doughlang: Expressions in patterns

ast:
- Document operators
- Parameterize Pat with Annotation
- Consolidate Path and Lit into "Value" (Expr)
- Consolidate NamedRecord/Namedtuple into PatOp::TypePrefixed
- Allow Pat<Pat,*> patterns
- Additional helper functions on Expr and Pat

lexer:
- Support inner-doc comment syntax `//!`
  - Cleans up `///` or `//!` prefix

parser:
- Make Parse::Prec `Default`
- Allow expression elision after `..`/`..=`
- Fix Parser::consume not updating elide_do state
- Add token splitting, to make `&&Expr` and `&&Pat` easier
- error: Embed Pat precedence in ParseError

examples/weight.rs

@@ -0,0 +1,140 @@
+//! Demonstrates the visitor pattern
+
+use std::{convert::Infallible, error::Error};
+
+use doughlang::{
+    ast::{
+        visit::{Visit, Walk},
+        *,
+    },
+    lexer::Lexer,
+    parser::{Parser, expr::Prec},
+};
+
+const CODE: &str = r#"
+let x: [i32; 4] = [1, 2, 3, 4];
+
+use foo::bar::baz::qux::{a, b, c, d, e};
+
+struct Point<T> {x: T, y: T}
+
+let v = Point {
+    x, y: 20 + x
+}
+
+fn main() -> (&str, bool, i128) {
+    // An if expression is like the ternary conditional operator in C
+    let y = if 10 < 50 {
+        "\u{1f988}"
+    } else {
+        "x"
+    }
+    
+    // A `while` expression is like the while-else construct in Python,
+    // but it returns a value via the `break` keyword
+    let z = while false {
+        // do a thing repeatedly
+        break true
+    } else {
+        // If `while` does not `break`, fall through to the `else` expression
+        false
+    }
+
+    // The same is true of `for` expressions!
+    let w = for idx in 0..100 {
+        if idx > 2 * 2 {
+            break idx
+        }
+    } else 12345; // semicolon operator required here
+
+
+    // desugars to
+    let w = match ((0..100).into_iter()) {
+        __iter => loop match (__iter.next()) {
+            None => break 12345,
+            Some (idx) => {
+                if (idx > (2 * 2)) {
+                    break idx
+                }
+            },
+        },
+    };
+
+    // A block evaluates to its last expression,
+    // or Empty if there is none
+    // (🦈, false, 5)
+    (y, z, w)
+}
+
+"#;
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let ast: Anno<Expr> = Parser::new(Lexer::new(CODE)).parse(Prec::MIN)?;
+    println!("{ast}");
+    let mut counters = Weight::new();
+    counters.visit(&ast);
+    println!("{counters:#?}");
+    println!("{}", CODE.len());
+
+    Ok(())
+}
+
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
+struct Weight {
+    size: usize,
+}
+
+impl Weight {
+    fn new() -> Self {
+        Self::default()
+    }
+}
+
+impl<'a> Visit<'a> for Weight {
+    type Error = Infallible;
+
+    fn visit_expr<A: Annotation>(&mut self, item: &'a Expr<A>) -> Result<(), Self::Error> {
+        self.size += size_of_val(item);
+        item.children(self)
+    }
+
+    fn visit_ident(&mut self, item: &'a str) -> Result<(), Self::Error> {
+        self.size += size_of_val(item);
+        item.children(self)
+    }
+
+    fn visit_path(&mut self, item: &'a Path) -> Result<(), Self::Error> {
+        self.size += size_of_val(item) + size_of_val(item.parts.as_slice());
+        item.children(self)
+    }
+
+    fn visit_literal(&mut self, item: &'a Literal) -> Result<(), Self::Error> {
+        self.size += size_of_val(item);
+        item.children(self)
+    }
+
+    fn visit_use(&mut self, item: &'a Use) -> Result<(), Self::Error> {
+        self.size += size_of_val(item);
+        item.children(self)
+    }
+
+    fn visit_pat<A: Annotation>(&mut self, item: &'a Pat<A>) -> Result<(), Self::Error> {
+        self.size += size_of_val(item);
+        item.children(self)
+    }
+
+    fn visit_bind<A: Annotation>(&mut self, item: &'a Bind<A>) -> Result<(), Self::Error> {
+        self.size += size_of_val(item);
+        item.children(self)
+    }
+
+    fn visit_make<A: Annotation>(&mut self, item: &'a Make<A>) -> Result<(), Self::Error> {
+        self.size += size_of_val(item);
+        item.children(self)
+    }
+
+    fn visit_makearm<A: Annotation>(&mut self, item: &'a MakeArm<A>) -> Result<(), Self::Error> {
+        self.size += size_of_val(item);
+        item.children(self)
+    }
+}