rustc stack overflow with self-recursive concat! macro

[Nemo157]

I tried this code:

#[macro_export]
macro_rules! n {() => (concat!("", n!()))}

fn main () {
    n!();
}

I expected to see this happen: An error message about exceeding some recursion limit

Instead, this happened:

thread 'rustc' has overflowed its stack
fatal runtime error: stack overflow

Meta

Tested on stable 1.51.0 + 1.53.0-nightly (2021-04-26 9684258936dabda2ba49)

[camelid]

Note that the crash also occurs without #[macro_export].

Based on testing with Godbolt, this doesn't seem to be a regression, or at least not a recent one.

[hameerabbasi]

Assigning P-medium as part of the prioritization working group discussion on Zulip and removing I-prioritize.

@rustbot modify labels -I-prioritize +P-medium

[fee1-dead]

Issue might be in this line:

rust/compiler/rustc_builtin_macros/src/concat.rs

Line 13 in 50ca3ac

let es = match base::get_exprs_from_tts(cx, sp, tts) {

Calls get_exprs_from_tts, which then calls fully_expand_fragment:

rust/compiler/rustc_expand/src/base.rs

Line 1191 in ba6275b

let expr = cx.expander().fully_expand_fragment(AstFragment::Expr(expr)).make_expr();

And sets current_expansion.depth to 0, which would result in stack overflow because the recursion limit is never reached.

rust/compiler/rustc_expand/src/expand.rs

Line 427 in ba6275b

self.cx.current_expansion.depth = 0;

[fee1-dead]

@rustbot claim

[fee1-dead]

Bug is confusing to track down, so I am no longer working on this.

For people who are interested, here is the log when the loop is encountered:

