Improve hint when provided array for destructuring has fewer elements than expected (#5139)

Co-authored-by: Laurenz <laurmaedje@gmail.com>

crates/typst/src/eval/binding.rs

@@ -1,6 +1,8 @@
 use std::collections::HashSet;
 
-use crate::diag::{bail, At, SourceResult};
+use ecow::eco_format;
+
+use crate::diag::{bail, error, At, SourceDiagnostic, SourceResult};
 use crate::eval::{Access, Eval, Vm};
 use crate::foundations::{Array, Dict, Value};
 use crate::syntax::ast::{self, AstNode};
@@ -96,10 +98,7 @@ where
         match p {
             ast::DestructuringItem::Pattern(pattern) => {
                 let Ok(v) = value.at(i as i64, None) else {
-                    bail!(
+                    bail!(wrong_number_of_elements(destruct, len));
-                        pattern.span(), "not enough elements to destructure";
-                        hint: "the provided array has a length of {len}",
-                    );
                 };
                 destructure_impl(vm, pattern, v, f)?;
                 i += 1;
@@ -108,10 +107,7 @@ where
                 let sink_size = (1 + len).checked_sub(destruct.items().count());
                 let sink = sink_size.and_then(|s| value.as_slice().get(i..i + s));
                 let (Some(sink_size), Some(sink)) = (sink_size, sink) else {
-                    bail!(
+                    bail!(wrong_number_of_elements(destruct, len));
-                        spread.span(), "not enough elements to destructure";
-                        hint: "the provided array has a length of {len}",
-                    );
                 };
                 if let Some(expr) = spread.sink_expr() {
                     f(vm, expr, Value::Array(sink.into()))?;
@@ -125,22 +121,7 @@ where
     }
 
     if i < len {
-        if i == 0 {
+        bail!(wrong_number_of_elements(destruct, len));
-            bail!(
-                destruct.span(), "too many elements to destructure";
-                hint: "the provided array has a length of {len}, but the pattern expects an empty array",
-            )
-        } else if i == 1 {
-            bail!(
-                destruct.span(), "too many elements to destructure";
-                hint: "the provided array has a length of {len}, but the pattern expects a single element",
-            );
-        } else {
-            bail!(
-                destruct.span(), "too many elements to destructure";
-                hint: "the provided array has a length of {len}, but the pattern expects {i} elements",
-            );
-        }
     }
 
     Ok(())
@@ -193,3 +174,36 @@ where
 
     Ok(())
 }
+
+/// The error message when the number of elements of the destructuring and the
+/// array is mismatched.
+#[cold]
+fn wrong_number_of_elements(
+    destruct: ast::Destructuring,
+    len: usize,
+) -> SourceDiagnostic {
+    let mut count = 0;
+    let mut spread = false;
+
+    for p in destruct.items() {
+        match p {
+            ast::DestructuringItem::Pattern(_) => count += 1,
+            ast::DestructuringItem::Spread(_) => spread = true,
+            ast::DestructuringItem::Named(_) => {}
+        }
+    }
+
+    let quantifier = if len > count { "too many" } else { "not enough" };
+    let expected = match (spread, count) {
+        (true, c) => eco_format!("at least {c} elements"),
+        (false, 0) => "an empty array".into(),
+        (false, 1) => "a single element".into(),
+        (false, c) => eco_format!("{c} elements",),
+    };
+
+    error!(
+        destruct.span(), "{quantifier} elements to destructure";
+        hint: "the provided array has a length of {len}, \
+               but the pattern expects {expected}",
+    )
+}

tests/suite/scripting/destructuring.typ

@@ -128,13 +128,13 @@
 #let () = (1, 2)
 
 --- destructuring-let-array-too-few-elements ---
-// Error: 13-14 not enough elements to destructure
+// Error: 6-15 not enough elements to destructure
-// Hint: 13-14 the provided array has a length of 2
+// Hint: 6-15 the provided array has a length of 2, but the pattern expects 3 elements
 #let (a, b, c) = (1, 2)
 
 --- destructuring-let-array-too-few-elements-with-sink ---
-// Error: 7-10 not enough elements to destructure
+// Error: 6-20 not enough elements to destructure
-// Hint: 7-10 the provided array has a length of 2
+// Hint: 6-20 the provided array has a length of 2, but the pattern expects at least 3 elements
 #let (..a, b, c, d) = (1, 2)
 
 --- destructuring-let-array-bool-invalid ---
@@ -192,8 +192,8 @@
 
 --- destructuring-let-array-trailing-placeholders ---
 // Trailing placeholders.
-// Error: 10-11 not enough elements to destructure
+// Error: 6-21 not enough elements to destructure
-// Hint: 10-11 the provided array has a length of 1
+// Hint: 6-21 the provided array has a length of 1, but the pattern expects 5 elements
 #let (a, _, _, _, _) = (1,)
 #test(a, 1)
 
@@ -360,8 +360,8 @@
 #for (x, y) in (1, 2) {}
 
 --- issue-3275-destructuring-loop-over-2d-array-1 ---
-// Error: 10-11 not enough elements to destructure
+// Error: 6-12 not enough elements to destructure
-// Hint: 10-11 the provided array has a length of 1
+// Hint: 6-12 the provided array has a length of 1, but the pattern expects 2 elements
 #for (x, y) in ((1,), (2,)) {}
 
 --- issue-3275-destructuring-loop-over-2d-array-2 ---