path: root/askama_derive/src/generator.rs

use super::{get_template_source, Context};
use input::TemplateInput;
use parser::{self, Cond, Expr, MatchParameter, MatchVariant, Node, Target, When, WS};
use shared::filters;

use proc_macro2::Span;

use quote::ToTokens;

use std::collections::{HashMap, HashSet};
use std::path::PathBuf;
use std::{cmp, hash, str};

use syn;

pub(crate) fn generate(input: &TemplateInput, contexts: &HashMap<&PathBuf, Context>) -> String {
    Generator::new(input, contexts, SetChain::new(), 0).build(&contexts[&input.path])
}

struct Generator<'a> {
    // The template input state: original struct AST and attributes
    input: &'a TemplateInput<'a>,
    // All contexts, keyed by the package-relative template path
    contexts: &'a HashMap<&'a PathBuf, Context<'a>>,
    // The buffer to generate the code into
    buf: String,
    // The current level of indentation (in spaces)
    indent: u8,
    // Whether the output buffer is currently at the start of a line
    start: bool,
    // Variables accessible directly from the current scope (not redirected to context)
    locals: SetChain<'a, &'a str>,
    // Suffix whitespace from the previous literal. Will be flushed to the
    // output buffer unless suppressed by whitespace suppression on the next
    // non-literal.
    next_ws: Option<&'a str>,
    // Whitespace suppression from the previous non-literal. Will be used to
    // determine whether to flush prefix whitespace from the next literal.
    skip_ws: bool,
    // Counter for askama-internal variable names allocated during code gen
    vars: usize,
    // If currently in a block, this will contain the name of a potential parent block
    super_block: Option<String>,
    // If the super macro is used; this determines whether code for the parent block
    // is generated or not.
    used_super: bool,
}

impl<'a> Generator<'a> {
    fn new<'n>(
        input: &'n TemplateInput,
        contexts: &'n HashMap<&'n PathBuf, Context<'n>>,
        locals: SetChain<'n, &'n str>,
        indent: u8,
    ) -> Generator<'n> {
        Generator {
            input,
            contexts,
            buf: String::new(),
            indent,
            start: true,
            locals,
            next_ws: None,
            skip_ws: false,
            vars: 0,
            super_block: None,
            used_super: false,
        }
    }

    fn child(&mut self) -> Generator {
        let locals = SetChain::with_parent(&self.locals);
        Self::new(self.input, self.contexts, locals, self.indent)
    }

    // Takes a Context and generates the relevant implementations.
    fn build(mut self, ctx: &'a Context) -> String {
        let heritage = if !ctx.blocks.is_empty() {
            if let Some(parent) = self.input.parent {
                self.deref_to_parent(parent);
            }
            let heritage = Heritage::new(ctx, self.contexts);
            self.trait_blocks(&heritage);
            Some(heritage)
        } else {
            None
        };

        self.impl_template(ctx, &heritage);
        self.impl_display();
        if cfg!(feature = "iron") {
            self.impl_modifier_response();
        }
        if cfg!(feature = "rocket") {
            self.impl_rocket_responder();
        }
        if cfg!(feature = "actix-web") {
            self.impl_actix_web_responder();
        }
        self.buf
    }

    // Implement `Template` for the given context struct.
    fn impl_template(&mut self, ctx: &'a Context, heritage: &Option<Heritage<'a>>) {
        self.write_header("::askama::Template", None);
        self.writeln(
            "fn render_into(&self, writer: &mut ::std::fmt::Write) -> \
             ::askama::Result<()> {",
        );

        if let Some(heritage) = heritage {
            self.handle(heritage.root, heritage.root.nodes, AstLevel::Top);
        } else {
            self.handle(ctx, &ctx.nodes, AstLevel::Top);
        }

        self.flush_ws(WS(false, false));
        self.writeln("Ok(())");
        self.writeln("}");
        self.writeln("}");
    }

    fn trait_blocks(&mut self, heritage: &Heritage<'a>) {
        let trait_name = format!("{}Blocks", self.input.ast.ident);
        let mut methods = vec![];