DEBUG rustc_resolve resolve_crate_root($crate#105)
DEBUG rustc_span::hygiene marks: getting parent of #105
DEBUG rustc_resolve resolve_crate_root: marks=[(ExpnData { kind: Macro(Bang, "n"), parent: ExpnId(2), call_site: issue-84632-recursive-builtin-macro.rs:1:36: 1:40 (#104), def_site: issue-84632-recursive-builtin-macro.rs:1:1: 1:43 (#0), allow_internal_unstable: None, allow_internal_unsafe: false, local_inner_macros: false, edition: Edition2015, macro_def_id: Some(DefId(0:3)), krate: crate0, orig_id: Some(206), disambiguator: 0 }, SemiTransparent)]
DEBUG rustc_span::hygiene marks: getting parent of #105
DEBUG rustc_resolve resolve_crate_root: found opaque mark None None
DEBUG rustc_resolve resolve_crate_root: found semi-transparent mark Some(ExpnId(206)) Some(ExpnData { kind: Macro(Bang, "n"), parent: ExpnId(2), call_site: issue-84632-recursive-builtin-macro.rs:1:36: 1:40 (#104), def_site: issue-84632-recursive-builtin-macro.rs:1:1: 1:43 (#0), allow_internal_unstable: None, allow_internal_unsafe: false, local_inner_macros: false, edition: Edition2015, macro_def_id: Some(DefId(0:3)), krate: crate0, orig_id: Some(206), disambiguator: 0 })
DEBUG rustc_resolve resolve_crate_root($crate#105): got module Some(Def(Mod, DefId(0:0))) (Some("")) (ident.span = issue-84632-recursive-builtin-macro.rs:1:1: 1:1 (#105))
DEBUG rustc_metadata::rmeta::table Table::lookup: index=DefIndex(18) len=192232
DEBUG rustc_parse::parser::attr parse_outer_attributes: self.token=Token { kind: Literal(Lit { kind: Str, symbol: "", suffix: None }), span: issue-84632-recursive-builtin-macro.rs:1:32: 1:34 (#105) }
DEBUG rustc_parse::parser::attr parse_outer_attributes: self.token=Token { kind: Ident("n", false), span: issue-84632-recursive-builtin-macro.rs:1:36: 1:37 (#105) }
DEBUG rustc_parse::parser::diagnostics check_trailing_angle_brackets: parsed_angle_bracket_args=false
DEBUG rustc_parse::parser::attr parse_outer_attributes: self.token=Token { kind: Ident("concat", false), span: issue-84632-recursive-builtin-macro.rs:1:24: 1:30 (#106) }
DEBUG rustc_parse::parser::diagnostics check_trailing_angle_brackets: parsed_angle_bracket_args=false
DEBUG rustc_resolve resolve_crate_root($crate#106)
DEBUG rustc_span::hygiene marks: getting parent of #106
DEBUG rustc_resolve resolve_crate_root: marks=[(ExpnData { kind: Macro(Bang, "n"), parent: ExpnId(2), call_site: issue-84632-recursive-builtin-macro.rs:1:36: 1:40 (#105), def_site: issue-84632-recursive-builtin-macro.rs:1:1: 1:43 (#0), allow_internal_unstable: None, allow_internal_unsafe: false, local_inner_macros: false, edition: Edition2015, macro_def_id: Some(DefId(0:3)), krate: crate0, orig_id: Some(208), disambiguator: 0 }, SemiTransparent)]
DEBUG rustc_span::hygiene marks: getting parent of #106
DEBUG rustc_resolve resolve_crate_root: found opaque mark None None
DEBUG rustc_resolve resolve_crate_root: found semi-transparent mark Some(ExpnId(208)) Some(ExpnData { kind: Macro(Bang, "n"), parent: ExpnId(2), call_site: issue-84632-recursive-builtin-macro.rs:1:36: 1:40 (#105), def_site: issue-84632-recursive-builtin-macro.rs:1:1: 1:43 (#0), allow_internal_unstable: None, allow_internal_unsafe: false, local_inner_macros: false, edition: Edition2015, macro_def_id: Some(DefId(0:3)), krate: crate0, orig_id: Some(208), disambiguator: 0 })
DEBUG rustc_resolve resolve_crate_root($crate#106): got module Some(Def(Mod, DefId(0:0))) (Some("")) (ident.span = issue-84632-recursive-builtin-macro.rs:1:1: 1:1 (#106))
DEBUG rustc_metadata::rmeta::table Table::lookup: index=DefIndex(18) len=192232
DEBUG rustc_parse::parser::attr parse_outer_attributes: self.token=Token { kind: Literal(Lit { kind: Str, symbol: "", suffix: None }), span: issue-84632-recursive-builtin-macro.rs:1:32: 1:34 (#106) }
DEBUG rustc_parse::parser::attr parse_outer_attributes: self.token=Token { kind: Ident("n", false), span: issue-84632-recursive-builtin-macro.rs:1:36: 1:37 (#106) }
DEBUG rustc_parse::parser::diagnostics check_trailing_angle_brackets: parsed_angle_bracket_args=false
DEBUG rustc_parse::parser::attr parse_outer_attributes: self.token=Token { kind: Ident("concat", false), span: issue-84632-recursive-builtin-macro.rs:1:24: 1:30 (#107) }
DEBUG rustc_parse::parser::diagnostics check_trailing_angle_brackets: parsed_angle_bracket_args=false
DEBUG rustc_resolve resolve_crate_root($crate#107)
DEBUG rustc_span::hygiene marks: getting parent of #107
DEBUG rustc_resolve resolve_crate_root: marks=[(ExpnData { kind: Macro(Bang, "n"), parent: ExpnId(2), call_site: issue-84632-recursive-builtin-macro.rs:1:36: 1:40 (#106), def_site: issue-84632-recursive-builtin-macro.rs:1:1: 1:43 (#0), allow_internal_unstable: None, allow_internal_unsafe: false, local_inner_macros: false, edition: Edition2015, macro_def_id: Some(DefId(0:3)), krate: crate0, orig_id: Some(210), disambiguator: 0 }, SemiTransparent)]
DEBUG rustc_span::hygiene marks: getting parent of #107

@rustbot release-assignment

[fee1-dead]

The issue is part of a bigger problem, that concat! seems to ignore the recursion limit.
This snippet below compiles successfully instead of reporting an error: Playground

#![recursion_limit = "5"]
macro_rules! a {
    () => ("");
    (A) => (concat!("", a!()));
    (A, $($A:ident),*) => (concat!("", a!($($A),*)))
}

fn main() { 
    a! (A, A, A, A, A, A, A, A, A, A);
}