Implement support for turning .json files into Lua modules (#308)

* Stub implementation

* Flesh out feature and add tests. Other snapshots currently failing.

* Blacklist .meta.json in JSON handler

* Write to correct property (Source) instead of Value

* Update changelog

src/lua_ast.rs

@@ -0,0 +1,279 @@
+//! Defines module for defining a small Lua AST for simple codegen.
+
+use std::{
+    fmt::{self, Write},
+    num::FpCategory,
+};
+
+/// Trait that helps turn a type into an equivalent Lua snippet.
+///
+/// Designed to be similar to the `Display` trait from Rust's std.
+trait FmtLua {
+    fn fmt_lua(&self, output: &mut LuaStream<'_>) -> fmt::Result;
+
+    /// Used to override how this type will appear when used as a table key.
+    /// Some types, like strings, can have a shorter representation as a table
+    /// key than the default, safe approach.
+    fn fmt_table_key(&self, output: &mut LuaStream<'_>) -> fmt::Result {
+        write!(output, "[")?;
+        self.fmt_lua(output)?;
+        write!(output, "]")
+    }
+}
+
+pub(crate) enum Statement {
+    Return(Expression),
+}
+
+impl FmtLua for Statement {
+    fn fmt_lua(&self, output: &mut LuaStream<'_>) -> fmt::Result {
+        match self {
+            Self::Return(literal) => {
+                write!(output, "return ")?;
+                literal.fmt_lua(output)
+            }
+        }
+    }
+}
+
+impl fmt::Display for Statement {
+    fn fmt(&self, output: &mut fmt::Formatter) -> fmt::Result {
+        let mut stream = LuaStream::new(output);
+        FmtLua::fmt_lua(self, &mut stream)
+    }
+}
+
+pub(crate) enum Expression {
+    Nil,
+    Bool(bool),
+    Number(f64),
+    String(String),
+    Table(Table),
+
+    /// Arrays are not technically distinct from other tables in Lua, but this
+    /// representation is more convenient.
+    Array(Vec<Expression>),
+}
+
+impl Expression {
+    pub fn table(entries: Vec<(Expression, Expression)>) -> Self {
+        Self::Table(Table { entries })
+    }
+}
+
+impl FmtLua for Expression {
+    fn fmt_lua(&self, output: &mut LuaStream<'_>) -> fmt::Result {
+        match self {
+            Self::Nil => write!(output, "nil"),
+            Self::Bool(inner) => inner.fmt_lua(output),
+            Self::Number(inner) => inner.fmt_lua(output),
+            Self::String(inner) => inner.fmt_lua(output),
+            Self::Table(inner) => inner.fmt_lua(output),
+            Self::Array(inner) => inner.fmt_lua(output),
+        }
+    }
+
+    fn fmt_table_key(&self, output: &mut LuaStream<'_>) -> fmt::Result {
+        match self {
+            Self::Nil => panic!("nil cannot be a table key"),
+            Self::Bool(inner) => inner.fmt_table_key(output),
+            Self::Number(inner) => inner.fmt_table_key(output),
+            Self::String(inner) => inner.fmt_table_key(output),
+            Self::Table(inner) => inner.fmt_table_key(output),
+            Self::Array(inner) => inner.fmt_table_key(output),
+        }
+    }
+}
+
+impl From<String> for Expression {
+    fn from(value: String) -> Self {
+        Self::String(value)
+    }
+}
+
+impl From<&'_ str> for Expression {
+    fn from(value: &str) -> Self {
+        Self::String(value.to_owned())
+    }
+}
+
+impl From<Table> for Expression {
+    fn from(value: Table) -> Self {
+        Self::Table(value)
+    }
+}
+
+impl FmtLua for bool {
+    fn fmt_lua(&self, output: &mut LuaStream<'_>) -> fmt::Result {
+        write!(output, "{}", self)
+    }
+}
+
+impl FmtLua for f64 {
+    fn fmt_lua(&self, output: &mut LuaStream<'_>) -> fmt::Result {
+        match self.classify() {
+            FpCategory::Nan => write!(output, "0/0"),
+            FpCategory::Infinite => {
+                if self.is_sign_positive() {
+                    write!(output, "math.huge")
+                } else {
+                    write!(output, "-math.huge")
+                }
+            }
+            _ => write!(output, "{}", self),
+        }
+    }
+}
+
+impl FmtLua for String {