        self.write_header(&trait_name, None);
        for blocks in heritage.blocks.values() {
            for (gen, (ctx, def)) in blocks.iter().enumerate() {
                self.used_super = false;
                if let Node::BlockDef(ws1, name, nodes, ws2) = def {
                    let fname = if gen == 0 {
                        name.to_string()
                    } else {
                        format!("{}_g{}", name, gen)
                    };

                    self.writeln("#[allow(unused_variables)]");
                    self.writeln(&format!(
                        "fn render_block_{}_into(&self, writer: &mut ::std::fmt::Write) \
                         -> ::askama::Result<()> {{",
                        fname
                    ));
                    methods.push(fname);
                    self.prepare_ws(*ws1);

                    self.locals.push();
                    self.super_block = Some(format!("{}_g{}", name, gen + 1));
                    self.handle(ctx, nodes, AstLevel::Block);
                    self.super_block = None;
                    self.locals.pop();

                    self.flush_ws(*ws2);
                    self.writeln("Ok(())");
                    self.writeln("}");
                } else {
                    panic!("only block definitions allowed here");
                }

                if !self.used_super {
                    break;
                }
            }
        }
        self.writeln("}");

        self.writeln(&format!("pub trait {} {{", trait_name));
        for name in methods {
            self.writeln(&format!(
                "fn render_block_{}_into(&self, writer: &mut ::std::fmt::Write) \
                 -> ::askama::Result<()>;",
                name
            ));
        }
        self.writeln("}");
    }

    // Implement `Deref<Parent>` for an inheriting context struct.
    fn deref_to_parent(&mut self, parent_type: &syn::Type) {
        self.write_header("::std::ops::Deref", None);
        self.writeln(&format!(
            "type Target = {};",
            parent_type.into_token_stream()
        ));
        self.writeln("fn deref(&self) -> &Self::Target {");
        self.writeln("&self._parent");
        self.writeln("}");
        self.writeln("}");
    }

    // Implement `Display` for the given context struct.
    fn impl_display(&mut self) {
        self.write_header("::std::fmt::Display", None);
        self.writeln("fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {");
        self.writeln("self.render_into(f).map_err(|_| ::std::fmt::Error {})");
        self.writeln("}");
        self.writeln("}");
    }

    // Implement iron's Modifier<Response> if enabled
    fn impl_modifier_response(&mut self) {
        self.write_header("::askama::iron::Modifier<::askama::iron::Response>", None);
        self.writeln("fn modify(self, res: &mut ::askama::iron::Response) {");
        self.writeln("res.body = Some(Box::new(self.render().unwrap().into_bytes()));");

        let ext = self.input
            .path
            .extension()
            .map_or("", |s| s.to_str().unwrap_or(""));
        match ext {
            "html" | "htm" => {
                self.writeln("::askama::iron::ContentType::html().0.modify(res);");
            }
            _ => (),
        };

        self.writeln("}");
        self.writeln("}");
    }

    // Implement Rocket's `Responder`.
    fn impl_rocket_responder(&mut self) {
        let lifetime = syn::Lifetime::new("'askama", Span::call_site());
        let param = syn::GenericParam::Lifetime(syn::LifetimeDef::new(lifetime));
        self.write_header("::askama::rocket::Responder<'askama>", Some(vec![param]));
        self.writeln(
            "fn respond_to(self, _: &::askama::rocket::Request) \
             -> ::askama::rocket::Result<'askama> {",
        );

        let ext = match self.input.path.extension() {
            Some(s) => s.to_str().unwrap(),
            None => "txt",
        };
        self.writeln(&format!("::askama::rocket::respond(&self, {:?})", ext));

        self.writeln("}");
        self.writeln("}");
    }

    // Implement Actix-web's `Responder`.
    fn impl_actix_web_responder(&mut self) {
        self.write_header("::askama::actix_web::Responder", None);
        self.writeln("type Item = ::askama::actix_web::HttpResponse;");
        self.writeln("type Error = ::askama::actix_web::Error;");
        self.writeln(
            "fn respond_to<S>(self, _req: &::askama::actix_web::HttpRequest<S>) \
             -> Result<Self::Item, Self::Error> {",
        );

