Rust: Improve CFG for let expressions

[hvitved]

Example:

    fn test_and_if_let(a: bool, b: Option<bool>, c: bool) -> bool {
        if a && let Some(d) = b {
            d
        } else {
            false
        }
    }

CFG before:

flowchart TD
1["enter test_and_if_let"]
10["BlockExpr"]
11["IfExpr"]
12["a"]
13["... && ..."]
14["[boolean(false)] ... && ..."]
15["LetExpr"]
16["TupleStructPat"]
17["d"]
18["b"]
19["BlockExpr"]
2["exit test_and_if_let"]
20["d"]
21["BlockExpr"]
22["false"]
3["exit test_and_if_let (normal)"]
4["a"]
5["Param"]
6["b"]
7["Param"]
8["c"]
9["Param"]

1 --> 4
3 --> 2
4 -- match --> 5
5 --> 6
6 -- match --> 7
7 --> 8
8 -- match --> 9
9 --> 12
10 --> 3
11 --> 10
12 -- false --> 14
12 -- true --> 15
13 -- false --> 22
13 -- true --> 20
14 -- false --> 22
15 --> 18
16 -- match --> 17
16 -- no-match --> 13
17 -- match --> 13
18 --> 16
19 --> 11
20 --> 19
21 --> 11
22 --> 21

Loading

CFG after:

flowchart TD
1["enter test_and_if_let"]
10["BlockExpr"]
11["IfExpr"]
12["a"]
13["[boolean(false)] ... && ..."]
14["[boolean(true)] ... && ..."]
15["LetExpr"]
16["TupleStructPat"]
17["d"]
18["b"]
19["BlockExpr"]
2["exit test_and_if_let"]
20["d"]
21["BlockExpr"]
22["false"]
3["exit test_and_if_let (normal)"]
4["a"]
5["Param"]
6["b"]
7["Param"]
8["c"]
9["Param"]

1 --> 4
3 --> 2
4 -- match --> 5
5 --> 6
6 -- match --> 7
7 --> 8
8 -- match --> 9
9 --> 12
10 --> 3
11 --> 10
12 -- false --> 13
12 -- true --> 15
13 -- false --> 22
14 -- true --> 20
15 --> 18
16 -- match --> 17
16 -- no-match --> 13
17 -- match --> 14
18 --> 16
19 --> 11
20 --> 19
21 --> 11
22 --> 21

Loading

[paldepind]

Do we want to support this? Using let as expressions is experimental and not stable syntax.

When I was looking at CFG inconsistencies in the projects we have on DCA the Rust compiler was the only project using this syntax.

[hvitved]

Do we want to support this? Using let as expressions is experimental and not stable syntax.

When I was looking at CFG inconsistencies in the projects we have on DCA the Rust compiler was the only project using this syntax.

Oh, I had no idea that this was experimental. Given the simplicity of supporting it, I think we should do it.

[paldepind]

Oh, I had no idea that this was experimental. Given the simplicity of supporting it, I think we should do it.

The Rust compiler complains about it by default. There's an issue tracking the feature here. But yes, if it is simple then it won't hurt to support. And it should fix quite a few CFG inconsistencies on the Rust compiler.

[paldepind]

Here's another example that I think results in a dead-end. As far as I recall, when I looked at the Rust code base, this pattern of starting with let and then having some conditions was by far the most common.

Suggested change

	fn test_and_if_let3(a: bool, b: Option<i64>, c: bool) -> bool {
	if let Some(d) = b && c && c {
	d > 0
	} else {
	false
	}
	}

[hvitved]

Let's perhaps do that in a follow-up PR?

[paldepind]

I don't think there's much value in adding partial support for let expressions without handling that (most common) case as well.

Is that change orthogonal to the one in this PR? If yes, then doing it in a follow up PR seems fine, otherwise I think I'd be easier to review the changes together.

[hvitved]

Yes, it's orthogonal to this PR; this PR merely improves CFG splitting a bit (see example in the PR description).

[paldepind]

Ok, great :)

[paldepind]

Suggested change

	if a && let d = b && c{
	if a && let d = b && c {