only allow placeholder shorthand in monadic index macros

Author: kaikalii

parser/src/parse.rs

@@ -1998,18 +1998,26 @@ pub fn ident_modifier_args(ident: &str) -> usize {
     count
 }
 
-/// Get the maximum placeholder index in a list of words
-pub fn max_placeholder(words: &[Sp<Word>]) -> Option<usize> {
+/// Get the maximum placeholder index in a list of words,
+/// as well as the span of a placeholder shorthand
+pub fn max_placeholder(words: &[Sp<Word>]) -> Option<(usize, Option<CodeSpan>)> {
     let mut max: Option<usize> = None;
-    let mut set = |i: Option<usize>| {
-        if let Some(i) = i {
+    let mut shorthand_span = None;
+    let mut set = |is: Option<(usize, Option<CodeSpan>)>| {
+        if let Some((i, s)) = is {
             let max = max.get_or_insert(0);
             *max = (*max).max(i);
+            if shorthand_span.is_none() && s.is_some() {
+                shorthand_span = s;
+            }
         }
     };
     for word in words {
         match &word.value {
-            Word::Placeholder(i) => set(Some(i.unwrap_or(0))),
+            Word::Placeholder(i) => set(Some((
+                i.unwrap_or(0),
+                i.is_none().then(|| word.span.clone()),
+            ))),
             Word::Strand(items) => set(max_placeholder(items)),
             Word::Array(arr) => {
                 for line in arr.word_lines() {
@@ -2033,7 +2041,7 @@ pub fn max_placeholder(words: &[Sp<Word>]) -> Option<usize> {
             _ => {}
         }
     }
-    max
+    max.map(|i| (i, shorthand_span))
 }
 
 /// Trim space words

src/compile/binding.rs

@@ -183,7 +183,7 @@ impl Compiler {
             return Ok(());
         }
         // Index macro
-        match (ident_margs, max_placeholder) {
+        match (ident_margs, &max_placeholder) {
             (0, None) => {}
             (_, None) => {
                 self.add_error(
@@ -204,7 +204,7 @@ impl Compiler {
                 );
                 return Ok(());
             }
-            (n, Some(max)) => {
+            (n, Some((max, shorthand_span))) => {
                 if max + 1 > n {
                     self.emit_diagnostic(
                         format!(
@@ -216,6 +216,17 @@ impl Compiler {
                         span.clone(),
                     );
                 }
+                if let Some(span) = shorthand_span {
+                    if *max > 0 {
+                        self.add_error(
+                            span.clone(),
+                            format!(
+                                "`{name}` cannot use the placeholder shorthand \
+                                because it contains a ^{max}. Use explicit ^0 instead."
+                            ),
+                        )
+                    }
+                }
             }
         }
         if max_placeholder.is_some() || ident_margs > 0 {

tests_special/error.ua

@@ -186,4 +186,6 @@ F ← |0.1000 ⍢(F F F|1)
 
 |
 
-+⊸¤⇡1e5
++⊸¤⇡1e5
+
+F‼ ← ^^1