GCC Rust Monthly Report #16 April 2022

Thanks again to Open Source Security, inc and Embecosm for their ongoing support for this project.

Milestone Progress

Four months into 2022, that was quick! For gcc rust we have been working diligently on our current milestone of imports and visibility. As we have mentioned before, this milestone breaks down into a few tracks:

  1. Module support
  2. Complex paths
  3. use statements
  4. Metadata exports
  5. Privacy checks

Some parts of this milestone were already started by the community and from overlapping work in previous milestones. So the remaining work includes:

  1. Metadata exports
  2. Privacy checks
  3. use statements

As well as fixing some more complex cases when it comes to module handling.

We have split the work up for the milestone between Philbert and Arthur Cohen so that Arthur is working on Privacy checks which have overlapping work in use statements, necessary for proper metadata handling. Philbert has taken advantage of the work Arthur is doing to dedicate himself to bug fixing since Arthur is making good progress. This meant we got all the bugs fixed to allow us to support generating slices from arrays required for our goal test case: The rust blake3 implementation). With more time for bug fixing, we hope to close this goal test case out soon. However, testing the compiler against actual code is very valuable to find the gaps early, and we want to continue to invest time into this as best we can.

We got quite a few new contributors this month who also tackled many bugs, including David Faust adding support for repr options on algebraic data types. In addition, Simon Cook fixed our build issues on macOS. Or liushuyu, who has fixed bugs all over the codebase. Thank you so much for all the contributions this month!

Monthly Community Call

Its time for our next community call, feel free to join in! 🙂

Note we have moved this by one week than our usual first Friday of the Month.

Testing project

As announced last month, we have started working on a new project which allows us to test gccrs in a variety of scenarios.

We have added three new passes to the project, which enable us to verify that gccrs can compile valid rust code:

  • Run gccrs on all valid test cases of rustc’s testsuite (test cases without an ERROR directive)
  • Run gccrs on all valid #![no_std] cases.
  • Run gccrs on all valid #![no_core] cases.

The last two passes are extremely important as we are not yet capable of compiling and adding the standard library to the rust code we compile.

The results for all five passes as of this month are as follow:

TestsuiteTotal Test CasesPassesFailures
gccrs parsing15481127832698
rustc on gccrs dejagnu563390173
gccrs success66038775726
gccrs success (no_std)27646982066
gccrs success (no_core)276414533
gccrs testing

The project’s source code is available here. It is still in its early days, and we’d love it if you found something to correct or enhance!

Completed Activities

  • Bugfix makefile not installing compiler driver when cross-compiling PR1092
  • Fix out of memory issue on huge array constructors PR1087
  • Add compile_error! builtin macro PR1080
  • Cleanup and bugfix of -frust-crate-name being overriden PR1083
  • Move `cfg!()` macro to builtins. Fixes #1039 PR1116
  • rust: Use -Otarget when building and logging warnings PR1114
  • macros: Add env! macro PR1113
  • rust: Clang/macOS Testing PR1112
  • Add AST Private Visibilities PR1111
  • Add Reachability visitors for items with generics PR1110
  • rust: Allow gccrs to build on x86_64-apple-darwin with clang/libc++ PR1109
  • Add missing unify rules for inference variables PR1108
  • macros: fix an infinite loop in `concat!` macro parser PR1106
  • Lower AST::Visibility to HIR::Visibility properly PR1103
  • Add helper as_string for DefIds PR1101
  • Add known lang item const_slice_ptr mappings PR1100
  • Fix bad inherent overlap error PR1099
  • Ensure unsize method resolutions actually unsize PR1098
  • Support mangling \*const ptr and slices like \*const [T] PR1097
  • Add -frust-edition flag and possible values PR1091
  • macros: add concat! macro PR1090
  • rust-session-manager: better crate name handling logic PR1088
  • Slice support PR1086
  • Add base for privacy visitor PR1082
  • Implement the builtin include! macro PR1096
  • Don’t do docker image builds on forks PR1124
  • Bug fix projection substitution PR1121
  • Fix ICE during HIR lowering PR1134
  • Refactor header and cleanup PR1137
  • Support patterns in function signatures PR1138
  • Add tests for cfg! macro PR1119
  • Add name and type resolution to TuplePatterns PR1144
  • Improve doc attribute support PR1139
  • Fix bug with generic parameters in extern declarations PR1145
  • Add missing coercion rule from array to slice PR1147
  • Handle cold attribute PR1148
  • Add support for isize and usize type hints on literals PR1154
  • Add base for visibility pass PR1155
  • Support link_section attribute PR1150
  • Add support for simple visibility checks PR1157
  • Support the no_mangle attribute PR1149
  • Refactor code from headers into impl files PR1160
  • Fix bug in missing macro transcribing ComparisonExpr LazyBooleanExpr and AssignmentExpr PR1161
  • CI catch malformed test cases PR1162
  • Handle crate_name attribute PR1163
  • Add NodeId to AST::SimplePaths PR1164
  • Support type inference of generic parameters on paths behind references PR1166
  • Fix nullptr when resolving root of a path PR1167
  • Refactor ResolveItem into impl file PR1168
  • Implement macro expansion on IfExpr, IfExprConseqElse, IfExprConseqIf IfExprConseqIfLet PR1170
  • CI Update base image for Dockerfile PR1171
  • Resolve visibility paths PR1172
  • Fix bad location info on SimplePaths PR1174
  • Add mappings for SimplePathSegments PR1176
  • Fix bad name resolution on path with generic segments PR1184
  • Refactor ResolvePath into impl file PR1186
  • Support align and packed repr options on ADTTypes PR1188
  • Fix bad location info on SimplePaths PR1189
  • Fix ICE in reachability pass PR1190
  • Add assertion on peeking compile context PR1192
  • Remove unused parameters on function calls to allow for more complex const calls PR1193
  • Add support for transmute PR1194
  • Resolve Simple path in use items PR1191
  • Support recursive coercion sites PR1197
  • Add new as_name interface for Dynamic types to improve debug names PR1199

