common/code_gen_utils.rs - crubit - Git at Google

 // Part of the Crubit project, under the Apache License v2.0 with LLVM
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

 use arc_anyhow::{anyhow, ensure, Result};
 use itertools::Itertools;
 use once_cell::sync::Lazy;
 use proc_macro2::{Ident, TokenStream};
 use quote::{format_ident, quote, ToTokens};
 use std::collections::{BTreeSet, HashSet};
 use std::rc::Rc;

 /// Formats a C++ (qualified) identifier. Returns an error when `ident` is a C++
 /// reserved keyword or is an invalid identifier.
 pub fn format_cc_ident(ident: &str) -> Result<TokenStream> {
     ensure!(!ident.is_empty(), "Empty string is not a valid C++ identifier");

     // C++ doesn't have an equivalent of
     // https://doc.rust-lang.org/rust-by-example/compatibility/raw_identifiers.html and therefore
     // an error is returned when `ident` is a C++ reserved keyword.
     ensure!(
         !RESERVED_CC_KEYWORDS.contains(ident),
         "`{}` is a C++ reserved keyword and can't be used as a C++ identifier",
         ident
     );

     // https://en.cppreference.com/w/cpp/language/identifiers says that "A valid identifier must
     // begin with a non-digit character (Latin letter, underscore, or Unicode
     // character of class XID_Start)".  One motivation for this check is to
     // explicitly catch names of tuple fields (e.g. `some_tuple.0`).
     let first_char = ident.chars().next().expect("!is_empty checked above");
     ensure!(
         unicode_ident::is_xid_start(first_char) || first_char == '_',
         "The following character can't be used as a start of a C++ identifier: {first_char}",
     );

     ident.parse().map_err(
         // Explicitly mapping the error via `anyhow!`, because `LexError` is not `Sync`
         // (required for `anyhow::Error` to implement `From<LexError>`) and
         // therefore we can't just use `?`.
         |lex_error| anyhow!("Can't format `{ident}` as a C++ identifier: {lex_error}"),
     )
 }

 /// Makes an 'Ident' to be used in the Rust source code. Escapes Rust keywords.
 /// Panics if `ident` is empty or is otherwise an invalid identifier.
 pub fn make_rs_ident(ident: &str) -> Ident {
     // TODO(https://github.com/dtolnay/syn/pull/1098): Remove the hardcoded list once syn recognizes
     // 2018 and 2021 keywords.
     if ["async", "await", "try", "dyn"].contains(&ident) {
         return format_ident!("r#{}", ident);
     }
     match syn::parse_str::<syn::Ident>(ident) {
         Ok(_) => format_ident!("{}", ident),
         Err(_) => format_ident!("r#{}", ident),
     }
 }

 /// Escapes characters that may not appear in a C++ or Rust identifier.
 ///
 /// The implemented escaping algorithm guarantess that different inputs will
 /// always produce different outputs (i.e. unique symbols will remain unique
 /// after escaping).  Other than that, the implemented escaping algorithm is
 /// somewhat arbitrary and should be treated as an implementation detail and not
 /// depended upon.
 ///
 /// This transformation allows using escaped symbol names as part of Rust and/or
 /// C++ identifiers. In particular note that in practice Rust uses `$` and `.`
 /// characters in symbols - for example: "_ZN58_$LT$rust_out..
 /// Point$u20$as$u20$core..default..Default$GT$7default17h144069f0ad7be325E".
 pub fn escape_non_identifier_chars(symbol: &str) -> String {
     // EXTRA_CAPACITY_PREDICTION has been haphazardly chosen based on a single
     // example encountered in practice where there were 16 characters that needed
     // escaping: 2 x '_', 8 x '$', 6 x '.': "_ZN58_$LT$rust_out..
     // Point$u20$as$u20$core..default..Default$GT$7default17h144069f0ad7be325E"
     const EXTRA_CAPACITY_PREDICTION: usize = 20;
     let mut result = String::with_capacity(symbol.len() + EXTRA_CAPACITY_PREDICTION);

     for (i, c) in symbol.chars().enumerate() {
         match c {
             '_' => result.push_str("_u"),
             '$' => result.push_str("_d"),
             '.' => result.push_str("_p"),
             c => {
                 let is_valid_identifier_char = if i == 0 {
                     // `is_xid_start` doesn't cover `'_'` character, but it is okay because we
                     // explicitly handle this character in a match branch above.
                     unicode_ident::is_xid_start(c)
                 } else {
                     unicode_ident::is_xid_continue(c)
                 };
                 if is_valid_identifier_char {
                     result.push(c);
                 } else {
                     result.push_str("_x");
                     result.push_str(&format!("{:08x}", c as u32));
                 };
             }
         }
     }

     result
 }

 /// Representation of `foo::bar::baz` where each component is either the name
 /// of a C++ namespace, or the name of a Rust module.
 #[derive(Debug, PartialEq, Eq, Clone, Hash, PartialOrd, Ord)]
 pub struct NamespaceQualifier(pub Vec<Rc<str>>);

 impl NamespaceQualifier {
     /// Constructs a new `NamespaceQualifier` from a sequence of names.
     pub fn new<T: Into<Rc<str>>>(iter: impl IntoIterator<Item = T>) -> Self {
         // TODO(b/258265044): Catch most (all if possible) error conditions early.  For
         // example:
         // - Panic early if any strings are empty, or are not Rust identifiers
         // - Report an error early if any strings are C++ reserved keywords
         // This may make `format_for_cc`, `format_with_cc_body`, and
         // `format_namespace_bound_cc_tokens` infallible.
         Self(iter.into_iter().map(Into::into).collect())
     }

     /// Returns `foo::bar::baz::` (escaping Rust keywords as needed).
     pub fn format_for_rs(&self) -> TokenStream {
         let namespace_rs_idents = self.0.iter().map(|ns| make_rs_ident(ns));
         quote! { #(#namespace_rs_idents::)* }
     }

