Let for-loop iterate Dict with an iterator (#3318)

crates/typst/src/eval/flow.rs

@@ -145,8 +145,8 @@ impl Eval for ast::ForLoop<'_> {
                 iter!(for pattern in array);
             }
             (_, Value::Dict(dict)) => {
-                // Iterate over pairs of dict.
-                iter!(for pattern in dict.pairs());
+                // Iterate over key-value pairs of dict.
+                iter!(for pattern in dict.iter());
             }
             (Pattern::Normal(_) | Pattern::Placeholder(_), Value::Str(str)) => {
                 // Iterate over graphemes of string.

crates/typst/src/foundations/cast.rs

@@ -9,7 +9,7 @@ use smallvec::SmallVec;
 use unicode_math_class::MathClass;
 
 use crate::diag::{At, SourceResult, StrResult};
-use crate::foundations::{repr, NativeElement, Packed, Repr, Type, Value};
+use crate::foundations::{array, repr, NativeElement, Packed, Repr, Str, Type, Value};
 use crate::syntax::{Span, Spanned};
 
 #[rustfmt::skip]
@@ -182,6 +182,12 @@ impl IntoValue for Value {
     }
 }
 
+impl IntoValue for (&Str, &Value) {
+    fn into_value(self) -> Value {
+        Value::Array(array![self.0.clone(), self.1.clone()])
+    }
+}
+
 impl<T: IntoValue + Clone> IntoValue for Cow<'_, T> {
     fn into_value(self) -> Value {
         self.into_owned().into_value()

docs/reference/scripting.md

@@ -195,6 +195,8 @@ For loops can iterate over a variety of collections:
 - `{for pair in dict {..}}` \
   Iterates over the key-value pairs of the [dictionary]($dictionary).
   The pairs can also be destructured by using `{for (key, value) in dict {..}}`.
+  It is more efficient than `{for pair in dict.pairs() {..}}` because it doesn't
+  create a temporary array of all key-value pairs.
 
 - `{for letter in "abc" {..}}` \
   Iterates over the characters of the [string]($str). Technically, it iterates