+    fn fmt_lua(&self, output: &mut LuaStream<'_>) -> fmt::Result {
+        write!(output, "\"{}\"", self)
+    }
+
+    fn fmt_table_key(&self, output: &mut LuaStream<'_>) -> fmt::Result {
+        if is_valid_ident(self) {
+            write!(output, "{}", self)
+        } else {
+            write!(output, "[\"{}\"]", self)
+        }
+    }
+}
+
+impl FmtLua for Vec<Expression> {
+    fn fmt_lua(&self, output: &mut LuaStream<'_>) -> fmt::Result {
+        write!(output, "{{")?;
+
+        for (index, value) in self.iter().enumerate() {
+            value.fmt_lua(output)?;
+
+            if index < self.len() - 1 {
+                write!(output, ", ")?;
+            }
+        }
+
+        write!(output, "}}")
+    }
+}
+
+pub(crate) struct Table {
+    pub entries: Vec<(Expression, Expression)>,
+}
+
+impl FmtLua for Table {
+    fn fmt_lua(&self, output: &mut LuaStream<'_>) -> fmt::Result {
+        writeln!(output, "{{")?;
+        output.indent();
+
+        for (key, value) in &self.entries {
+            key.fmt_table_key(output)?;
+            write!(output, " = ")?;
+            value.fmt_lua(output)?;
+            writeln!(output, ",")?;
+        }
+
+        output.unindent();
+        write!(output, "}}")
+    }
+}
+
+fn is_valid_ident_char_start(value: char) -> bool {
+    value.is_ascii_alphabetic() || value == '_'
+}
+
+fn is_valid_ident_char(value: char) -> bool {
+    value.is_ascii_alphanumeric() || value == '_'
+}
+
+fn is_keyword(value: &str) -> bool {
+    match value {
+        "and" | "break" | "do" | "else" | "elseif" | "end" | "false" | "for" | "function"
+        | "if" | "in" | "local" | "nil" | "not" | "or" | "repeat" | "return" | "then" | "true"
+        | "until" | "while" => true,
+        _ => false,
+    }
+}
+
+/// Tells whether the given string is a valid Lua identifier.
+fn is_valid_ident(value: &str) -> bool {
+    if is_keyword(value) {
+        return false;
+    }
+
+    let mut chars = value.chars();
+
+    match chars.next() {
+        Some(first) => {
+            if !is_valid_ident_char_start(first) {
+                return false;
+            }
+        }
+        None => return false,
+    }
+
+    chars.all(is_valid_ident_char)
+}
+
+/// Wraps a `fmt::Write` with additional tracking to do pretty-printing of Lua.
+///
+/// Behaves similarly to `fmt::Formatter`. This trait's relationship to `LuaFmt`
+/// is very similar to `Formatter`'s relationship to `Display`.
+struct LuaStream<'a> {
+    indent_level: usize,
+    is_start_of_line: bool,
+    inner: &'a mut (dyn fmt::Write + 'a),
+}
+
+impl fmt::Write for LuaStream<'_> {
+    /// Method to support the `write!` and `writeln!` macros. Instead of using a
+    /// trait directly, these macros just call `write_str` on their first
+    /// argument.
+    ///
+    /// This method is also available on `io::Write` and `fmt::Write`.
+    fn write_str(&mut self, value: &str) -> fmt::Result {
+        let mut is_first_line = true;
+
+        for line in value.split('\n') {
+            if is_first_line {
+                is_first_line = false;
+            } else {
+                self.line()?;
+            }
+
+            if !line.is_empty() {
+                if self.is_start_of_line {
+                    self.is_start_of_line = false;
+                    let indentation = "\t".repeat(self.indent_level);
+                    self.inner.write_str(&indentation)?;
+                }
+
+                self.inner.write_str(line)?;
+            }
+        }
+
+        Ok(())
+    }
+}
+
+impl<'a> LuaStream<'a> {
+    fn new(inner: &'a mut (dyn fmt::Write + 'a)) -> Self {
+        LuaStream {
+            indent_level: 0,
+            is_start_of_line: true,
+            inner,
+        }
+    }
+
+    fn indent(&mut self) {
+        self.indent_level += 1;
+    }
+
+    fn unindent(&mut self) {
+        assert!(self.indent_level > 0);
+        self.indent_level -= 1;
+    }
+
+    fn line(&mut self) -> fmt::Result {
+        self.is_start_of_line = true;
+        self.inner.write_str("\n")
+    }
+}