     /// Returns `foo::bar::baz::` (reporting errors for C++ keywords).
     pub fn format_for_cc(&self) -> Result<TokenStream> {
         let namespace_cc_idents = self.cc_idents()?;
         Ok(quote! { #(#namespace_cc_idents::)* })
     }

     pub fn format_with_cc_body(
         &self,
         body: TokenStream,
         attributes: Vec<TokenStream>,
     ) -> Result<TokenStream> {
         if self.0.is_empty() {
             Ok(body)
         } else {
             let namespace_cc_idents = self.cc_idents()?;
             Ok(quote! {
                 __NEWLINE__ #(#attributes)* namespace #(#namespace_cc_idents)::* { __NEWLINE__
                     #body
                 __NEWLINE__ }  __NEWLINE__
             })
         }
     }

     pub fn cc_idents(&self) -> Result<Vec<TokenStream>> {
         self.0.iter().map(|ns| format_cc_ident(ns)).collect()
     }
 }

 /// `CcInclude` represents a single `#include ...` directive in C++.
 #[derive(Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
 pub enum CcInclude {
     /// Represents a system header, e.g., `cstdef`, which will be included by
     /// angular brackets.
     SystemHeader(&'static str),
     /// Represents a user header, which will be included by quotes.
     UserHeader(Rc<str>),
     /// Represents an `#include` for Crubit C++ support library headers: the
     /// format specifier for what comes after `#include` and path of the support
     /// library header.
     SupportLibHeader(Rc<str>, Rc<str>),
 }

 impl CcInclude {
     /// Creates a `CcInclude` that represents `#include <cstddef>` and provides
     /// C++ types like `std::size_t` or `std::ptrdiff_t`.  See
     /// https://en.cppreference.com/w/cpp/header/cstddef
     pub fn cstddef() -> Self {
         Self::SystemHeader("cstddef")
     }

     /// Creates a `CcInclude` that represents `#include <cstdint>` and provides
     /// C++ types like `std::int16_t` or `std::uint32_t`.  See
     /// https://en.cppreference.com/w/cpp/header/cstdint
     pub fn cstdint() -> Self {
         Self::SystemHeader("cstdint")
     }

     /// Creates a `CcInclude` that represents `#include <memory>`.
     /// See https://en.cppreference.com/w/cpp/header/memory
     pub fn memory() -> Self {
         Self::SystemHeader("memory")
     }

     /// Creates a `CcInclude` that represents `#include <utility>` and provides
     /// C++ functions like `std::move` and C++ types like `std::tuple`.
     /// See https://en.cppreference.com/w/cpp/header/utility
     pub fn utility() -> Self {
         Self::SystemHeader("utility")
     }

     /// Creates a `CcInclude` that represents `#include <type_traits>` and
     /// provides C++ APIs like `std::is_trivially_copy_constructible_v`.
     /// See https://en.cppreference.com/w/cpp/header/type_traits
     pub fn type_traits() -> Self {
         Self::SystemHeader("type_traits")
     }

     /// Creates a user include: `#include "some/path/to/header.h"`.
     pub fn user_header(path: Rc<str>) -> Self {
         Self::UserHeader(path)
     }

     /// Creates a support library header include based on the specified format.
     /// E.g., `\"{header}\"` and `hdr.h` produces `#include "hdr.h"`.
     pub fn support_lib_header(format: Rc<str>, path: Rc<str>) -> Self {
         Self::SupportLibHeader(format, path)
     }
 }

 impl ToTokens for CcInclude {
     fn to_tokens(&self, tokens: &mut TokenStream) {
         match self {
             Self::SystemHeader(path) => {
                 let path: TokenStream = path
                     .parse()
                     .expect("`pub` API of `CcInclude` guarantees validity of system includes");
                 quote! { __HASH_TOKEN__ include < #path > __NEWLINE__ }.to_tokens(tokens)
             }
             Self::UserHeader(path) => {
                 quote! { __HASH_TOKEN__ include #path __NEWLINE__ }.to_tokens(tokens)
             }
             Self::SupportLibHeader(format, path) => {
                 let full_path: TokenStream = format
                     .replace("{header}", path)
                     .parse()
                     .expect("Failed to parse support lib `#include` path");
                 quote! { __HASH_TOKEN__ include #full_path __NEWLINE__ }.to_tokens(tokens)
             }
         }
     }
 }

 /// Formats a set of `CcInclude`s, trying to follow the guidance from
 /// [the Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html#Names_and_Order_of_Includes).
 pub fn format_cc_includes(set_of_includes: &BTreeSet<CcInclude>) -> TokenStream {
     let mut tokens = TokenStream::default();
     let mut iter = set_of_includes.iter().peekable();
     while let Some(include) = iter.next() {
         include.to_tokens(&mut tokens);

         // Add an empty line between system headers and user headers.
         if let (CcInclude::SystemHeader(_), Some(CcInclude::UserHeader(_))) = (include, iter.peek())
         {
             quote! { __NEWLINE__ }.to_tokens(&mut tokens)
         }
     }
     tokens
 }

 static RESERVED_CC_KEYWORDS: Lazy<HashSet<&'static str>> = Lazy::new(|| {
     // `RESERVED_CC_KEYWORDS` are based on https://en.cppreference.com/w/cpp/keyword
     [
         "alignas",
         "alignof",
         "and",
         "and_eq",
         "asm",
         "atomic_cancel",
         "atomic_commit",
         "atomic_noexcept",
         "auto",
         "bitand",
         "bitor",
         "bool",
         "break",
         "case",
         "catch",
         "char",
         "char8_t",
         "char16_t",
         "char32_t",
         "class",
         "compl",
         "concept",
         "const",
         "consteval",
         "constexpr",
         "constinit",
         "const_cast",
         "continue",
         "co_await",
         "co_return",
         "co_yield",
         "decltype",
         "default",
         "delete",
         "do",
         "double",
         "dynamic_cast",
         "else",
         "enum",
         "explicit",
         "export",
         "extern",
         "false",
         "float",
         "for",
         "friend",
         "goto",
         "if",
         "inline",
         "int",
         "long",
         "mutable",
         "namespace",
         "new",
         "noexcept",
         "not",
         "not_eq",
         "nullptr",
         "operator",
         "or",
         "or_eq",
         "private",
         "protected",
         "public",
         "reflexpr",
         "register",
         "reinterpret_cast",
         "requires",
         "return",
         "short",
         "signed",
         "sizeof",
         "static",
         "static_assert",
         "static_cast",
         "struct",
         "switch",
         "synchronized",
         "template",
         "this",
         "thread_local",
         "throw",
         "true",
         "try",
         "typedef",
         "typeid",
         "typename",
         "union",
         "unsigned",
         "using",
         "virtual",
         "void",
         "volatile",
         "wchar_t",
         "while",
         "xor",
         "xor_eq",
     ]
     .into_iter()
     .collect()
 });

