forbid underscore as identifier closes #513 (#837)

7 files changed, 114 insertions, 14 deletions

diff --git a/src/eval/func.rs b/src/eval/func.rs
index faf2d696..29b85f7a 100644
--- a/src/eval/func.rs
+++ b/src/eval/func.rs
@@ -274,6 +274,8 @@ pub enum Param {
     Named(Ident, Value),
     /// An argument sink: `..args`.
     Sink(Option<Ident>),
+    /// A placeholder: `_`.
+    Placeholder,
 }
 
 impl Closure {
@@ -334,6 +336,9 @@ impl Closure {
                         args.named::<Value>(ident)?.unwrap_or_else(|| default.clone());
                     vm.define(ident.clone(), value);
                 }
+                Param::Placeholder => {
+                    args.eat::<Value>()?;
+                }
             }
         }
 
diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index 797e1fef..0a963faf 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -1160,6 +1160,7 @@ impl Eval for ast::Closure {
                     params.push(Param::Named(named.name(), named.expr().eval(vm)?));
                 }
                 ast::Param::Sink(name) => params.push(Param::Sink(name)),
+                ast::Param::Placeholder => params.push(Param::Placeholder),
             }
         }
 
@@ -1184,6 +1185,7 @@ impl ast::Pattern {
                 vm.define(ident.clone(), value);
                 Ok(Value::None)
             }
+            ast::Pattern::Placeholder => Ok(Value::None),
             ast::Pattern::Destructuring(destruct) => {
                 match value {
                     Value::Array(value) => {
@@ -1215,6 +1217,7 @@ impl ast::Pattern {
                                         "cannot destructure named elements from an array"
                                     )
                                 }
+                                ast::DestructuringKind::Placeholder => i += 1,
                             }
                         }
                         if i < value.len() {
@@ -1243,6 +1246,7 @@ impl ast::Pattern {
                                     vm.define(ident.clone(), v.clone());
                                     used.insert(key.clone().take());
                                 }
+                                ast::DestructuringKind::Placeholder => {}
                             }
                         }
 
diff --git a/src/syntax/ast.rs b/src/syntax/ast.rs
index caf5319f..f22508e8 100644
--- a/src/syntax/ast.rs
+++ b/src/syntax/ast.rs
@@ -1568,6 +1568,8 @@ pub enum Param {
     Named(Named),
     /// An argument sink: `..args`.
     Sink(Option<Ident>),
+    /// A placeholder: `_`.
+    Placeholder,
 }
 
 impl AstNode for Param {
@@ -1576,6 +1578,7 @@ impl AstNode for Param {
             SyntaxKind::Ident => node.cast().map(Self::Pos),
             SyntaxKind::Named => node.cast().map(Self::Named),
             SyntaxKind::Spread => Some(Self::Sink(node.cast_first_match())),
+            SyntaxKind::Underscore => Some(Self::Placeholder),
             _ => Option::None,
         }
     }
@@ -1585,6 +1588,7 @@ impl AstNode for Param {
             Self::Pos(v) => v.as_untyped(),
             Self::Named(v) => v.as_untyped(),
             Self::Sink(_) => self.as_untyped(),
+            Self::Placeholder => self.as_untyped(),
         }
     }
 }
@@ -1603,6 +1607,8 @@ pub enum DestructuringKind {
     Sink(Option<Ident>),
     /// Named arguments: `x: 1`.
     Named(Ident, Ident),
+    /// A placeholder: `_`.
+    Placeholder,
 }
 
 impl Destructuring {
@@ -1619,6 +1625,7 @@ impl Destructuring {
                 let ident = filtered.next().unwrap_or_default();
                 Some(DestructuringKind::Named(key, ident))
             }
+            SyntaxKind::Underscore => Some(DestructuringKind::Placeholder),
             _ => Option::None,
         })
     }
@@ -1629,6 +1636,7 @@ impl Destructuring {
             DestructuringKind::Ident(ident) => Some(ident),
             DestructuringKind::Sink(ident) => ident,
             DestructuringKind::Named(_, ident) => Some(ident),
+            DestructuringKind::Placeholder => Option::None,
         })
     }
 }
@@ -1638,6 +1646,8 @@ impl Destructuring {
 pub enum Pattern {
     /// A single identifier: `x`.
     Ident(Ident),
+    /// A placeholder: `_`.
+    Placeholder,
     /// A destructuring pattern: `(x, _, ..y)`.
     Destructuring(Destructuring),
 }
@@ -1647,6 +1657,7 @@ impl AstNode for Pattern {
         match node.kind() {
             SyntaxKind::Ident => node.cast().map(Self::Ident),
             SyntaxKind::Destructuring => node.cast().map(Self::Destructuring),
+            SyntaxKind::Underscore => Some(Self::Placeholder),
             _ => Option::None,
         }
     }
@@ -1655,6 +1666,7 @@ impl AstNode for Pattern {
         match self {
             Self::Ident(v) => v.as_untyped(),
             Self::Destructuring(v) => v.as_untyped(),
+            Self::Placeholder => self.as_untyped(),
         }
     }
 }
@@ -1665,6 +1677,7 @@ impl Pattern {
         match self {
             Pattern::Ident(ident) => vec![ident.clone()],
             Pattern::Destructuring(destruct) => destruct.idents().collect(),
+            Pattern::Placeholder => vec![],
         }
     }
 }
@@ -1722,9 +1735,7 @@ impl LetBinding {
             LetBindingKind::Normal(Pattern::Ident(_)) => {
                 self.0.children().filter_map(SyntaxNode::cast).nth(1)
             }
-            LetBindingKind::Normal(Pattern::Destructuring(_)) => {
-                self.0.cast_first_match()
-            }
+            LetBindingKind::Normal(_) => self.0.cast_first_match(),
             LetBindingKind::Closure(_) => self.0.cast_first_match(),
         }
     }
diff --git a/src/syntax/lexer.rs b/src/syntax/lexer.rs
index dcd2509a..dc75a902 100644
--- a/src/syntax/lexer.rs
+++ b/src/syntax/lexer.rs
@@ -527,7 +527,11 @@ impl Lexer<'_> {
             }
         }
 