Contributors this month

Overall Task Status

CategoryLast MonthThis MonthDelta
In Progress2325+2
GitHub Issues

Test Cases

CategoryLast MonthThis MonthDelta
make check-rust


CategoryLast MonthThis MonthDelta
In Progress1012+2
GitHub Bugs

Milestone Progress

MilestoneLast MonthThis MonthDeltaStart DateCompletion DateTarget
Data Structures 1 – Core100%100%30th Nov 202027th Jan 202129th Jan 2021
Control Flow 1 – Core100%100%28th Jan 202110th Feb 202126th Feb 2021
Data Structures 2 – Generics100%100%11th Feb 202114th May 202128th May 2021
Data Structures 3 – Traits100%100%20th May 202117th Sept 202127th Aug 2021
Control Flow 2 – Pattern Matching100%%10020th Sept 20219th Dec 202129th Nov 2021
Macros and cfg expansion100100%1st Dec 202128th Mar 2022
Imports and Visibility0%48%+4829th Mar 202227th May 2022
Const Generics0%0%30th May 202225th Jul 2022
Intrinsics0%0%6th Sept 202230th Sept 2022
GitHub Milestones


RiskImpact (1-3)Likelihood (0-10)Risk (I * L)Mitigation
Rust Language Changes3721Keep up to date with the Rust language on a regular basis
Going over target dates3515Maintain status reports and issue tracking to stakeholders

Planned Activities

We plan on continuing to work on the various privacy parts: This includes linting for private items in public settings, using private items outside of their modules or reporting type privacy violations in complex traits and associated types. We will also work on implementing a first working version of metadata exports, which will allow us to start compiling multiple crates together. Finally, a lot of time will also be spent on fixing bugs in various areas of the compiler.

Detailed changelog

Array Constructors

Recently as part of our testing effort to use the rustc testsuite we hit upon a rustc testcase that tries to allocate a 4tb array on the stack. This testcase was designed to detect an out-of-memory case in the rustc compiler rust-lang/rust#66342 we also had this failure in our implementation. The error here is due to the fact for copied array constructors we created a constructor expression of the specified number of elements. This means we create a huge vector in memory at compile time which is inefficent. Though if we follow how the GCC D front-end handles this we can use a loop to initilize the memory and allow the GCC middle-end to optimize this using a memset. The only caveat here is that this is not possible in a const context.

For more information see:

Slices support

We finally got slice generation support merged, this is the extracted code from Rustc libcore 1.49.0. The key thing here is that this test-case exposed lots of bugs in our type resolution system so working through this was pretty key. We are working on a blog post to explain how this works, as slice generation is actually implemented via the trick of unsized method resolution and clever use of libcore. For now please review the code below and you can see the slice generation via passing a range to the array index expression kicks off the array index operator overload for a Range<usize> as the entry point which uses the generic higher ranked trait bound.

