format_args! does not respect integer literals' type

[endorpersand]

When using the 1234i8 syntax in the Display format in formatting macros, the i8 is ignored and treated as a u128.

I tried this code:

fn main() {
    println!("{}", 0x8Fi8);
}

I expected to see this happen: Getting an error that 0x8F is out of the bounds for type i8.

Instead, this happened: This function compiles and prints 143.

Meta

rustc --version --verbose:

rustc 1.71.1 (eb26296b5 2023-08-03)
binary: rustc
commit-hash: eb26296b556cef10fb713a38f3d16b9886080f26
commit-date: 2023-08-03
host: x86_64-apple-darwin
release: 1.71.1
LLVM version: 16.0.5

[konnorandrews]

I went and looked through the format_args!() implementation and found where the issue comes from. The check at https://github.com/rust-lang/rust/blob/master/compiler/rustc_ast_lowering/src/format.rs#L131 ignores the integer type, and it blindly calls n.string() on the u128 value of the literal.

[gurry]

@rustbot claim

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium

[apiraino]

Did run a bisection on the reproducible provided (and duplicated #116631)

Regression in nightly-2023-04-22, specifically I think commit 8bdcc62 or (as pointed out in PR #116633) #106824

found 11 bors merge commits in the specified range
commit[0] 2023-04-20: Auto merge of #110616 - m-ou-se:fmt-lang-items, r=jyn514
commit[1] 2023-04-20: Auto merge of #109999 - m-ou-se:flatten-format-args, r=oli-obk
commit[2] 2023-04-21: Auto merge of #110370 - c410-f3r:dqewdas, r=petrochenkov
commit[3] 2023-04-21: Auto merge of #110431 - jsoref:spelling-src-etc, r=Mark-Simulacrum
commit[4] 2023-04-21: Auto merge of #110636 - matthiaskrgr:rollup-faa33c6, r=matthiaskrgr
commit[5] 2023-04-21: Auto merge of #96840 - cjgillot:query-feed, r=oli-obk
commit[6] 2023-04-21: Auto merge of #109002 - michaelvanstraten:master, r=petrochenkov
commit[7] 2023-04-21: Auto merge of #110542 - petrochenkov:qcstore4, r=cjgillot
commit[8] 2023-04-21: Auto merge of #110569 - saethlin:mir-pass-cooperation, r=cjgillot
commit[9] 2023-04-21: Auto merge of #110107 - cjgillot:const-prop-lint-junk, r=oli-obk
commit[10] 2023-04-21: Auto merge of #110648 - Dylan-DPC:rollup-em3ovcq, r=Dylan-DPC

cc @m-ou-se

[m-ou-se]

Looks like it's not easy invoke the same integer-literal-range-checking code from the place where format_args!() is expanded. So the easiest solution is probably to expand to something like { let _ = literal; let _ = literal; fmt::Arguments::new(..) } when literals are removed, such that the literals are still be checked later.

[m-ou-se]

Actually, looks like there is a much easier fix that mostly just involves removing code. ^^'

Will send a PR soon, probably tomorrow.

[m-ou-se]

Fix: #118659