type {
 
 /// Returns a vector containing the first `len` integers in order from 0 to `len`-1.
 /// For example, `iota(i32, 8)` will return a vector containing `.{0, 1, 2, 3, 4, 5, 6, 7}`.
-pub fn iota(comptime T: type, comptime len: usize) @Vector(len, T) {
-    var out: [len]T = undefined;
-    for (out) |*element, i| {
-        element.* = switch (@typeInfo(T)) {
-            .Int => @intCast(T, i),
-            .Float => @intToFloat(T, i),
-            else => @compileError("Can't use type " ++ @typeName(T) ++ " in iota."),
-        };
+pub inline fn iota(comptime T: type, comptime len: usize) @Vector(len, T) {
+    comptime {
+        var out: [len]T = undefined;
+        for (out) |*element, i| {
+            element.* = switch (@typeInfo(T)) {
+                .Int => @intCast(T, i),
+                .Float => @intToFloat(T, i),
+                else => @compileError("Can't use type " ++ @typeName(T) ++ " in iota."),
+            };
+        }
+        return @as(@Vector(len, T), out);
     }
-    return @as(@Vector(len, T), out);
 }
 
 /// Returns a vector containing the same elements as the input, but repeated until the desired length is reached.