        let ext = match self.input.path.extension() {
            Some(s) => s.to_str().unwrap(),
            None => "txt",
        };
        self.writeln(&format!("::askama::actix_web::respond(&self, {:?})", ext));

        self.writeln("}");
        self.writeln("}");
    }

    // Writes header for the `impl` for `TraitFromPathName` or `Template`
    // for the given context struct.
    fn write_header(&mut self, target: &str, params: Option<Vec<syn::GenericParam>>) {
        let mut generics = self.input.ast.generics.clone();
        if let Some(params) = params {
            for param in params {
                generics.params.push(param);
            }
        }
        let (_, orig_ty_generics, _) = self.input.ast.generics.split_for_impl();
        let (impl_generics, _, where_clause) = generics.split_for_impl();
        self.writeln(
            format!(
                "{} {} for {}{} {{",
                quote!(impl#impl_generics),
                target,
                self.input.ast.ident,
                quote!(#orig_ty_generics #where_clause),
            ).as_ref(),
        );
    }

    /* Helper methods for handling node types */

    fn handle(&mut self, ctx: &'a Context, nodes: &'a [Node], level: AstLevel) {
        for n in nodes {
            match *n {
                Node::Lit(lws, val, rws) => {
                    self.write_lit(lws, val, rws);
                }
                Node::Comment(ws) => {
                    self.write_comment(ws);
                }
                Node::Expr(ws, ref val) => {
                    self.write_expr(ws, val);
                }
                Node::LetDecl(ws, ref var) => {
                    self.write_let_decl(ws, var);
                }
                Node::Let(ws, ref var, ref val) => {
                    self.write_let(ws, var, val);
                }
                Node::Cond(ref conds, ws) => {
                    self.write_cond(ctx, conds, ws);
                }
                Node::Match(ws1, ref expr, inter, ref arms, ws2) => {
                    self.write_match(ctx, ws1, expr, inter, arms, ws2);
                }
                Node::Loop(ws1, ref var, ref iter, ref body, ws2) => {
                    self.write_loop(ctx, ws1, var, iter, body, ws2);
                }
                Node::BlockDef(ws1, name, _, ws2) => {
                    if let AstLevel::Nested = level {
                        panic!(
                            "blocks ('{}') are only allowed at the top level of a template \
                             or another block",
                            name
                        );
                    }
                    self.write_block(ws1, name, ws2);
                }
                Node::Include(ws, path) => {
                    self.handle_include(ctx, ws, path);
                }
                Node::Call(ws, scope, name, ref args) => {
                    self.write_call(ctx, ws, scope, name, args);
                }
                Node::Macro(_, ref m) => {
                    if level != AstLevel::Top {
                        panic!("macro blocks only allowed at the top level");
                    }
                    self.flush_ws(m.ws1);
                    self.prepare_ws(m.ws2);
                }
                Node::Import(ws, _, _) => {
                    if level != AstLevel::Top {
                        panic!("import blocks only allowed at the top level");
                    }
                    self.handle_ws(ws);
                }
                Node::Extends(_) => {
                    if level != AstLevel::Top {
                        panic!("extend blocks only allowed at the top level");
                    }
                    // No whitespace handling: child template top-level is not used,
                    // except for the blocks defined in it.
                }
            }
        }
    }

    fn write_cond(&mut self, ctx: &'a Context, conds: &'a [Cond], ws: WS) {
        for (i, &(cws, ref cond, ref nodes)) in conds.iter().enumerate() {
            self.handle_ws(cws);
            match *cond {
                Some(ref expr) => {
                    let expr_code = self.visit_expr_root(expr);
                    if i == 0 {
                        self.write("if ");
                    } else {
                        self.dedent();
                        self.write("} else if ");
                    }
                    self.write(&expr_code);
                }
                None => {
                    self.dedent();
                    self.write("} else");
                }
            }
            self.writeln(" {");
            self.locals.push();
            self.handle(ctx, nodes, AstLevel::Nested);
            self.locals.pop();
        }
        self.handle_ws(ws);
        self.writeln("}");
    }

