feat: add a warning when using unsafe blocks without safety comments

[guipublic]

Description

Problem*

Resolves #6478

Summary*

Warn when an unsafe block does not start with a '//@safety' comment.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Peak Memory Sample

Program	Peak Memory
keccak256	78.57M
workspace	123.76M
regression_4709	423.68M
ram_blowup_regression	1.58G
private-kernel-tail	207.31M
private-kernel-reset	720.90M
private-kernel-inner	292.52M
parity-root	172.27M

[github-actions]

Compilation Sample

Program	Compilation Time	%
sha256_regression	0m1.400s	-4%
regression_4709	0m0.831s	2%
ram_blowup_regression	0m14.792s	-3%
rollup-base-public	3m26.617s	-2%
rollup-base-private	2m58.295s	-5%
private-kernel-tail	0m1.276s	9%
private-kernel-reset	0m8.194s	10%
private-kernel-inner	0m2.481s	-11%
parity-root	0m0.881s	-12%
noir-contracts	2m45.952s	1%

[github-actions]

Execution Sample

Program	Execution Time	%
sha256_regression	0m0.627s	-1%
regression_4709	0m0.397s	0%
ram_blowup_regression	0m4.342s	-1%
rollup-base-public	0m27.787s	-2%
rollup-base-private	0m25.891s	-2%
private-kernel-tail	0m0.708s	-7%
private-kernel-reset	0m1.482s	1%
private-kernel-inner	0m1.010s	2%
parity-root	0m0.574s	4%

[github-actions]

🚀 Deployed on https://6764601f16d2690dfcb11c81--noir-docs.netlify.app

[michaeljklein]

LGTM besides some docs/nits

[asterite]

Have you considered alternatives to a new comment type?

One alternative is using doc comments, either on top of unsafe:

/// This is a safety comment
unsafe { ... }

or inside it:

unsafe {
    /// This is a safety comment
}

Mainly because those doc comments already reach the parser and go into the AST (in some cases).

[guipublic]

Have you considered alternatives to a new comment type?

One alternative is using doc comments, either on top of unsafe:

/// This is a safety comment
unsafe { ... }

or inside it:

unsafe {
    /// This is a safety comment
}

Mainly because those doc comments already reach the parser and go into the AST (in some cases).

Yes I did:

• The comment on top of unsafe is harder to implement because the parser does not expect to have any comment left inside blocks, and I have don't have much preference regarding top vs inside.
• I am already using doc comments (but with a specific 'style'), so I believe it would be easy to switch to normal block comments. However I was thinking their semantic is a bit different.

So basically, I don't have strong opinion on this, I'm happy to change if you think it would be better.

[github-actions]

FYI @noir-lang/developerrelations on Noir doc changes.

[michaeljklein]

LGTM