 #[cfg(test)]
 pub mod tests {
     use super::*;
     use quote::quote;
     use token_stream_matchers::{assert_cc_matches, assert_rs_matches};
     use token_stream_printer::cc_tokens_to_formatted_string_for_tests;

     #[test]
     fn test_format_cc_ident_basic() {
         assert_cc_matches!(format_cc_ident("foo").unwrap(), quote! { foo });
     }

     #[test]
     fn test_format_cc_ident_exotic_xid_start() {
         assert_cc_matches!(format_cc_ident("Łukasz").unwrap(), quote! { Łukasz });
     }

     #[test]
     fn test_format_cc_ident_underscore() {
         assert_cc_matches!(format_cc_ident("_").unwrap(), quote! { _ });
     }

     #[test]
     fn test_format_cc_ident_reserved_rust_keyword() {
         assert_cc_matches!(format_cc_ident("impl").unwrap(), quote! { impl });
     }

     #[test]
     fn test_format_cc_ident_reserved_cc_keyword() {
         let err = format_cc_ident("reinterpret_cast").unwrap_err();
         let msg = err.to_string();
         assert!(msg.contains("`reinterpret_cast`"));
         assert!(msg.contains("C++ reserved keyword"));
     }

     #[test]
     fn test_format_cc_ident_unparseable_identifier() {
         let err = format_cc_ident("foo)").unwrap_err();
         let msg = err.to_string();
         assert!(msg.contains("Can't format `foo)` as a C++ identifier"));
         assert!(msg.contains("cannot parse"));
     }

     #[test]
     fn test_format_cc_ident_unqualified_identifiers() {
         // https://en.cppreference.com/w/cpp/language/identifiers#Unqualified_identifiers

         // These may appear in `IR::Func::name`.
         assert_cc_matches!(format_cc_ident("operator==").unwrap(), quote! { operator== });
         assert_cc_matches!(format_cc_ident("operator new").unwrap(), quote! { operator new });

         // This may appear in `IR::Record::cc_name` (although in practice these will
         // be namespace-qualified most of the time).
         assert_cc_matches!(format_cc_ident("MyTemplate<int>").unwrap(), quote! { MyTemplate<int> });
     }

     /// https://en.cppreference.com/w/cpp/language/identifiers#Qualified_identifiers
     ///
     /// This may appear in `IR::Record::cc_name`, or in
     /// `__crubit::annotate(cpp_type=...)`.
     #[test]
     fn test_format_cc_ident_qualified_identifiers() {
         assert_cc_matches!(
             format_cc_ident("std::vector<int>").unwrap(),
             quote! { std::vector<int> }
         );
     }

     #[test]
     fn test_format_cc_ident_empty() {
         let err = format_cc_ident("").unwrap_err();
         let msg = err.to_string();
         assert_eq!(msg, "Empty string is not a valid C++ identifier");
     }

     #[test]
     fn test_format_cc_ident_invalid_first_char() {
         let tests = vec![
             // `0` and `1 are field names in `struct RustStruct(i32, u16)`.
             "0",
             // `~MyClass` is a valid unqualified identifier in C++, but it is okay if
             // `format_cc_ident` rejects it, because `format_cc_ident` is not used to format
             // destructor names.
             "~MyClass",
             // We used to trim leading and/or trailing whitespace, but stricter validation
             // of leading whitespace seems desirable.
             r#" operator "" _km "#,
             " foo",
             // Other tests
             "(foo",
             "(foo)",
         ];
         for test in tests.into_iter() {
             let err = format_cc_ident(test).unwrap_err();
             let actual_msg = err.to_string();
             let c = test.chars().next().unwrap();
             let expected_msg = format!(
                 "The following character can't be used as a start of a C++ identifier: {c}"
             );
             assert_eq!(actual_msg, expected_msg);
         }
     }

     #[test]
     fn test_make_rs_ident_basic() {
         let id = make_rs_ident("foo");
         assert_rs_matches!(quote! { #id }, quote! { foo });
     }

     #[test]
     fn test_make_rs_ident_reserved_cc_keyword() {
         let id = make_rs_ident("reinterpret_cast");
         assert_rs_matches!(quote! { #id }, quote! { reinterpret_cast });
     }

     #[test]
     fn test_make_rs_ident_reserved_rust_keyword() {
         let id = make_rs_ident("impl");
         assert_rs_matches!(quote! { #id }, quote! { r#impl });
     }

     #[test]
     #[should_panic]
     fn test_make_rs_ident_unfinished_group() {
         make_rs_ident("(foo"); // No closing `)`.
     }

     #[test]
     #[should_panic]
     fn test_make_rs_ident_empty() {
         make_rs_ident("");
     }

     #[test]
     fn test_cc_include_to_tokens_for_system_header() {
         let include = CcInclude::cstddef();
         assert_cc_matches!(
             quote! { #include },
             quote! {
                 __HASH_TOKEN__ include <cstddef>
             }
         );
     }

     #[test]
     fn test_cc_include_to_tokens_for_user_header() {
         let include = CcInclude::user_header("some/path/to/header.h".into());
         assert_cc_matches!(
             quote! { #include },
             quote! {
                 __HASH_TOKEN__ include "some/path/to/header.h"
             }
         );
     }

