Warn about C-style octal literals

[GrigorenkoPV]

Uplifts clippy::zero_prefixed_literal as leading_zeros_in_decimal_literals, warn-by-default.

---

The leading_zeros_in_decimal_literals lint detects decimal integral literals with leading zeros.

Example

fn is_executable(unix_mode: u32) -> bool {
    unix_mode & 0111 != 0

Explanation

In some languages (including the infamous C language and most of its family), a leading zero marks an octal constant. In Rust however, a 0o prefix is used instead.
Thus, a leading zero can be confusing for both the writer and a reader.

In Rust:

fn main() {
    let a = 0123;
    println!("{}", a);
}

prints 123, while in C:

#include <stdio.h>

int main() {
    int a = 0123;
    printf("%d\n", a);
}

prints 83 (as 83 == 0o123 while 123 == 0o173).

---

Notes

Compared to the Clippy lint the only difference in behavior is that the suggestion to remove leading zeros from unambiguously decimal literals (those with 8's or 9's in them) is marked as machine-applicable.

So, the current approach thus is:

• If a non-zero decimal literal starts with a 0, issue a warning.
• Suggest to remove leading zeros.
  • If the literal cannot possibly be octal, mark the suggestion as machine-applicable.
  • If it can, mark the suggestion as maybe-incorrect and also maybe-incorrectly suggest to use 0o.

Alternative approaches to consider:

1. Only lint ambiguous cases (do not warn about literals with 8's or 9's in them at all)
2. Only lint ambiguous cases where it makes a difference (at least 2 significant digits, none of them are 8 or 9).

Closes #33448
r? @ghost

[GrigorenkoPV]

@rustbot ready

r? @Urgau

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[Urgau]

This looks fine to me. Let's nominate it for T-lang fcp (which could unfortunatly take months, due to T-lang backlog).

I've taken the liberty to adjust the PR description to include the lint description as well, in order to clearly see what the lint does. Feel free to update it if you update the one in the PR.

@rustbot labels -S-waiting-on-review +S-waiting-on-team +I-lang-nominated

[Noratrieb]

Adding easy decision as I expect this to be accepted without much discussion, I was honestly surprised to see that this didn't exist already in rustc.

[joshtriplett]

Starting a lang FCP for this.

@rfcbot merge

[rfcbot]

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[traviscross]

I don't know. This still feels maybe more like a clippy lint to me. That is, it seems maybe odd to lint against this in Rust when it's entirely unambiguous in our language, and when I've intentionally written decimals with leading zeros for reasons that are spiritually similar to why we might write hexidecimals or binary literals with leading zeros.

Perhaps if we had a 0d42 syntax for being explicit it's a decimal literal, the case here would seem stronger to me, as then at least you could switch to using that syntax rather than just having to allow the warning.