// { dg-additional-options "-w" }
extern "rust-intrinsic" {
    pub fn offset<T>(dst: *const T, offset: isize) -> *const T;

struct FatPtr<T> {
    data: *const T,
    len: usize,

union Repr<T> {
    rust: *const [T],
    rust_mut: *mut [T],
    raw: FatPtr<T>,

#[lang = "Range"]
pub struct Range<Idx> {
    pub start: Idx,
    pub end: Idx,

#[lang = "const_slice_ptr"]
impl<T> *const [T] {
    pub const fn len(self) -> usize {
        let a = unsafe { Repr { rust: self }.raw };

    pub const fn as_ptr(self) -> *const T {
        self as *const T

#[lang = "const_ptr"]
impl<T> *const T {
    pub const unsafe fn offset(self, count: isize) -> *const T {
        unsafe { offset(self, count) }

    pub const unsafe fn add(self, count: usize) -> Self {
        unsafe { self.offset(count as isize) }

    pub const fn as_ptr(self) -> *const T {
        self as *const T

const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
    unsafe {
        Repr {
            raw: FatPtr { data, len },

#[lang = "index"]
trait Index<Idx> {
    type Output;

    fn index(&self, index: Idx) -> &Self::Output;

pub unsafe trait SliceIndex<T> {
    type Output;

    unsafe fn get_unchecked(self, slice: *const T) -> *const Self::Output;

    fn index(self, slice: &T) -> &Self::Output;

unsafe impl<T> SliceIndex<[T]> for Range<usize> {
    type Output = [T];

    unsafe fn get_unchecked(self, slice: *const [T]) -> *const [T] {
        unsafe {
            let a: *const T = slice.as_ptr();
            let b: *const T = a.add(self.start);
            slice_from_raw_parts(b, self.end - self.start)

    fn index(self, slice: &[T]) -> &[T] {
        unsafe { &*self.get_unchecked(slice) }

impl<T, I> Index<I> for [T]
    I: SliceIndex<[T]>,
    type Output = I::Output;

    fn index(&self, index: I) -> &I::Output {

fn main() -> i32 {
    let a = [1, 2, 3, 4, 5];
    let b = &a[1..3];


see: https://godbolt.org/z/csn8hMej8

More built-in macros

Our first builtin macro, cfg!, has been moved with the rest of the builtin macros: It can now benefit from the other helper functions we have developed in this module to help consuming tokens and generating AST fragments. Two new macros have also been added:

  1. concat!, which allows the concatenation of literal tokens at compile-time
concat!("hey", 'n', 0, "w"); // expands to "heyn0w"
  1. env!, which allows fetching environment variables during compilation.
let path = env!("PATH");
// expands to the content of the user's path when they invoked `gccrs`

env! is interesting as it is one of the first built-in with an optional extra argument: You can specify an extra error message to display if the variable is not present.

macro_rules! env {
    ($name:expr $(,)?) => { ... };
    ($name:expr, $error_msg:expr $(,)?) => { ... };

This enables us to start looking into properly checking for multiple “matchers” in builtins, and parse and fetch them accordingly.

A lot of built-in macros remain to implement, and we’d love for you to have a go at them if you’re interested! Feel free to drop by our Zulip or ask on GitHub for any question you might have.

Patterns in function parameters

As part of our bug fixing this week we realized we could unify the code paths for handling match arms such that we can support patterns everywhere. There are bugs in code-generation for more complex patterns which need to be fixed but we are correctly name and type resolving them which is the starting point. As our support for Match Expression improves over time so will our support for patterns.

struct Pattern(i32);

fn pattern_as_arg(Pattern(value): Pattern) -> i32 {

fn main() -> i32 {
    pattern_as_arg(Pattern(15)) - 15


We added support for transmute which is an interesting intrinsic to allow the reinterpretation of types. This test case is a snippet from this bug report Rust-GCC/gccrs#1130

mod mem {
    extern "rust-intrinsic" {
        fn size_of<T>() -> usize;
        fn transmute<U, V>(_: U) -> V;

impl u16 {
    fn to_ne_bytes(self) -> [u8; mem::size_of::<Self>()] {
        unsafe { mem::transmute(self) }

pub trait Hasher {
    fn finish(&self) -> u64;

    fn write(&mut self, bytes: &[u8]);

    fn write_u8(&mut self, i: u8) {

    fn write_i8(&mut self, i: i8) {
        self.write_u8(i as u8)

    fn write_u16(&mut self, i: u16) {

    fn write_i16(&mut self, i: i16) {
        self.write_u16(i as u16)

pub struct SipHasher;

impl Hasher for SipHasher {
    fn write(&mut self, msg: &[u8]) {}

    fn finish(&self) -> u64 {

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