     #[test]
     fn test_cc_include_ord() {
         let cstddef = CcInclude::cstddef();
         let memory = CcInclude::memory();
         let a = CcInclude::user_header("a.h".into());
         let b = CcInclude::user_header("b.h".into());
         assert!(cstddef < memory);
         assert!(cstddef < a);
         assert!(cstddef < b);
         assert!(memory < a);
         assert!(memory < b);
         assert!(a < b);
     }

     #[test]
     fn test_format_cc_includes() {
         let includes = [
             CcInclude::cstddef(),
             CcInclude::memory(),
             CcInclude::user_header("a.h".into()),
             CcInclude::user_header("b.h".into()),
         ]
         .into_iter()
         .collect::<BTreeSet<_>>();

         let tokens = format_cc_includes(&includes);
         let actual =
             cc_tokens_to_formatted_string_for_tests(quote! { __NEWLINE__ #tokens }).unwrap();
         assert_eq!(
             actual,
             r#"
 #include <cstddef>
 #include <memory>

 #include "a.h"
 #include "b.h"
 "#
         );
     }

     #[test]
     fn test_namespace_qualifier_empty() {
         let ns = NamespaceQualifier::new::<&str>([]);
         let actual_rs = ns.format_for_rs();
         assert!(actual_rs.is_empty());
         let actual_cc = ns.format_for_cc().unwrap();
         assert!(actual_cc.is_empty());
     }

     #[test]
     fn test_namespace_qualifier_basic() {
         let ns = NamespaceQualifier::new(["foo", "bar"]);
         let actual_rs = ns.format_for_rs();
         assert_rs_matches!(actual_rs, quote! { foo::bar:: });
         let actual_cc = ns.format_for_cc().unwrap();
         assert_cc_matches!(actual_cc, quote! { foo::bar:: });
     }

     #[test]
     fn test_namespace_qualifier_reserved_cc_keyword() {
         let ns = NamespaceQualifier::new(["foo", "impl", "bar"]);
         let actual_rs = ns.format_for_rs();
         assert_rs_matches!(actual_rs, quote! { foo :: r#impl :: bar :: });
         let actual_cc = ns.format_for_cc().unwrap();
         assert_cc_matches!(actual_cc, quote! { foo::impl::bar:: });
     }

     #[test]
     fn test_namespace_qualifier_reserved_rust_keyword() {
         let ns = NamespaceQualifier::new(["foo", "reinterpret_cast", "bar"]);
         let actual_rs = ns.format_for_rs();
         assert_rs_matches!(actual_rs, quote! { foo :: reinterpret_cast :: bar :: });
         let cc_error = ns.format_for_cc().unwrap_err();
         let msg = cc_error.to_string();
         assert!(msg.contains("`reinterpret_cast`"));
         assert!(msg.contains("C++ reserved keyword"));
     }

     #[test]
     fn test_namespace_qualifier_format_with_cc_body_top_level_namespace() {
         let ns = NamespaceQualifier::new::<&str>([]);
         assert_cc_matches!(
             ns.format_with_cc_body(quote! { cc body goes here }, vec![]).unwrap(),
             quote! { cc body goes here },
         );
     }

     #[test]
     fn test_namespace_qualifier_format_with_cc_body_nested_namespace() {
         let ns = NamespaceQualifier::new(["foo", "bar", "baz"]);
         assert_cc_matches!(
             ns.format_with_cc_body(quote! { cc body goes here }, vec![]).unwrap(),
             quote! {
                 namespace foo::bar::baz {
                     cc body goes here
                 }  // namespace foo::bar::baz
             },
         );
     }

     #[test]
     fn test_format_cc_include_support_lib_header() {
         let tests = vec![
             (
                 "\"crubit/support/path/for/test/{header}\"",
                 "header.h",
                 "\"crubit/support/path/for/test/header.h\"",
             ),
             (
                 "\"crubit/support/path/for/test/{header}\"",
                 "subdir/header.h",
                 "\"crubit/support/path/for/test/subdir/header.h\"",
             ),
             (
                 "<crubit/support/path/for/test/{header}>",
                 "header.h",
                 "<crubit/support/path/for/test/header.h>",
             ),
             ("\"{header}\"", "header.h", "\"header.h\""),
         ];

         for (support_path_format, header, expected_output) in tests.iter() {
             let header = CcInclude::support_lib_header(
                 support_path_format.to_string().into(),
                 header.to_string().into(),
             );
             let mut actual_tokens = TokenStream::default();
             header.to_tokens(&mut actual_tokens);

             let expected_output: TokenStream =
                 expected_output.parse().expect("Failed to convert expected_output to TokenStream");

             assert_cc_matches!(
                 actual_tokens,
                 quote! {
                   __HASH_TOKEN__ include #expected_output
                 }
             );
         }
     }

     #[test]
     fn test_escape_non_identifier_chars() {
         let tests = vec![
             ("", ""),
             ("foo", "foo"),
             ("0abc", "_x00000030abc"),
             ("abc$xyz", "abc_dxyz"),
             ("abc.xyz", "abc_pxyz"),
             ("abc_xyz", "abc_uxyz"),
             ("abc🦀xyz", "abc_x0001f980xyz"),
             // With an escaping scheme like `$` => "_d", `<utf8 dd80 char>` => "_dd80", the
             // following 2 tests would fail the injectivity requirement (they both would map to
             // "_dd80"):
             ("$d80", "_dd80"),
             ("\u{740}", "_x00000740"),
         ];

         for (input, expected_output) in tests.iter() {
             let actual_output = escape_non_identifier_chars(input);
             assert_eq!(&actual_output, expected_output);
         }