    fn write_match(
        &mut self,
        ctx: &'a Context,
        ws1: WS,
        expr: &Expr,
        inter: Option<&'a str>,
        arms: &'a [When],
        ws2: WS,
    ) {
        self.flush_ws(ws1);
        if let Some(inter) = inter {
            if !inter.is_empty() {
                self.next_ws = Some(inter);
            }
        }

        let expr_code = self.visit_expr_root(expr);
        self.writeln(&format!("match &{} {{", expr_code));
        for arm in arms {
            let &(ws, ref variant, ref params, ref body) = arm;
            self.locals.push();
            match *variant {
                Some(ref param) => {
                    self.visit_match_variant(param);
                }
                None => self.write("_"),
            };
            if !params.is_empty() {
                self.write("(");
                for (i, param) in params.iter().enumerate() {
                    if let MatchParameter::Name(p) = *param {
                        self.locals.insert(p);
                    }
                    if i > 0 {
                        self.write(", ");
                    }
                    self.visit_match_param(param);
                }
                self.write(")");
            }
            self.writeln(" => {");
            self.handle_ws(ws);
            self.handle(ctx, body, AstLevel::Nested);
            self.writeln("}");
            self.locals.pop();
        }

        self.writeln("}");
        self.handle_ws(ws2);
    }

    fn write_loop(
        &mut self,
        ctx: &'a Context,
        ws1: WS,
        var: &'a Target,
        iter: &Expr,
        body: &'a [Node],
        ws2: WS,
    ) {
        self.handle_ws(ws1);
        self.locals.push();

        let expr_code = self.visit_expr_root(iter);
        self.write("for (_loop_index, ");
        let targets = self.visit_target(var);
        for name in &targets {
            self.locals.insert(name);
            self.write(name);
        }
        self.writeln(&format!(") in (&{}).into_iter().enumerate() {{", expr_code));

        self.handle(ctx, body, AstLevel::Nested);
        self.handle_ws(ws2);
        self.writeln("}");
        self.locals.pop();
    }

    fn write_call(
        &mut self,
        ctx: &'a Context,
        ws: WS,
        scope: Option<&str>,
        name: &str,
        args: &[Expr],
    ) {
        if name == "super" {
            self.flush_ws(ws);
            let line = match self.super_block {
                Some(ref name) => format!("self.render_block_{}_into(writer)?;", name),
                None => panic!("cannot call 'super()' outside block"),
            };
            self.writeln(&line);
            self.prepare_ws(ws);
            self.used_super = true;
            return;
        }

        let def = if let Some(s) = scope {
            let path = ctx.imports
                .get(s)
                .unwrap_or_else(|| panic!("no import found for scope '{}'", s));
            let mctx = self.contexts
                .get(path)
                .unwrap_or_else(|| panic!("context for '{:?}' not found", path));
            mctx.macros
                .get(name)
                .unwrap_or_else(|| panic!("macro '{}' not found in scope '{}'", s, name))
        } else {
            ctx.macros
                .get(name)
                .unwrap_or_else(|| panic!("macro '{}' not found", name))
        };

        self.flush_ws(ws); // Cannot handle_ws() here: whitespace from macro definition comes first
        self.locals.push();
        self.writeln("{");
        self.prepare_ws(def.ws1);

        for (i, arg) in def.args.iter().enumerate() {
            let expr_code = self.visit_expr_root(
                args.get(i)
                    .unwrap_or_else(|| panic!("macro '{}' takes more than {} arguments", name, i)),
            );
            self.write(&format!("let {} = &{};", arg, expr_code));
            self.locals.insert(arg);
        }
        self.handle(ctx, &def.nodes, AstLevel::Nested);

        self.flush_ws(def.ws2);
        self.writeln("}");
        self.locals.pop();
        self.prepare_ws(ws);
    }

