thread 'rustc' panicked at 'unexpected components in a float token

[Badel2]

I'm seeing an internal compiler error on the following input, found by fuzz-rustc:

Code

fn main() {
    a.5.2E+
}

Error output

error: expected at least one digit in exponent
 --> src/main.rs:2:7
  |
2 |     a.5.2E+
  |       ^^^^^

thread 'rustc' panicked at 'unexpected components in a float token: [IdentLike("5"), Punct('.'), IdentLike("2E"), Punct('+')]', compiler/rustc_parse/src/parser/expr.rs:1041:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The ICE happens after correctly reporting a malformed float error, so low priority. Affected versions: probably all versions since that panic was introduced, I tried nightly 2021-11-07 and stable 1.56.1 and they both show the same ICE.

Backtrace

thread 'rustc' panicked at 'unexpected components in a float token: [IdentLike("5"), Punct('.'), IdentLike("2E"), Punct('+')]', compiler/rustc_parse/src/parser/expr.rs:1041:18
stack backtrace:
   0: rust_begin_unwind
             at /rustc/46b8e7488eae116722196e8390c1bd2ea2e396cf/library/std/src/panicking.rs:498:5
   1: core::panicking::panic_fmt
             at /rustc/46b8e7488eae116722196e8390c1bd2ea2e396cf/library/core/src/panicking.rs:106:14
   2: <rustc_parse::parser::Parser>::parse_dot_or_call_expr_with_
   3: <rustc_parse::parser::Parser>::parse_stmt_path_start
   4: <rustc_parse::parser::Parser>::parse_stmt_without_recovery
   5: <rustc_parse::parser::Parser>::parse_block_tail
   6: <rustc_parse::parser::Parser>::parse_fn_body
   7: <rustc_parse::parser::Parser>::parse_item_common
   8: <rustc_parse::parser::Parser>::parse_mod
   9: rustc_parse::parse_crate_from_file
  10: <rustc_session::session::Session>::time::<core::result::Result<rustc_ast::ast::Crate, rustc_errors::diagnostic_builder::DiagnosticBuilder>, rustc_interface::passes::parse::{closure#0}>
  11: rustc_interface::passes::parse
  12: <rustc_interface::queries::Queries>::parse
  13: <rustc_interface::interface::Compiler>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_errors::ErrorReported>>
  14: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorReported>, rustc_interface::interface::create_compiler_and_run<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>::{closure#0}>
  15: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorReported>, rustc_driver::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorReported>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_errors::ErrorReported>>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

A quick inspection of the code shows that the similar case 1e+ is handled the same as 1e+2:

rust/compiler/rustc_parse/src/parser/expr.rs

Lines 1031 to 1041 in 87df4dd

	// 1e+ | 1e- (recovered)
	[IdentLike(_), Punct('+' | '-')] |
	// 1e+2 | 1e-2
	[IdentLike(_), Punct('+' | '-'), IdentLike(_)] |
	// 1.2e+3 | 1.2e-3
	[IdentLike(_), Punct('.'), IdentLike(_), Punct('+' | '-'), IdentLike(_)] => {
	// See the FIXME about `TokenCursor` above.
	self.error_unexpected_after_dot();
	base
	}
	_ => panic!("unexpected components in a float token: {:?}", components),

So if I had to guess I would say that the match is missing this one case, so 1.2e+ should be handled the same as 1.2e+3:

            // 1.2e+ | 1.2e- (recovered)
            [IdentLike(_), Punct('+' | '-'), IdentLike(_), Punct('+' | '-')] |

But no idea what does that code even do, it has a FIXME by @petrochenkov so maybe he knows better.

[terrarier2111]

@rustbot claim