         // Asserting that each distinct, unique test input should result in a unique,
         // non-duplicated output.  (This can be seen as a rather lightweight and
         // indirect verification of the injectivity requirement.)
         let duplicate_expectations =
             tests.iter().map(|(_, expected)| *expected).duplicates().collect_vec();
         let empty_vec: Vec<&'static str> = vec![];
         assert_eq!(empty_vec, duplicate_expectations);
     }
 }
	// Part of the Crubit project, under the Apache License v2.0 with LLVM
	// Exceptions. See /LICENSE for license information.
	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

	use arc_anyhow::{anyhow, ensure, Result};
	use itertools::Itertools;
	use once_cell::sync::Lazy;
	use proc_macro2::{Ident, TokenStream};
	use quote::{format_ident, quote, ToTokens};
	use std::collections::{BTreeSet, HashSet};
	use std::rc::Rc;

	/// Formats a C++ (qualified) identifier. Returns an error when `ident` is a C++
	/// reserved keyword or is an invalid identifier.
	pub fn format_cc_ident(ident: &str) -> Result<TokenStream> {
	ensure!(!ident.is_empty(), "Empty string is not a valid C++ identifier");

	// C++ doesn't have an equivalent of
	// https://doc.rust-lang.org/rust-by-example/compatibility/raw_identifiers.html and therefore
	// an error is returned when `ident` is a C++ reserved keyword.
	ensure!(
	!RESERVED_CC_KEYWORDS.contains(ident),
	"`{}` is a C++ reserved keyword and can't be used as a C++ identifier",
	ident
	);

	// https://en.cppreference.com/w/cpp/language/identifiers says that "A valid identifier must
	// begin with a non-digit character (Latin letter, underscore, or Unicode
	// character of class XID_Start)". One motivation for this check is to
	// explicitly catch names of tuple fields (e.g. `some_tuple.0`).
	let first_char = ident.chars().next().expect("!is_empty checked above");
	ensure!(
	unicode_ident::is_xid_start(first_char) \|\| first_char == '_',
	"The following character can't be used as a start of a C++ identifier: {first_char}",
	);

	ident.parse().map_err(
	// Explicitly mapping the error via `anyhow!`, because `LexError` is not `Sync`
	// (required for `anyhow::Error` to implement `From<LexError>`) and
	// therefore we can't just use `?`.
	\|lex_error\| anyhow!("Can't format `{ident}` as a C++ identifier: {lex_error}"),
	)
	}

	/// Makes an 'Ident' to be used in the Rust source code. Escapes Rust keywords.
	/// Panics if `ident` is empty or is otherwise an invalid identifier.
	pub fn make_rs_ident(ident: &str) -> Ident {
	// TODO(https://github.com/dtolnay/syn/pull/1098): Remove the hardcoded list once syn recognizes
	// 2018 and 2021 keywords.
	if ["async", "await", "try", "dyn"].contains(&ident) {
	return format_ident!("r#{}", ident);
	}
	match syn::parse_str::<syn::Ident>(ident) {
	Ok(_) => format_ident!("{}", ident),
	Err(_) => format_ident!("r#{}", ident),
	}
	}

	/// Escapes characters that may not appear in a C++ or Rust identifier.
	///
	/// The implemented escaping algorithm guarantess that different inputs will
	/// always produce different outputs (i.e. unique symbols will remain unique
	/// after escaping). Other than that, the implemented escaping algorithm is
	/// somewhat arbitrary and should be treated as an implementation detail and not
	/// depended upon.
	///
	/// This transformation allows using escaped symbol names as part of Rust and/or
	/// C++ identifiers. In particular note that in practice Rust uses `$` and `.`
	/// characters in symbols - for example: "_ZN58_$LT$rust_out..
	/// Point$u20$as$u20$core..default..Default$GT$7default17h144069f0ad7be325E".
	pub fn escape_non_identifier_chars(symbol: &str) -> String {
	// EXTRA_CAPACITY_PREDICTION has been haphazardly chosen based on a single
	// example encountered in practice where there were 16 characters that needed
	// escaping: 2 x '_', 8 x '$', 6 x '.': "_ZN58_$LT$rust_out..
	// Point$u20$as$u20$core..default..Default$GT$7default17h144069f0ad7be325E"
	const EXTRA_CAPACITY_PREDICTION: usize = 20;
	let mut result = String::with_capacity(symbol.len() + EXTRA_CAPACITY_PREDICTION);

	for (i, c) in symbol.chars().enumerate() {
	match c {
	'_' => result.push_str("_u"),
	'$' => result.push_str("_d"),
	'.' => result.push_str("_p"),
	c => {
	let is_valid_identifier_char = if i == 0 {
	// `is_xid_start` doesn't cover `'_'` character, but it is okay because we
	// explicitly handle this character in a match branch above.
	unicode_ident::is_xid_start(c)
	} else {
	unicode_ident::is_xid_continue(c)
	};
	if is_valid_identifier_char {
	result.push(c);
	} else {
	result.push_str("_x");
	result.push_str(&format!("{:08x}", c as u32));
	};
	}
	}
	}

	result
	}

	/// Representation of `foo::bar::baz` where each component is either the name
	/// of a C++ namespace, or the name of a Rust module.
	#[derive(Debug, PartialEq, Eq, Clone, Hash, PartialOrd, Ord)]
	pub struct NamespaceQualifier(pub Vec<Rc<str>>);

	impl NamespaceQualifier {
	/// Constructs a new `NamespaceQualifier` from a sequence of names.
	pub fn new<T: Into<Rc<str>>>(iter: impl IntoIterator<Item = T>) -> Self {
	// TODO(b/258265044): Catch most (all if possible) error conditions early. For
	// example:
	// - Panic early if any strings are empty, or are not Rust identifiers
	// - Report an error early if any strings are C++ reserved keywords
	// This may make `format_for_cc`, `format_with_cc_body`, and
	// `format_namespace_bound_cc_tokens` infallible.
	Self(iter.into_iter().map(Into::into).collect())
	}

	/// Returns `foo::bar::baz::` (escaping Rust keywords as needed).
	pub fn format_for_rs(&self) -> TokenStream {
	let namespace_rs_idents = self.0.iter().map(\|ns\| make_rs_ident(ns));
	quote! { #(#namespace_rs_idents::)* }
	}

