Parser clean up. (nushell#16668)

- Rewrite `if matches!(val, pat) { .. }` expressions as `if let pat =
val { .. }`
- Replace `if matches!(..) {..}` with `match` expressions where it makes
code easier to understand
- Use combinator methods instead of imperative code in a few places.
- Slay the
[beast](nushell/nushell@69857c1...c430bcd)

## Release notes summary - What our users need to know
N/A

## Tasks after submitting
N/A

Author: Bahex

crates/nu-cli/src/completions/completer.rs

@@ -394,15 +394,14 @@ impl NuCompleter {
                             //   - "--foo ..a" => ["--foo", "..a"] => "..a"
                             //   - "--foo=..a" => ["--foo", "..a"] => "..a"
                             // - strip placeholder (`a`) if present
-                            let (new_span, prefix) = if matches!(arg, Argument::Named(_)) {
-                                strip_placeholder_with_rsplit(
+                            let (new_span, prefix) = match arg {
+                                Argument::Named(_) => strip_placeholder_with_rsplit(
                                     working_set,
                                     &span,
                                     |b| *b == b'=' || *b == b' ',
                                     strip,
-                                )
-                            } else {
-                                strip_placeholder_if_any(working_set, &span, strip)
+                                ),
+                                _ => strip_placeholder_if_any(working_set, &span, strip),
                             };
                             let ctx = Context::new(working_set, new_span, prefix, offset);
 

crates/nu-cli/src/completions/operator_completions.rs

@@ -229,7 +229,7 @@ impl Completer for OperatorCompletion<'_> {
             Type::Any => match &self.left_hand_side.expr {
                 Expr::FullCellPath(path) => {
                     // for `$ <tab>`
-                    if matches!(path.head.expr, Expr::Garbage) {
+                    if let Expr::Garbage = path.head.expr {
                         return vec![];
                     }
                     let value =

crates/nu-cmd-extra/src/extra/bits/and.rs

@@ -73,7 +73,7 @@ impl Command for BitsAnd {
         };
 
         // This doesn't match explicit nulls
-        if matches!(input, PipelineData::Empty) {
+        if let PipelineData::Empty = input {
             return Err(ShellError::PipelineEmpty { dst_span: head });
         }
 

crates/nu-cmd-extra/src/extra/bits/not.rs

@@ -73,7 +73,7 @@ impl Command for BitsNot {
         let number_size = get_number_bytes(number_bytes, head)?;
 
         // This doesn't match explicit nulls
-        if matches!(input, PipelineData::Empty) {
+        if let PipelineData::Empty = input {
             return Err(ShellError::PipelineEmpty { dst_span: head });
         }
 

crates/nu-cmd-extra/src/extra/bits/or.rs

@@ -74,7 +74,7 @@ impl Command for BitsOr {
         };
 
         // This doesn't match explicit nulls
-        if matches!(input, PipelineData::Empty) {
+        if let PipelineData::Empty = input {
             return Err(ShellError::PipelineEmpty { dst_span: head });
         }
 

crates/nu-cmd-extra/src/extra/bits/rotate_left.rs

@@ -75,7 +75,7 @@ impl Command for BitsRol {
         let number_size = get_number_bytes(number_bytes, head)?;
 
         // This doesn't match explicit nulls
-        if matches!(input, PipelineData::Empty) {
+        if let PipelineData::Empty = input {
             return Err(ShellError::PipelineEmpty { dst_span: head });
         }
 

crates/nu-cmd-extra/src/extra/bits/rotate_right.rs

@@ -75,7 +75,7 @@ impl Command for BitsRor {
         let number_size = get_number_bytes(number_bytes, head)?;
 
         // This doesn't match explicit nulls
-        if matches!(input, PipelineData::Empty) {
+        if let PipelineData::Empty = input {
             return Err(ShellError::PipelineEmpty { dst_span: head });
         }
 

crates/nu-cmd-extra/src/extra/bits/shift_left.rs

@@ -80,7 +80,7 @@ impl Command for BitsShl {
         let number_size = get_number_bytes(number_bytes, head)?;
 
         // This doesn't match explicit nulls
-        if matches!(input, PipelineData::Empty) {
+        if let PipelineData::Empty = input {
             return Err(ShellError::PipelineEmpty { dst_span: head });
         }
 

crates/nu-cmd-extra/src/extra/bits/shift_right.rs

@@ -77,7 +77,7 @@ impl Command for BitsShr {
         let number_size = get_number_bytes(number_bytes, head)?;
 
         // This doesn't match explicit nulls
-        if matches!(input, PipelineData::Empty) {
+        if let PipelineData::Empty = input {
             return Err(ShellError::PipelineEmpty { dst_span: head });
         }
 

crates/nu-cmd-extra/src/extra/bits/xor.rs

@@ -74,7 +74,7 @@ impl Command for BitsXor {
         };
 
         // This doesn't match explicit nulls
-        if matches!(input, PipelineData::Empty) {
+        if let PipelineData::Empty = input {
             return Err(ShellError::PipelineEmpty { dst_span: head });
         }