Allow for-loop to iterate over bytes (#3317)

crates/typst/src/eval/flow.rs

@@ -152,7 +152,11 @@ impl Eval for ast::ForLoop<'_> {
                 // Iterate over graphemes of string.
                 iter!(for pattern in str.as_str().graphemes(true));
             }
-            (Pattern::Destructuring(_), Value::Str(_)) => {
+            (Pattern::Normal(_) | Pattern::Placeholder(_), Value::Bytes(bytes)) => {
+                // Iterate over the integers of bytes.
+                iter!(for pattern in bytes.as_slice());
+            }
+            (Pattern::Destructuring(_), Value::Str(_) | Value::Bytes(_)) => {
                 bail!(pattern.span(), "cannot destructure values of {}", iterable_type);
             }
             _ => {

crates/typst/src/foundations/bytes.rs

@@ -13,12 +13,13 @@ use crate::foundations::{cast, func, scope, ty, Array, Reflect, Repr, Str, Value
 /// A sequence of bytes.
 ///
 /// This is conceptually similar to an array of [integers]($int) between `{0}`
-/// and `{255}`, but represented much more efficiently.
+/// and `{255}`, but represented much more efficiently. You can iterate over it
+/// using a [for loop]($scripting/#loops).
 ///
 /// You can convert
 /// - a [string]($str) or an [array]($array) of integers to bytes with the
 ///   [`bytes`]($bytes) constructor
-/// - bytes to a string with the [`str`]($str) constructor
+/// - bytes to a string with the [`str`]($str) constructor, with UTF-8 encoding
 /// - bytes to an array of integers with the [`array`]($array) constructor
 ///
 /// When [reading]($read) data from a file, you can decide whether to load it

docs/reference/scripting.md

@@ -188,13 +188,6 @@ together into one larger array.
 
 For loops can iterate over a variety of collections:
 
-- `{for letter in "abc" {..}}` \
-  Iterates over the characters of the [string]($str).
-  (Technically, iterates over the grapheme clusters of the string. Most of the
-  time, a grapheme cluster is just a single character/codepoint. However, some
-  constructs like flag emojis that consist of multiple codepoints are still only
-  one cluster.)
-
 - `{for value in array {..}}` \
   Iterates over the items in the [array]($array). The destructuring syntax
   described in [Let binding]($scripting/#bindings) can also be used here.
@@ -203,6 +196,17 @@ For loops can iterate over a variety of collections:
   Iterates over the key-value pairs of the [dictionary]($dictionary).
   The pairs can also be destructured by using `{for (key, value) in dict {..}}`.
 
+- `{for letter in "abc" {..}}` \
+  Iterates over the characters of the [string]($str). Technically, it iterates
+  over the grapheme clusters of the string. Most of the time, a grapheme cluster
+  is just a single codepoint. However, a grapheme cluster could contain multiple
+  codepoints, like a flag emoji.
+
+- `{for byte in bytes("😀") {..}}` \
+  Iterates over the [bytes]($bytes), which can be converted from a [string]($str)
+  or [read]($read) from a file without encoding. Each byte value is an
+  [integer]($int) between `{0}` and `{255}`.
+
 To control the execution of the loop, Typst provides the `{break}` and
 `{continue}` statements. The former performs an early exit from the loop while
 the latter skips ahead to the next iteration of the loop.

tests/typ/bugs/3275-loop-errors.typ

@@ -6,17 +6,19 @@
 #for x in (1, 2) {}
 #for x in (a: 1, b: 2) {}
 #for x in "foo" {}
+#for x in bytes("😊") {}
 
 ---
 // Placeholder.
 #for _ in (1, 2) {}
 #for _ in (a: 1, b: 2) {}
 #for _ in "foo" {}
+#for _ in bytes("😊") {}
 
 ---
 // Destructuring.
-#for (k, v)  in (("a", 1), ("b", 2), ("c", 3)) {}
-#for (k, ..) in (("a", 1), ("b", 2), ("c", 3)) {}
+#for (a,b,c) in (("a", 1, bytes(())), ("b", 2, bytes(""))) {}
+#for (a, ..) in (("a", 1, bytes(())), ("b", 2, bytes(""))) {}
 #for (k, v)  in (a: 1, b: 2, c: 3) {}
 #for (.., v) in (a: 1, b: 2, c: 3) {}
 
@@ -25,8 +27,8 @@
 #for x in [1, 2] {}
 
 ---
-// Error: 11-25 cannot loop over bytes
-#for _ in bytes((22, 0)) {}
+// Error: 11-25 cannot loop over arguments
+#for _ in arguments("a") {}
 
 ---
 // Error: 16-21 cannot loop over integer
@@ -44,6 +46,22 @@
 // Error: 6-12 cannot destructure string
 #for (x, y) in ("foo", "bar") {}
 
+---
+// Error: 6-12 cannot destructure values of bytes
+#for (x, y) in bytes("😊") {}
+
+---
+// Error: 6-12 cannot destructure bytes
+#for (x, y) in (bytes((1,2)), bytes((1,2))) {}
+
 ---
 // Error: 6-12 cannot destructure integer
 #for (x, y) in (1, 2) {}
+
+---
+// Error: 10-11 not enough elements to destructure
+#for (x, y) in ((1,), (2,)) {}
+
+---
+// Error: 6-12 too many elements to destructure
+#for (x, y) in ((1,2,3), (4,5,6)) {}