	/// Returns `foo::bar::baz::` (reporting errors for C++ keywords).
	pub fn format_for_cc(&self) -> Result<TokenStream> {
	let namespace_cc_idents = self.cc_idents()?;
	Ok(quote! { #(#namespace_cc_idents::)* })
	}

	pub fn format_with_cc_body(
	&self,
	body: TokenStream,
	attributes: Vec<TokenStream>,
	) -> Result<TokenStream> {
	if self.0.is_empty() {
	Ok(body)
	} else {
	let namespace_cc_idents = self.cc_idents()?;
	Ok(quote! {
	__NEWLINE__ #(#attributes)* namespace #(#namespace_cc_idents)::* { __NEWLINE__
	#body
	__NEWLINE__ } __NEWLINE__
	})
	}
	}

	pub fn cc_idents(&self) -> Result<Vec<TokenStream>> {
	self.0.iter().map(\|ns\| format_cc_ident(ns)).collect()
	}
	}

	/// `CcInclude` represents a single `#include ...` directive in C++.
	#[derive(Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
	pub enum CcInclude {
	/// Represents a system header, e.g., `cstdef`, which will be included by
	/// angular brackets.
	SystemHeader(&'static str),
	/// Represents a user header, which will be included by quotes.
	UserHeader(Rc<str>),
	/// Represents an `#include` for Crubit C++ support library headers: the
	/// format specifier for what comes after `#include` and path of the support
	/// library header.
	SupportLibHeader(Rc<str>, Rc<str>),
	}

	impl CcInclude {
	/// Creates a `CcInclude` that represents `#include <cstddef>` and provides
	/// C++ types like `std::size_t` or `std::ptrdiff_t`. See
	/// https://en.cppreference.com/w/cpp/header/cstddef
	pub fn cstddef() -> Self {
	Self::SystemHeader("cstddef")
	}

	/// Creates a `CcInclude` that represents `#include <cstdint>` and provides
	/// C++ types like `std::int16_t` or `std::uint32_t`. See
	/// https://en.cppreference.com/w/cpp/header/cstdint
	pub fn cstdint() -> Self {
	Self::SystemHeader("cstdint")
	}

	/// Creates a `CcInclude` that represents `#include <memory>`.
	/// See https://en.cppreference.com/w/cpp/header/memory
	pub fn memory() -> Self {
	Self::SystemHeader("memory")
	}

	/// Creates a `CcInclude` that represents `#include <utility>` and provides
	/// C++ functions like `std::move` and C++ types like `std::tuple`.
	/// See https://en.cppreference.com/w/cpp/header/utility
	pub fn utility() -> Self {
	Self::SystemHeader("utility")
	}

	/// Creates a `CcInclude` that represents `#include <type_traits>` and
	/// provides C++ APIs like `std::is_trivially_copy_constructible_v`.
	/// See https://en.cppreference.com/w/cpp/header/type_traits
	pub fn type_traits() -> Self {
	Self::SystemHeader("type_traits")
	}

	/// Creates a user include: `#include "some/path/to/header.h"`.
	pub fn user_header(path: Rc<str>) -> Self {
	Self::UserHeader(path)
	}

	/// Creates a support library header include based on the specified format.
	/// E.g., `\"{header}\"` and `hdr.h` produces `#include "hdr.h"`.
	pub fn support_lib_header(format: Rc<str>, path: Rc<str>) -> Self {
	Self::SupportLibHeader(format, path)
	}
	}

	impl ToTokens for CcInclude {
	fn to_tokens(&self, tokens: &mut TokenStream) {
	match self {
	Self::SystemHeader(path) => {
	let path: TokenStream = path
	.parse()
	.expect("`pub` API of `CcInclude` guarantees validity of system includes");
	quote! { __HASH_TOKEN__ include < #path > __NEWLINE__ }.to_tokens(tokens)
	}
	Self::UserHeader(path) => {
	quote! { __HASH_TOKEN__ include #path __NEWLINE__ }.to_tokens(tokens)
	}
	Self::SupportLibHeader(format, path) => {
	let full_path: TokenStream = format
	.replace("{header}", path)
	.parse()
	.expect("Failed to parse support lib `#include` path");
	quote! { __HASH_TOKEN__ include #full_path __NEWLINE__ }.to_tokens(tokens)
	}
	}
	}
	}

	/// Formats a set of `CcInclude`s, trying to follow the guidance from
	/// [the Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html#Names_and_Order_of_Includes).
	pub fn format_cc_includes(set_of_includes: &BTreeSet<CcInclude>) -> TokenStream {
	let mut tokens = TokenStream::default();
	let mut iter = set_of_includes.iter().peekable();
	while let Some(include) = iter.next() {
	include.to_tokens(&mut tokens);

	// Add an empty line between system headers and user headers.
	if let (CcInclude::SystemHeader(_), Some(CcInclude::UserHeader(_))) = (include, iter.peek())
	{
	quote! { __NEWLINE__ }.to_tokens(&mut tokens)
	}
	}
	tokens
	}

	static RESERVED_CC_KEYWORDS: Lazy<HashSet<&'static str>> = Lazy::new(\|\| {
	// `RESERVED_CC_KEYWORDS` are based on https://en.cppreference.com/w/cpp/keyword
	[
	"alignas",
	"alignof",
	"and",
	"and_eq",
	"asm",
	"atomic_cancel",
	"atomic_commit",
	"atomic_noexcept",
	"auto",
	"bitand",
	"bitor",
	"bool",
	"break",
	"case",
	"catch",
	"char",
	"char8_t",
	"char16_t",
	"char32_t",
	"class",
	"compl",
	"concept",
	"const",
	"consteval",
	"constexpr",
	"constinit",
	"const_cast",
	"continue",
	"co_await",
	"co_return",
	"co_yield",
	"decltype",
	"default",
	"delete",
	"do",
	"double",
	"dynamic_cast",
	"else",
	"enum",
	"explicit",
	"export",
	"extern",
	"false",
	"float",
	"for",
	"friend",
	"goto",
	"if",
	"inline",
	"int",
	"long",
	"mutable",
	"namespace",
	"new",
	"noexcept",
	"not",
	"not_eq",
	"nullptr",
	"operator",
	"or",
	"or_eq",
	"private",
	"protected",
	"public",
	"reflexpr",
	"register",
	"reinterpret_cast",
	"requires",
	"return",
	"short",
	"signed",
	"sizeof",
	"static",
	"static_assert",
	"static_cast",
	"struct",
	"switch",
	"synchronized",
	"template",
	"this",
	"thread_local",
	"throw",
	"true",
	"try",
	"typedef",
	"typeid",
	"typename",
	"union",
	"unsigned",
	"using",
	"virtual",
	"void",
	"volatile",
	"wchar_t",
	"while",
	"xor",
	"xor_eq",
	]
	.into_iter()
	.collect()
	});