-        SyntaxKind::Ident
+        if ident == "_" {
+            SyntaxKind::Underscore
+        } else {
+            SyntaxKind::Ident
+        }
     }
 
     fn number(&mut self, mut start: usize, c: char) -> SyntaxKind {
diff --git a/src/syntax/parser.rs b/src/syntax/parser.rs
index 801fdfc8..389cf026 100644
--- a/src/syntax/parser.rs
+++ b/src/syntax/parser.rs
@@ -636,6 +636,17 @@ fn code_primary(p: &mut Parser, atomic: bool) {
                 p.wrap(m, SyntaxKind::Closure);
             }
         }
+        SyntaxKind::Underscore if !atomic => {
+            p.eat();
+            if p.at(SyntaxKind::Arrow) {
+                p.wrap(m, SyntaxKind::Params);
+                p.eat();
+                code_expr(p);
+                p.wrap(m, SyntaxKind::Closure);
+            } else if let Some(underscore) = p.node_mut(m) {
+                underscore.convert_to_error("expected expression, found underscore");
+            }
+        }
 
         SyntaxKind::LeftBrace => code_block(p),
         SyntaxKind::LeftBracket => content_block(p),
@@ -718,6 +729,7 @@ fn with_paren(p: &mut Parser) {
     match kind {
         SyntaxKind::Array => validate_array(p, m),
         SyntaxKind::Dict => validate_dict(p, m),
+        SyntaxKind::Parenthesized => validate_parenthesized(p, m),
         _ => {}
     }
     p.wrap(m, kind);
@@ -787,13 +799,19 @@ fn item(p: &mut Parser, keyed: bool) -> SyntaxKind {
         return SyntaxKind::Spread;
     }
 
-    code_expr(p);
+    if !p.eat_if(SyntaxKind::Underscore) {
+        code_expr(p);
+    } else {
+        return SyntaxKind::Underscore;
+    }
 
     if !p.eat_if(SyntaxKind::Colon) {
         return SyntaxKind::Int;
     }
 
-    code_expr(p);
+    if !p.eat_if(SyntaxKind::Underscore) {
+        code_expr(p);
+    }
 
     let kind = match p.node(m).map(SyntaxNode::kind) {
         Some(SyntaxKind::Ident) => SyntaxKind::Named,
@@ -840,12 +858,12 @@ fn args(p: &mut Parser) {
 
 enum PatternKind {
     Ident,
+    Placeholder,
     Destructuring,
 }
 
 fn pattern(p: &mut Parser) -> PatternKind {
     let m = p.marker();
-
     if p.at(SyntaxKind::LeftParen) {
         let kind = collection(p, false);
         validate_destruct_pattern(p, m);
@@ -856,6 +874,8 @@ fn pattern(p: &mut Parser) -> PatternKind {
             p.wrap(m, SyntaxKind::Destructuring);
             PatternKind::Destructuring
         }
+    } else if p.eat_if(SyntaxKind::Underscore) {
+        PatternKind::Placeholder
     } else {
         p.expect(SyntaxKind::Ident);
         PatternKind::Ident
@@ -879,6 +899,7 @@ fn let_binding(p: &mut Parser) {
                 p.wrap(m3, SyntaxKind::Params);
             }
         }
+        PatternKind::Placeholder => {}
         PatternKind::Destructuring => destructuring = true,
     }
 
@@ -961,7 +982,9 @@ fn for_loop(p: &mut Parser) {
     pattern(p);
     if p.at(SyntaxKind::Comma) {
         p.expected("keyword `in`. did you mean to use a destructuring pattern?");
-        p.eat_if(SyntaxKind::Ident);
+        if !p.eat_if(SyntaxKind::Ident) {
+            p.eat_if(SyntaxKind::Underscore);
+        }
         p.eat_if(SyntaxKind::In);
     } else {
         p.expect(SyntaxKind::In);
@@ -1023,10 +1046,25 @@ fn return_stmt(p: &mut Parser) {
     p.wrap(m, SyntaxKind::FuncReturn);
 }
 
