Add if-then-else statement support

[stringhandler]

Built on #185, so that would need to be merged first.

Addresses #181

[gerau]

Could we make the else expression optional? Maybe we could evaluate a missing else as the unit type (), so this would prevent conflicts when using an if statement in an assignment, for example.

[KyrylR]

#185 is merged, so you can rebase

[uncomputable]

This is an if-then-else expression, by the way.

[apoelstra]

Sorry for the long delay on this. I thought it would be much harder to review than it was, and I had a big pile of notifications to climb out from under. I should be available closer to real-time from here on out.

In ddf467b:

Are we intentionally allowing expressions that are not blocks? For example tests still pass with this diff:

diff --git a/src/ast.rs b/src/ast.rs
index 3c32866..0ca53ba 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -1733,7 +1733,7 @@ mod test {

     #[test]
     fn test_if_expression_with_complex_arms() {
-        let input = "if false { let x: u8 = 5; x } else { 10 }";
+        let input = "if false { let x: u8 = 5; x } else 10";

         let parsed_expr = parse::Expression::parse_from_str(input).expect("Failed to parse");

This seems okay to me, but it does differ from Rust and if it's intentional we should call it out.

[stringhandler]

Oh interesting. I thought that was valid in Rust, but it explicitly requires blocks to avoid mistakes. I think it's worth sticking with Rust's rule of requiring brackets. I'll update it

[stringhandler]

rebased with brace validation

[apoelstra]

Sorry for the delay -- my fuzzer rejected this. But it looks like actually the fuzz failure is on master.

echo 1dVgWywL1VvV1dVAPQAAPTE= | base64 -d > hmm
cargo +nightly fuzz run display_parse_tree hmm

thread '<unnamed>' (2590207) panicked at fuzz/fuzz_targets/display_parse_tree.rs:9:10:
Output of fmt::Display should be parseable: RichError { error: Syntax { expected: ["something else"], label: Some("alias name"), found: Some("fn") }, span: Span { start: 5, end: 7 }, file: None }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[gerau]

Sorry for the delay -- my fuzzer rejected this. But it looks like actually the fuzz failure is on master.

Seems like an issue with lexer -- it trying to parse type fn = ((),);, but fn is reserved keyword, so it can't be used as alias name. When I wrote the lexer, I tried to imitate rust, and it would reject keywords as identifier name.

[stringhandler]

Converting back to draft for now. I just realised that the Value::from_const_expr will not exclude If. Which means they can be included in witness files.