	#[cfg(test)]
	pub mod tests {
	use super::*;
	use quote::quote;
	use token_stream_matchers::{assert_cc_matches, assert_rs_matches};
	use token_stream_printer::cc_tokens_to_formatted_string_for_tests;

	#[test]
	fn test_format_cc_ident_basic() {
	assert_cc_matches!(format_cc_ident("foo").unwrap(), quote! { foo });
	}

	#[test]
	fn test_format_cc_ident_exotic_xid_start() {
	assert_cc_matches!(format_cc_ident("Łukasz").unwrap(), quote! { Łukasz });
	}

	#[test]
	fn test_format_cc_ident_underscore() {
	assert_cc_matches!(format_cc_ident("_").unwrap(), quote! { _ });
	}

	#[test]
	fn test_format_cc_ident_reserved_rust_keyword() {
	assert_cc_matches!(format_cc_ident("impl").unwrap(), quote! { impl });
	}

	#[test]
	fn test_format_cc_ident_reserved_cc_keyword() {
	let err = format_cc_ident("reinterpret_cast").unwrap_err();
	let msg = err.to_string();
	assert!(msg.contains("`reinterpret_cast`"));
	assert!(msg.contains("C++ reserved keyword"));
	}

	#[test]
	fn test_format_cc_ident_unparseable_identifier() {
	let err = format_cc_ident("foo)").unwrap_err();
	let msg = err.to_string();
	assert!(msg.contains("Can't format `foo)` as a C++ identifier"));
	assert!(msg.contains("cannot parse"));
	}

	#[test]
	fn test_format_cc_ident_unqualified_identifiers() {
	// https://en.cppreference.com/w/cpp/language/identifiers#Unqualified_identifiers

	// These may appear in `IR::Func::name`.
	assert_cc_matches!(format_cc_ident("operator==").unwrap(), quote! { operator== });
	assert_cc_matches!(format_cc_ident("operator new").unwrap(), quote! { operator new });

	// This may appear in `IR::Record::cc_name` (although in practice these will
	// be namespace-qualified most of the time).
	assert_cc_matches!(format_cc_ident("MyTemplate<int>").unwrap(), quote! { MyTemplate<int> });
	}

	/// https://en.cppreference.com/w/cpp/language/identifiers#Qualified_identifiers
	///
	/// This may appear in `IR::Record::cc_name`, or in
	/// `__crubit::annotate(cpp_type=...)`.
	#[test]
	fn test_format_cc_ident_qualified_identifiers() {
	assert_cc_matches!(
	format_cc_ident("std::vector<int>").unwrap(),
	quote! { std::vector<int> }
	);
	}

	#[test]
	fn test_format_cc_ident_empty() {
	let err = format_cc_ident("").unwrap_err();
	let msg = err.to_string();
	assert_eq!(msg, "Empty string is not a valid C++ identifier");
	}

	#[test]
	fn test_format_cc_ident_invalid_first_char() {
	let tests = vec![
	// `0` and `1 are field names in `struct RustStruct(i32, u16)`.
	"0",
	// `~MyClass` is a valid unqualified identifier in C++, but it is okay if
	// `format_cc_ident` rejects it, because `format_cc_ident` is not used to format
	// destructor names.
	"~MyClass",
	// We used to trim leading and/or trailing whitespace, but stricter validation
	// of leading whitespace seems desirable.
	r#" operator "" _km "#,
	" foo",
	// Other tests
	"(foo",
	"(foo)",
	];
	for test in tests.into_iter() {
	let err = format_cc_ident(test).unwrap_err();
	let actual_msg = err.to_string();
	let c = test.chars().next().unwrap();
	let expected_msg = format!(
	"The following character can't be used as a start of a C++ identifier: {c}"
	);
	assert_eq!(actual_msg, expected_msg);
	}
	}

	#[test]
	fn test_make_rs_ident_basic() {
	let id = make_rs_ident("foo");
	assert_rs_matches!(quote! { #id }, quote! { foo });
	}

	#[test]
	fn test_make_rs_ident_reserved_cc_keyword() {
	let id = make_rs_ident("reinterpret_cast");
	assert_rs_matches!(quote! { #id }, quote! { reinterpret_cast });
	}

	#[test]
	fn test_make_rs_ident_reserved_rust_keyword() {
	let id = make_rs_ident("impl");
	assert_rs_matches!(quote! { #id }, quote! { r#impl });
	}

	#[test]
	#[should_panic]
	fn test_make_rs_ident_unfinished_group() {
	make_rs_ident("(foo"); // No closing `)`.
	}

	#[test]
	#[should_panic]
	fn test_make_rs_ident_empty() {
	make_rs_ident("");
	}

	#[test]
	fn test_cc_include_to_tokens_for_system_header() {
	let include = CcInclude::cstddef();
	assert_cc_matches!(
	quote! { #include },
	quote! {
	__HASH_TOKEN__ include <cstddef>
	}
	);
	}

	#[test]
	fn test_cc_include_to_tokens_for_user_header() {
	let include = CcInclude::user_header("some/path/to/header.h".into());
	assert_cc_matches!(
	quote! { #include },
	quote! {
	__HASH_TOKEN__ include "some/path/to/header.h"
	}
	);
	}

