Better error messages for keywords in place of identifiers

Fixes #1123

crates/typst/src/syntax/kind.rs

@@ -304,6 +304,32 @@ impl SyntaxKind {
         )
     }
 
+    /// Is this node is a keyword.
+    pub fn is_keyword(self) -> bool {
+        matches!(
+            self,
+            Self::Not
+                | Self::And
+                | Self::Or
+                | Self::None
+                | Self::Auto
+                | Self::Let
+                | Self::Set
+                | Self::Show
+                | Self::If
+                | Self::Else
+                | Self::For
+                | Self::In
+                | Self::While
+                | Self::Break
+                | Self::Continue
+                | Self::Return
+                | Self::Import
+                | Self::Include
+                | Self::As
+        )
+    }
+
     /// Whether this kind of node is automatically skipped by the parser in
     /// code and math mode.
     pub fn is_trivia(self) -> bool {

crates/typst/src/syntax/parser.rs

@@ -1583,11 +1583,16 @@ impl<'s> Parser<'s> {
             self.current = SyntaxKind::Eof;
         }
     }
+}
 
+impl<'s> Parser<'s> {
+    /// Consume the given syntax `kind` or produce an error.
     fn expect(&mut self, kind: SyntaxKind) -> bool {
         let at = self.at(kind);
         if at {
             self.eat();
+        } else if kind == SyntaxKind::Ident && self.current.is_keyword() {
+            self.expected_found(kind.name(), self.current.name());
         } else {
             self.balanced &= !kind.is_grouping();
             self.expected(kind.name());
@@ -1595,26 +1600,75 @@ impl<'s> Parser<'s> {
         at
     }
 
+    /// Produce an error that the given `thing` was expected.
+    fn expected(&mut self, thing: &str) {
+        self.unskip();
+        if !self.after_error() {
+            let message = eco_format!("expected {thing}");
+            self.nodes.push(SyntaxNode::error(message, ""));
+        }
+        self.skip();
+    }
+
+    /// Produce an error that the given `thing` was expected but another
+    /// thing was `found` and consumethe next token.
+    fn expected_found(&mut self, thing: &str, found: &str) {
+        self.unskip();
+        if !self.after_error() {
+            self.skip();
+            self.convert_to_error(eco_format!("expected {thing}, found {found}"));
+        }
+        self.skip();
+    }
+
+    /// Produce an error that the given `thing` was expected at the position
+    /// of the marker `m`.
+    fn expected_at(&mut self, m: Marker, thing: &str) {
+        let message = eco_format!("expected {}", thing);
+        let error = SyntaxNode::error(message, "");
+        self.nodes.insert(m.0, error);
+    }
+
+    /// Produce an error for the unclosed delimiter `kind` at the position
+    /// `open`.
     fn expect_closing_delimiter(&mut self, open: Marker, kind: SyntaxKind) {
         if !self.eat_if(kind) {
             self.nodes[open.0].convert_to_error("unclosed delimiter");
         }
     }
 
-    fn expected(&mut self, thing: &str) {
+    /// Consume the next token and produce an error stating that it was
+    /// unexpected.
+    fn unexpected(&mut self) {
         self.unskip();
-        if self
+        while self
             .nodes
             .last()
-            .map_or(true, |child| child.kind() != SyntaxKind::Error)
+            .map_or(false, |child| child.kind().is_error() && child.is_empty())
         {
-            let message = eco_format!("expected {}", thing);
-            self.nodes.push(SyntaxNode::error(message, ""));
+            self.nodes.pop();
         }
         self.skip();
+        self.convert_to_error(eco_format!("unexpected {}", self.current.name()));
     }
 
-    // Adds a hint to the last node, if the last node is an error.
+    /// Whether the last node is an error.
+    fn after_error(&self) -> bool {
+        self.nodes.last().map_or(false, |child| child.kind().is_error())
+    }
+
+    /// Consume the next token and turn it into an error.
+    fn convert_to_error(&mut self, message: EcoString) {
+        let kind = self.current;
+        let offset = self.nodes.len();
+        self.eat();
+        self.balanced &= !kind.is_grouping();
+        if !kind.is_error() {
+            self.nodes[offset].convert_to_error(message);
+        }
+    }
+
+    /// Adds a hint to the last node, if the last node is an error.
     fn hint(&mut self, hint: impl Into<EcoString>) {
         self.unskip();
         if let Some(last) = self.nodes.last_mut() {
@@ -1622,32 +1676,4 @@ impl<'s> Parser<'s> {
         }
         self.skip();
     }
-
-    fn expected_at(&mut self, m: Marker, thing: &str) {
-        let message = eco_format!("expected {}", thing);
-        let error = SyntaxNode::error(message, "");
-        self.nodes.insert(m.0, error);
-    }
-
-    fn unexpected(&mut self) {
-        self.unskip();
-        while self
-            .nodes
-            .last()
-            .map_or(false, |child| child.kind() == SyntaxKind::Error && child.is_empty())
-        {
-            self.nodes.pop();
-        }
-        self.skip();
-
-        let kind = self.current;
-        let offset = self.nodes.len();
-        self.eat();
-        self.balanced &= !kind.is_grouping();
-
-        if !kind.is_error() {
-            self.nodes[offset]
-                .convert_to_error(eco_format!("unexpected {}", kind.name()));
-        }
-    }
 }

tests/typ/compiler/embedded-expr.typ

@@ -0,0 +1,12 @@
+// Test embedded expressions.
+// Ref: false
+
+---
+// Error: 6-8 expected identifier, found keyword `as`
+#let as = 1 + 2
+
+---
+#{
+  // Error: 7-9 expected identifier, found keyword `as`
+  let as = 10
+}