    fn handle_include(&mut self, ctx: &'a Context, ws: WS, path: &str) {
        self.flush_ws(ws);
        let path = self.input
            .config
            .find_template(path, Some(&self.input.path));
        let src = get_template_source(&path);
        let nodes = parser::parse(&src);
        let nested = {
            let mut gen = self.child();
            gen.handle(ctx, &nodes, AstLevel::Nested);
            gen.buf
        };
        self.buf.push_str(&nested);
        self.prepare_ws(ws);
    }

    fn write_let_decl(&mut self, ws: WS, var: &'a Target) {
        self.handle_ws(ws);
        self.write("let ");
        match *var {
            Target::Name(name) => {
                self.locals.insert(name);
                self.write(name);
            }
        }
        self.writeln(";");
    }

    fn write_let(&mut self, ws: WS, var: &'a Target, val: &Expr) {
        self.handle_ws(ws);
        let mut code = String::new();
        self.visit_expr(val, &mut code);

        match *var {
            Target::Name(name) => {
                if !self.locals.contains(name) {
                    self.write("let ");
                    self.locals.insert(name);
                }
                self.write(name);
            }
        }
        self.write(&format!(" = {};", &code));
    }

    fn write_block(&mut self, ws1: WS, name: &str, ws2: WS) {
        self.flush_ws(ws1);
        self.writeln(&format!("self.render_block_{}_into(writer)?;", name));
        self.prepare_ws(ws2);
    }

    fn write_expr(&mut self, ws: WS, s: &Expr) {
        self.handle_ws(ws);
        let mut code = String::new();
        let wrapped = self.visit_expr(s, &mut code);
        self.writeln(&format!("let askama_expr = &{};", code));

        use self::DisplayWrap::*;
        use super::input::EscapeMode::*;
        self.write("writer.write_fmt(format_args!(\"{}\", ");
        self.write(match (wrapped, &self.input.escaping) {
            (Wrapped, &Html) | (Wrapped, &None) | (Unwrapped, &None) => "askama_expr",
            (Unwrapped, &Html) => "&::askama::MarkupDisplay::from(askama_expr)",
        });
        self.writeln("))?;");
    }

    fn write_lit(&mut self, lws: &'a str, val: &str, rws: &'a str) {
        assert!(self.next_ws.is_none());
        if !lws.is_empty() {
            if self.skip_ws {
                self.skip_ws = false;
            } else if val.is_empty() {
                assert!(rws.is_empty());
                self.next_ws = Some(lws);
            } else {
                self.writeln(&format!("writer.write_str({:#?})?;", lws));
            }
        }
        if !val.is_empty() {
            self.writeln(&format!("writer.write_str({:#?})?;", val));
        }
        if !rws.is_empty() {
            self.next_ws = Some(rws);
        }
    }

    fn write_comment(&mut self, ws: WS) {
        self.handle_ws(ws);
    }

    /* Visitor methods for expression types */

    fn visit_expr_root(&mut self, expr: &Expr) -> String {
        let mut code = String::new();
        self.visit_expr(expr, &mut code);
        code
    }

    fn visit_expr(&mut self, expr: &Expr, code: &mut String) -> DisplayWrap {
        match *expr {
            Expr::NumLit(s) => self.visit_num_lit(s, code),
            Expr::StrLit(s) => self.visit_str_lit(s, code),
            Expr::Var(s) => self.visit_var(s, code),
            Expr::Path(ref path) => self.visit_path(path, code),
            Expr::Array(ref elements) => self.visit_array(elements, code),
            Expr::Attr(ref obj, name) => self.visit_attr(obj, name, code),
            Expr::Index(ref obj, ref key) => self.visit_index(obj, key, code),
            Expr::Filter(name, ref args) => self.visit_filter(name, args, code),
            Expr::Unary(op, ref inner) => self.visit_unary(op, inner, code),
            Expr::BinOp(op, ref left, ref right) => self.visit_binop(op, left, right, code),
            Expr::Range(op, ref left, ref right) => self.visit_range(op, left, right, code),
            Expr::Group(ref inner) => self.visit_group(inner, code),
            Expr::MethodCall(ref obj, method, ref args) => {
                self.visit_method_call(obj, method, args, code)
            }
        }
    }

