When compiling a pattern match like
```
[] -> ..
[_] -> ..
[_, ..] -> ..
```
to a decision tree, we must make sure that the last test (len >= 1)
does not touch the branch reached by the second test (len == 1). It is
enough to ban (len >=) tests from ever touching exact-sized list
patterns, because a spread test (len >=) can never reach an exact-sized
test.
On the other hand, an exact-sized test can reach a spread pattern,
because in
```
[_, _] -> ..
[..] -> ..
```
the last branch generates tests for patterns `[]` and `[_]`, and we would
like those patterns to be covered by the spread test (len >= 0)!
Closes#4685
This patch provides errors for defs that are used only in
possibly-mutual recursion, and are not reachable outside of their
recursive closures. For example:
```
test_report!(
mutual_recursion_not_reached_nested,
indoc!(
r#"
app "test" provides [main] to "./platform"
main =
f = \{} -> if Bool.true then "" else g {}
g = \{} -> if Bool.true then "" else f {}
""
"#
),
@r###"
── DEFINITIONs ONLY USED IN RECURSION ──────────────────── /code/proj/Main.roc ─
These 2 definitions are only used in mutual recursion with themselves:
4│> f = \{} -> if Bool.true then "" else g {}
5│> g = \{} -> if Bool.true then "" else f {}
If you don't intend to use or export any of them, they should all be
removed!
"###
);
```
Previously, a program like
```
main =
f =
n = 1
\{} -[#lam]-> n # suppose lambda set = #lam
f {}
```
would be transformed to
```
main =
n = 1
f = \{} -[#lam]-> n
f {}
```
However, the IR lowering procedure is such that we would then associate
`f` as definining the procedure given the lambda set `#lam`. This is not
correct, as `f` is really a function pointer in this circumstance,
rather than the definer of `#lam`.
Instead, the transformation we want to perform is
```
main =
n = 1
#lam = \{} -[#lam]-> n
f = #lam
f {}
```
Which is what this patch does
Closes#2403
Previously this was a bit hacky, we worked around having to update
symbol substitutions by special-casing the rest of the program. There's
not need to do that.