	#[test]
	fn test_cc_include_ord() {
	let cstddef = CcInclude::cstddef();
	let memory = CcInclude::memory();
	let a = CcInclude::user_header("a.h".into());
	let b = CcInclude::user_header("b.h".into());
	assert!(cstddef < memory);
	assert!(cstddef < a);
	assert!(cstddef < b);
	assert!(memory < a);
	assert!(memory < b);
	assert!(a < b);
	}

	#[test]
	fn test_format_cc_includes() {
	let includes = [
	CcInclude::cstddef(),
	CcInclude::memory(),
	CcInclude::user_header("a.h".into()),
	CcInclude::user_header("b.h".into()),
	]
	.into_iter()
	.collect::<BTreeSet<_>>();

	let tokens = format_cc_includes(&includes);
	let actual =
	cc_tokens_to_formatted_string_for_tests(quote! { __NEWLINE__ #tokens }).unwrap();
	assert_eq!(
	actual,
	r#"
	#include <cstddef>
	#include <memory>

	#include "a.h"
	#include "b.h"
	"#
	);
	}

	#[test]
	fn test_namespace_qualifier_empty() {
	let ns = NamespaceQualifier::new::<&str>([]);
	let actual_rs = ns.format_for_rs();
	assert!(actual_rs.is_empty());
	let actual_cc = ns.format_for_cc().unwrap();
	assert!(actual_cc.is_empty());
	}

	#[test]
	fn test_namespace_qualifier_basic() {
	let ns = NamespaceQualifier::new(["foo", "bar"]);
	let actual_rs = ns.format_for_rs();
	assert_rs_matches!(actual_rs, quote! { foo::bar:: });
	let actual_cc = ns.format_for_cc().unwrap();
	assert_cc_matches!(actual_cc, quote! { foo::bar:: });
	}

	#[test]
	fn test_namespace_qualifier_reserved_cc_keyword() {
	let ns = NamespaceQualifier::new(["foo", "impl", "bar"]);
	let actual_rs = ns.format_for_rs();
	assert_rs_matches!(actual_rs, quote! { foo :: r#impl :: bar :: });
	let actual_cc = ns.format_for_cc().unwrap();
	assert_cc_matches!(actual_cc, quote! { foo::impl::bar:: });
	}

	#[test]
	fn test_namespace_qualifier_reserved_rust_keyword() {
	let ns = NamespaceQualifier::new(["foo", "reinterpret_cast", "bar"]);
	let actual_rs = ns.format_for_rs();
	assert_rs_matches!(actual_rs, quote! { foo :: reinterpret_cast :: bar :: });
	let cc_error = ns.format_for_cc().unwrap_err();
	let msg = cc_error.to_string();
	assert!(msg.contains("`reinterpret_cast`"));
	assert!(msg.contains("C++ reserved keyword"));
	}

	#[test]
	fn test_namespace_qualifier_format_with_cc_body_top_level_namespace() {
	let ns = NamespaceQualifier::new::<&str>([]);
	assert_cc_matches!(
	ns.format_with_cc_body(quote! { cc body goes here }, vec![]).unwrap(),
	quote! { cc body goes here },
	);
	}

	#[test]
	fn test_namespace_qualifier_format_with_cc_body_nested_namespace() {
	let ns = NamespaceQualifier::new(["foo", "bar", "baz"]);
	assert_cc_matches!(
	ns.format_with_cc_body(quote! { cc body goes here }, vec![]).unwrap(),
	quote! {
	namespace foo::bar::baz {
	cc body goes here
	} // namespace foo::bar::baz
	},
	);
	}

	#[test]
	fn test_format_cc_include_support_lib_header() {
	let tests = vec![
	(
	"\"crubit/support/path/for/test/{header}\"",
	"header.h",
	"\"crubit/support/path/for/test/header.h\"",
	),
	(
	"\"crubit/support/path/for/test/{header}\"",
	"subdir/header.h",
	"\"crubit/support/path/for/test/subdir/header.h\"",
	),
	(
	"<crubit/support/path/for/test/{header}>",
	"header.h",
	"<crubit/support/path/for/test/header.h>",
	),
	("\"{header}\"", "header.h", "\"header.h\""),
	];

	for (support_path_format, header, expected_output) in tests.iter() {
	let header = CcInclude::support_lib_header(
	support_path_format.to_string().into(),
	header.to_string().into(),
	);
	let mut actual_tokens = TokenStream::default();
	header.to_tokens(&mut actual_tokens);

	let expected_output: TokenStream =
	expected_output.parse().expect("Failed to convert expected_output to TokenStream");

	assert_cc_matches!(
	actual_tokens,
	quote! {
	__HASH_TOKEN__ include #expected_output
	}
	);
	}
	}

	#[test]
	fn test_escape_non_identifier_chars() {
	let tests = vec![
	("", ""),
	("foo", "foo"),
	("0abc", "_x00000030abc"),
	("abc$xyz", "abc_dxyz"),
	("abc.xyz", "abc_pxyz"),
	("abc_xyz", "abc_uxyz"),
	("abc🦀xyz", "abc_x0001f980xyz"),
	// With an escaping scheme like `$` => "_d", `<utf8 dd80 char>` => "_dd80", the
	// following 2 tests would fail the injectivity requirement (they both would map to
	// "_dd80"):
	("$d80", "_dd80"),
	("\u{740}", "_x00000740"),
	];

	for (input, expected_output) in tests.iter() {
	let actual_output = escape_non_identifier_chars(input);
	assert_eq!(&actual_output, expected_output);
	}

	// Asserting that each distinct, unique test input should result in a unique,
	// non-duplicated output. (This can be seen as a rather lightweight and
	// indirect verification of the injectivity requirement.)
	let duplicate_expectations =
	tests.iter().map(\|(_, expected)\| *expected).duplicates().collect_vec();
	let empty_vec: Vec<&'static str> = vec![];
	assert_eq!(empty_vec, duplicate_expectations);
	}
	}