    fn visit_match_variant(&mut self, param: &MatchVariant) -> DisplayWrap {
        let mut code = String::new();
        let wrapped = match *param {
            MatchVariant::StrLit(s) => {
                code.push_str("&");
                self.visit_str_lit(s, &mut code)
            }
            MatchVariant::NumLit(s) => self.visit_num_lit(s, &mut code),
            MatchVariant::Name(s) => {
                code.push_str(s);
                DisplayWrap::Unwrapped
            }
            MatchVariant::Path(ref s) => {
                code.push_str(&s.join("::"));
                DisplayWrap::Unwrapped
            }
        };
        self.write(&code);
        wrapped
    }

    fn visit_match_param(&mut self, param: &MatchParameter) -> DisplayWrap {
        let mut code = String::new();
        let wrapped = match *param {
            MatchParameter::NumLit(s) => self.visit_num_lit(s, &mut code),
            MatchParameter::StrLit(s) => self.visit_str_lit(s, &mut code),
            MatchParameter::Name(s) => {
                code.push_str(s);
                DisplayWrap::Unwrapped
            }
        };
        self.write(&code);
        wrapped
    }

    fn visit_filter(&mut self, name: &str, args: &[Expr], code: &mut String) -> DisplayWrap {
        if name == "format" {
            self._visit_format_filter(args, code);
            return DisplayWrap::Unwrapped;
        } else if name == "join" {
            self._visit_join_filter(args, code);
            return DisplayWrap::Unwrapped;
        }

        if filters::BUILT_IN_FILTERS.contains(&name) {
            code.push_str(&format!("::askama::filters::{}(&", name));
        } else {
            code.push_str(&format!("filters::{}(&", name));
        }

        self._visit_args(args, code);
        code.push_str(")?");
        if name == "safe" || name == "escape" || name == "e" || name == "json" {
            DisplayWrap::Wrapped
        } else {
            DisplayWrap::Unwrapped
        }
    }

    fn _visit_format_filter(&mut self, args: &[Expr], code: &mut String) {
        code.push_str("format!(");
        self._visit_args(args, code);
        code.push_str(")");
    }

    // Force type coercion on first argument to `join` filter (see #39).
    fn _visit_join_filter(&mut self, args: &[Expr], code: &mut String) {
        code.push_str("::askama::filters::join((&");
        for (i, arg) in args.iter().enumerate() {
            if i > 0 {
                code.push_str(", &");
            }
            self.visit_expr(arg, code);
            if i == 0 {
                code.push_str(").into_iter()");
            }
        }
        code.push_str(")?");
    }

    fn _visit_args(&mut self, args: &[Expr], code: &mut String) {
        for (i, arg) in args.iter().enumerate() {
            if i > 0 {
                code.push_str(", &");
            }

            let intercept = match *arg {
                Expr::Filter(_, _) | Expr::MethodCall(_, _, _) => true,
                _ => false,
            };

            if intercept {
                let offset = code.len();
                self.visit_expr(arg, code);
                let idx = self.vars;
                self.vars += 1;
                self.writeln(&format!("let var{} = {};", idx, &code[offset..]));
                code.truncate(offset);
                code.push_str(&format!("var{}", idx));
            } else {
                self.visit_expr(arg, code);
            }
        }
    }

    fn visit_attr(&mut self, obj: &Expr, attr: &str, code: &mut String) -> DisplayWrap {
        if let Expr::Var(name) = *obj {
            if name == "loop" {
                code.push_str("_loop_index");
                if attr == "index" {
                    code.push_str(" + 1");
                    return DisplayWrap::Unwrapped;
                } else if attr == "index0" {
                    return DisplayWrap::Unwrapped;
                } else if attr == "first" {
                    code.push_str(" == 0");
                    return DisplayWrap::Unwrapped;
                } else {
                    panic!("unknown loop variable");
                }
            }
        }
        self.visit_expr(obj, code);
        code.push_str(&format!(".{}", attr));
        DisplayWrap::Unwrapped
    }