+fn validate_parenthesized(p: &mut Parser, m: Marker) {
+    for child in p.post_process(m) {
+        let kind = child.kind();
+        if kind == SyntaxKind::Underscore {
+            child.convert_to_error(eco_format!(
+                "expected expression, found {}",
+                kind.name()
+            ));
+        }
+    }
+}
+
 fn validate_array(p: &mut Parser, m: Marker) {
     for child in p.post_process(m) {
         let kind = child.kind();
-        if kind == SyntaxKind::Named || kind == SyntaxKind::Keyed {
+        if kind == SyntaxKind::Named
+            || kind == SyntaxKind::Keyed
+            || kind == SyntaxKind::Underscore
+        {
             child.convert_to_error(eco_format!(
                 "expected expression, found {}",
                 kind.name()
@@ -1114,7 +1152,10 @@ fn validate_params(p: &mut Parser, m: Marker) {
                     child.make_erroneous();
                 }
             }
-            SyntaxKind::LeftParen | SyntaxKind::RightParen | SyntaxKind::Comma => {}
+            SyntaxKind::LeftParen
+            | SyntaxKind::RightParen
+            | SyntaxKind::Comma
+            | SyntaxKind::Underscore => {}
             kind => {
                 child.convert_to_error(eco_format!(
                     "expected identifier, named pair or argument sink, found {}",
@@ -1137,6 +1178,8 @@ fn validate_args(p: &mut Parser, m: Marker) {
                 ));
                 child.make_erroneous();
             }
+        } else if child.kind() == SyntaxKind::Underscore {
+            child.convert_to_error("unexpected underscore");
         }
     }
 }
@@ -1147,7 +1190,7 @@ fn validate_destruct_pattern(p: &mut Parser, m: Marker) {
     for child in p.post_process(m) {
         match child.kind() {
             SyntaxKind::Ident => {
-                if child.text() != "_" && !used.insert(child.text().clone()) {
+                if !used.insert(child.text().clone()) {
                     child.convert_to_error(
                         "at most one binding per identifier is allowed",
                     );
@@ -1172,7 +1215,7 @@ fn validate_destruct_pattern(p: &mut Parser, m: Marker) {
                     continue;
                 }
 
-                if within.text() != "_" && !used.insert(within.text().clone()) {
+                if !used.insert(within.text().clone()) {
                     within.convert_to_error(
                         "at most one binding per identifier is allowed",
                     );
@@ -1189,7 +1232,9 @@ fn validate_destruct_pattern(p: &mut Parser, m: Marker) {
                 }
 
                 let Some(within) = child.children_mut().last_mut() else { return };
-                if within.kind() != SyntaxKind::Ident {
+                if within.kind() != SyntaxKind::Ident
+                    && within.kind() != SyntaxKind::Underscore
+                {
                     within.convert_to_error(eco_format!(
                         "expected identifier, found {}",
                         within.kind().name(),
@@ -1197,7 +1242,10 @@ fn validate_destruct_pattern(p: &mut Parser, m: Marker) {
                     child.make_erroneous();
                 }
             }
-            SyntaxKind::LeftParen | SyntaxKind::RightParen | SyntaxKind::Comma => {}
+            SyntaxKind::LeftParen
+            | SyntaxKind::RightParen
+            | SyntaxKind::Comma
+            | SyntaxKind::Underscore => {}
             kind => {
                 child.convert_to_error(eco_format!(
                     "expected identifier or destructuring sink, found {}",
@@ -1313,6 +1361,10 @@ impl<'s> Parser<'s> {
         self.nodes.get(m.0)
     }
 
+    fn node_mut(&mut self, m: Marker) -> Option<&mut SyntaxNode> {
+        self.nodes.get_mut(m.0)
+    }
+
     fn post_process(&mut self, m: Marker) -> impl Iterator<Item = &mut SyntaxNode> {
         self.nodes[m.0..]
             .iter_mut()
diff --git a/tests/typ/compiler/closure.typ b/tests/typ/compiler/closure.typ
index 915de850..2d12a2bc 100644
--- a/tests/typ/compiler/closure.typ
+++ b/tests/typ/compiler/closure.typ
@@ -170,3 +170,7 @@
 ---
 // Error: 10-14 expected identifier, found `none`
 #let foo(none: b) = key
+
+---
+// Error: 11 expected comma
+#let foo(_: 3) = none
diff --git a/tests/typ/compiler/let.typ b/tests/typ/compiler/let.typ
index 70657617..160d2147 100644
--- a/tests/typ/compiler/let.typ
+++ b/tests/typ/compiler/let.typ
@@ -222,6 +222,26 @@ Three
 #let (..true) = false
 
 ---
+#let _ = 4
+
+#for _ in range(2) []
+
+// Error: 2-3 unexpected underscore
+#_
+
+// Error: 8-9 unexpected underscore
+#lorem(_)
+
+// Error: 3-4 expected expression, found underscore
+#(_,)
+
+// Error: 3-4 expected expression, found underscore
+#{_}
+
+// Error: 8-9 expected expression, found underscore
+#{ 1 + _ }
+
+---
 // Error: 13 expected equals sign
 #let func(x)