Emit unreachable arm warning for OR match arms (#6505)

## Description

This PR fixes #6388. If a whole OR match arm is unreachable a warning
will be emitted like in this case:

```Sway
match v {
    1 => (),
    2 => (),
    1 | 2 => (), <<<--- Warning emitted here.
    _ => (),
};
```

The PR strictly fixes the issue reported in #6388. It does not fix a
previously known, more general issue, described in #5097 where we expect
warnings to be emitted also on individual elements of an OR arm, even if
the whole arm is reachable. E.g.:

```Sway
match v {
    1 => (),
    2 => (),
    1 | 3 => (), <<<--- Warning should be emitted here that `1` is not reachable.
    2 | 4 => (), <<<--- Warning should be emitted here that `2` is not reachable.
    _ => (),
};
```

Fixing this issue requires more invasive restructuring of the
propagation of the collected witness information and will be done in a
separate PR.

Closes #6388.

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [ ] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [ ] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

sway-core/src/semantic_analysis/ast_node/expression/match_expression/analysis/usefulness.rs

@@ -129,7 +129,7 @@ use super::{
 /// `2`, any cases that the pattern `2` covers would previously be caught by the
 /// catchall pattern.
 ///
-/// Usefulness used in the exhaustivity algorithm.
+/// Usefulness is used in the exhaustivity algorithm.
 ///
 /// # Witnesses
 ///
@@ -148,11 +148,11 @@ use super::{
 /// }
 /// ```
 ///
-/// the witness for pattern `1` would be the pattern "1" as the pattern `1`
+/// the witness for pattern `1` would be the value "1" as the pattern `1`
 /// would catch the concrete hypothetical matched value "1" and no other
 /// previous cases would have caught it. The witness for pattern `_` is an
 /// or-pattern of all of the remaining integers they wouldn't be caught by `0`
-/// and `1`, so "3 | .. | MAX".
+/// and `1`, so "2 | .. | MAX".
 ///
 /// Given:
 ///
@@ -166,7 +166,7 @@ use super::{
 /// }
 /// ```
 ///
-/// the pattern `1` (noted with an arrow) would not have any witnesses as there
+/// the pattern `1` (noted with an arrow) would not have any witnesses
 /// that it catches that are not caught by previous patterns.
 ///
 /// # Putting it all together
@@ -205,7 +205,7 @@ pub(crate) fn check_match_expression_usefulness(
     let mut matrix = Matrix::empty();
     let mut arms_reachability = vec![];
 
-    // If the provided type does has no valid constructor and there are no
+    // If the provided type does not have a valid constructor and there are no
     // branches in the match expression (i.e. no scrutinees to check), then
     // every scrutinee (i.e. 0 scrutinees) are useful! We return early in this
     // case.
@@ -489,7 +489,7 @@ fn is_useful_or(
 ) -> Result<WitnessReport, ErrorEmitted> {
     let (_, q_rest) = q.split_first(handler, span)?;
     let mut p = p.clone();
-    let mut witness_report = WitnessReport::Witnesses(PatStack::empty());
+    let mut witness_report = WitnessReport::NoWitnesses;
     for pat in pats.into_iter() {
         // 1. For each *k* 0..*a* compute *q'* as \[*rₖ q₂ ... qₙ*\].
         let mut v = PatStack::from_pattern(pat);

test/src/e2e_vm_tests/test_programs/should_pass/language/match_expressions_unreachable_or/Forc.lock

@@ -0,0 +1,8 @@
+[[package]]
+name = "core"
+source = "path+from-root-7FA7C6C8132DFBE7"
+
+[[package]]
+name = "match_expressions_unreachable_or"
+source = "member"
+dependencies = ["core"]

test/src/e2e_vm_tests/test_programs/should_pass/language/match_expressions_unreachable_or/Forc.toml

@@ -0,0 +1,8 @@
+[project]
+authors = ["Fuel Labs <[email protected]>"]
+license = "Apache-2.0"
+name = "match_expressions_unreachable_or"
+entry = "main.sw"
+
+[dependencies]
+core = { path = "../../../../../../../sway-lib-core" }

test/src/e2e_vm_tests/test_programs/should_pass/language/match_expressions_unreachable_or/src/main.sw

@@ -0,0 +1,42 @@
+// This test proves that https://github.com/FuelLabs/sway/issues/6388 is fixed.
+script;
+
+struct S {
+    x: bool
+}
+
+fn main() {
+    let v = 0u64;
+    // We want to have only one error in the case below,
+    // for the whole or-pattern elements and not for individual
+    // parts.
+    match v {
+        1 => (),
+        1 | 1 => (),
+        _ => (),
+    };
+
+    match v {
+        1 => (),
+        2 => (),
+        1 | 2 => (),
+        _ => (),
+    };
+
+    // TODO: Once https://github.com/FuelLabs/sway/issues/5097 is fixed, the below examples
+    //       will also emit warnings for unreacahbility of individual or-pattern elements.
+    //       Extend filecheck checks to cover those warning.
+
+    match v {
+        1 => (),
+        2 => (),
+        1 | 3 => (),
+        2 | 4 => (),
+        _ => (),
+    };
+
+    let s = S { x: false };
+    let _x = match s {
+        S { x } | S { x } => { x },
+    };
+}

test/src/e2e_vm_tests/test_programs/should_pass/language/match_expressions_unreachable_or/test.toml

@@ -0,0 +1,16 @@
+category = "compile"
+expected_warnings = 2
+
+#check: $()warning
+#sameln: $()Match arm is unreachable
+#check: $()1 => (),
+#nextln: $()Preceding match arms already match all the values that `1 | 1` can match.
+#check: $()1 | 1 => (),
+#nextln: $()Match arm `1 | 1` is unreachable.
+
+#check: $()warning
+#sameln: $()Match arm is unreachable
+#check: $()2 => (),
+#nextln: $()Preceding match arms already match all the values that `1 | 2` can match.
+#check: $()1 | 2 => (),
+#nextln: $()Match arm `1 | 2` is unreachable.