    fn visit_index(&mut self, obj: &Expr, key: &Expr, code: &mut String) -> DisplayWrap {
        self.visit_expr(obj, code);
        code.push_str("[");
        self.visit_expr(key, code);
        code.push_str("]");
        DisplayWrap::Unwrapped
    }

    fn visit_method_call(
        &mut self,
        obj: &Expr,
        method: &str,
        args: &[Expr],
        code: &mut String,
    ) -> DisplayWrap {
        if let Expr::Var("self") = obj {
            code.push_str("self");
        } else {
            self.visit_expr(obj, code);
        }

        code.push_str(&format!(".{}(", method));
        self._visit_args(args, code);
        code.push_str(")");
        DisplayWrap::Unwrapped
    }

    fn visit_unary(&mut self, op: &str, inner: &Expr, code: &mut String) -> DisplayWrap {
        code.push_str(op);
        self.visit_expr(inner, code);
        DisplayWrap::Unwrapped
    }

    fn visit_range(
        &mut self,
        op: &str,
        left: &Option<Box<Expr>>,
        right: &Option<Box<Expr>>,
        code: &mut String,
    ) -> DisplayWrap {
        if let Some(left) = left {
            self.visit_expr(left, code);
        }
        code.push_str(op);
        if let Some(right) = right {
            self.visit_expr(right, code);
        }
        DisplayWrap::Unwrapped
    }

    fn visit_binop(
        &mut self,
        op: &str,
        left: &Expr,
        right: &Expr,
        code: &mut String,
    ) -> DisplayWrap {
        self.visit_expr(left, code);
        code.push_str(&format!(" {} ", op));
        self.visit_expr(right, code);
        DisplayWrap::Unwrapped
    }

    fn visit_group(&mut self, inner: &Expr, code: &mut String) -> DisplayWrap {
        code.push_str("(");
        self.visit_expr(inner, code);
        code.push_str(")");
        DisplayWrap::Unwrapped
    }

    fn visit_array(&mut self, elements: &[Expr], code: &mut String) -> DisplayWrap {
        code.push_str("[");
        for (i, el) in elements.iter().enumerate() {
            if i > 0 {
                code.push_str(", ");
            }
            self.visit_expr(el, code);
        }
        code.push_str("]");
        DisplayWrap::Unwrapped
    }

    fn visit_path(&mut self, path: &[&str], code: &mut String) -> DisplayWrap {
        for (i, part) in path.iter().enumerate() {
            if i > 0 {
                code.push_str("::");
            }
            code.push_str(part);
        }
        DisplayWrap::Unwrapped
    }

    fn visit_var(&mut self, s: &str, code: &mut String) -> DisplayWrap {
        if self.locals.contains(s) {
            code.push_str(s);
        } else {
            code.push_str("self.");
            code.push_str(s);
        }
        DisplayWrap::Unwrapped
    }

    fn visit_str_lit(&mut self, s: &str, code: &mut String) -> DisplayWrap {
        code.push_str(&format!("\"{}\"", s));
        DisplayWrap::Unwrapped
    }

    fn visit_num_lit(&mut self, s: &str, code: &mut String) -> DisplayWrap {
        code.push_str(s);
        DisplayWrap::Unwrapped
    }

    fn visit_target_single<'t>(&mut self, name: &'t str) -> Vec<&'t str> {
        vec![name]
    }

    fn visit_target<'t>(&mut self, target: &'t Target) -> Vec<&'t str> {
        match *target {
            Target::Name(s) => self.visit_target_single(s),
        }
    }

    /* Helper methods for dealing with whitespace nodes */

    // Combines `flush_ws()` and `prepare_ws()` to handle both trailing whitespace from the
    // preceding literal and leading whitespace from the succeeding literal.
    fn handle_ws(&mut self, ws: WS) {
        self.flush_ws(ws);
        self.prepare_ws(ws);
    }

    // If the previous literal left some trailing whitespace in `next_ws` and the
    // prefix whitespace suppressor from the given argument, flush that whitespace.
    // In either case, `next_ws` is reset to `None` (no trailing whitespace).
    fn flush_ws(&mut self, ws: WS) {
        if self.next_ws.is_some() && !ws.0 {
            let val = self.next_ws.unwrap();
            if !val.is_empty() {
                self.writeln(&format!("writer.write_str({:#?})?;", val));
            }
        }
        self.next_ws = None;
    }

    // Sets `skip_ws` to match the suffix whitespace suppressor from the given
    // argument, to determine whether to suppress leading whitespace from the
    // next literal.
    fn prepare_ws(&mut self, ws: WS) {
        self.skip_ws = ws.1;
    }

    /* Helper methods for writing to internal buffer */

    fn writeln(&mut self, s: &str) {
        if s.is_empty() {
            return;
        }
        if s == "}" {
            self.dedent();
        }
        self.write(s);
        if s.ends_with('{') {
            self.indent();
        }
        self.buf.push('\n');
        self.start = true;
    }

    fn write(&mut self, s: &str) {
        if self.start {
            for _ in 0..(self.indent * 4) {
                self.buf.push(' ');
            }
            self.start = false;
        }
        self.buf.push_str(s);
    }

    fn indent(&mut self) {
        self.indent += 1;
    }

    fn dedent(&mut self) {
        if self.indent == 0 {
            panic!("dedent() called while indentation == 0");
        }
        self.indent -= 1;
    }
}

struct SetChain<'a, T: 'a>
where
    T: cmp::Eq + hash::Hash,
{
    parent: Option<&'a SetChain<'a, T>>,
    scopes: Vec<HashSet<T>>,
}

impl<'a, T: 'a> SetChain<'a, T>
where
    T: cmp::Eq + hash::Hash,
{
    fn new() -> SetChain<'a, T> {
        SetChain {
            parent: None,
            scopes: vec![HashSet::new()],
        }
    }
    fn with_parent<'p>(parent: &'p SetChain<T>) -> SetChain<'p, T> {
        SetChain {
            parent: Some(parent),
            scopes: vec![HashSet::new()],
        }
    }
    fn contains(&self, val: T) -> bool {
        self.scopes.iter().rev().any(|set| set.contains(&val)) || match self.parent {
            Some(set) => set.contains(val),
            None => false,
        }
    }
    fn insert(&mut self, val: T) {
        self.scopes.last_mut().unwrap().insert(val);
    }
    fn push(&mut self) {
        self.scopes.push(HashSet::new());
    }
    fn pop(&mut self) {
        self.scopes.pop().unwrap();
        assert!(!self.scopes.is_empty());
    }
}

struct Heritage<'a> {
    root: &'a Context<'a>,
    blocks: BlockAncestry<'a>,
}

impl<'a> Heritage<'a> {
    fn new<'n>(
        mut ctx: &'n Context<'n>,
        contexts: &'n HashMap<&'n PathBuf, Context<'n>>,
    ) -> Heritage<'n> {
        let mut blocks: BlockAncestry<'n> = ctx.blocks
            .iter()
            .map(|(name, def)| (*name, vec![(ctx, *def)]))
            .collect();

        while let Some(ref path) = ctx.extends {
            ctx = &contexts[&path];
            for (name, def) in &ctx.blocks {
                blocks
                    .entry(name)
                    .or_insert_with(|| vec![])
                    .push((ctx, def));
            }
        }

        Heritage { root: ctx, blocks }
    }
}

#[derive(Clone, PartialEq)]
enum AstLevel {
    Top,
    Block,
    Nested,
}

impl Copy for AstLevel {}

#[derive(Clone)]
enum DisplayWrap {
    Wrapped,
    Unwrapped,
}

impl Copy for DisplayWrap {}

type BlockAncestry<'a> = HashMap<&'a str, Vec<(&'a Context<'a>, &'a Node<'a>)>>;