Add rest and ignore-rest patterns (#8681)

# Description

Adds two more patterns when working with lists:

```
[1, ..$remainder]
```
and
```
[1, ..]
```
The first one collects the remaining items and assigns them into the
variable. The second one ignores any remaining values.

# User-Facing Changes

Adds more capability to list pattern matching.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass
- `cargo run -- crates/nu-utils/standard_library/tests.nu` to run the
tests for the standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/tests/commands/match_.rs

@@ -55,6 +55,28 @@ fn match_list() {
     assert_eq!(actual.out, "double: 1 2");
 }
 
+#[test]
+fn match_list_rest_ignore() {
+    let actual = nu!(
+        cwd: ".",
+        r#"match [1, 2] { [$a, ..] => { print $"single: ($a)" }, [$b, $c] => {print $"double: ($b) ($c)"}}"#
+    );
+    // Make sure we don't see any of these values in the output
+    // As we do not auto-print loops anymore
+    assert_eq!(actual.out, "single: 1");
+}
+
+#[test]
+fn match_list_rest() {
+    let actual = nu!(
+        cwd: ".",
+        r#"match [1, 2, 3] { [$a, ..$remainder] => { print $"single: ($a) ($remainder | math sum)" }, [$b, $c] => {print $"double: ($b) ($c)"}}"#
+    );
+    // Make sure we don't see any of these values in the output
+    // As we do not auto-print loops anymore
+    assert_eq!(actual.out, "single: 1 5");
+}
+
 #[test]
 fn match_constant_1() {
     let actual = nu!(

crates/nu-parser/src/flatten.rs

@@ -515,6 +515,9 @@ pub fn flatten_pattern(match_pattern: &MatchPattern) -> Vec<(Span, FlatShape)> {
         Pattern::IgnoreValue => {
             output.push((match_pattern.span, FlatShape::Nothing));
         }
+        Pattern::IgnoreRest => {
+            output.push((match_pattern.span, FlatShape::Nothing));
+        }
         Pattern::List(items) => {
             if let Some(first) = items.first() {
                 if let Some(last) = items.last() {
@@ -559,6 +562,9 @@ pub fn flatten_pattern(match_pattern: &MatchPattern) -> Vec<(Span, FlatShape)> {
         Pattern::Variable(_) => {
             output.push((match_pattern.span, FlatShape::Variable));
         }
+        Pattern::Rest(_) => {
+            output.push((match_pattern.span, FlatShape::Variable));
+        }
         Pattern::Or(patterns) => {
             for pattern in patterns {
                 output.extend(flatten_pattern(pattern));

crates/nu-parser/src/parse_patterns.rs

@@ -1,7 +1,7 @@
 use nu_protocol::{
     ast::{Expr, Expression, MatchPattern, Pattern},
     engine::StateWorkingSet,
-    Span, SyntaxShape, Type,
+    Span, SyntaxShape, Type, VarId,
 };
 
 use crate::{
@@ -72,32 +72,34 @@ pub fn parse_pattern(
     }
 }
 
-pub fn parse_variable_pattern(
-    working_set: &mut StateWorkingSet,
-    span: Span,
-) -> (MatchPattern, Option<ParseError>) {
+fn parse_variable_pattern_helper(working_set: &mut StateWorkingSet, span: Span) -> Option<VarId> {
     let bytes = working_set.get_span_contents(span);
 
     if is_variable(bytes) {
         if let Some(var_id) = working_set.find_variable_in_current_frame(bytes) {
-            (
-                MatchPattern {
-                    pattern: Pattern::Variable(var_id),
-                    span,
-                },
-                None,
-            )
+            Some(var_id)
         } else {
             let var_id = working_set.add_variable(bytes.to_vec(), span, Type::Any, false);
 
-            (
-                MatchPattern {
-                    pattern: Pattern::Variable(var_id),
-                    span,
-                },
-                None,
-            )
+            Some(var_id)
         }
+    } else {
+        None
+    }
+}
+
+pub fn parse_variable_pattern(
+    working_set: &mut StateWorkingSet,
+    span: Span,
+) -> (MatchPattern, Option<ParseError>) {
+    if let Some(var_id) = parse_variable_pattern_helper(working_set, span) {
+        (
+            MatchPattern {
+                pattern: Pattern::Variable(var_id),
+                span,
+            },
+            None,
+        )
     } else {
         (
             garbage(span),
@@ -143,10 +145,39 @@ pub fn parse_list_pattern(
 
             if let LiteElement::Command(_, command) = arg {
                 while spans_idx < command.parts.len() {
-                    let (arg, err) = parse_pattern(working_set, command.parts[spans_idx]);
-                    error = error.or(err);
+                    let contents = working_set.get_span_contents(command.parts[spans_idx]);
+                    if contents == b".." {
+                        args.push(MatchPattern {
+                            pattern: Pattern::IgnoreRest,
+                            span: command.parts[spans_idx],
+                        });
+                        break;
+                    } else if contents.starts_with(b"..$") {
+                        if let Some(var_id) = parse_variable_pattern_helper(
+                            working_set,
+                            Span::new(
+                                command.parts[spans_idx].start + 2,
+                                command.parts[spans_idx].end,
+                            ),
+                        ) {
+                            args.push(MatchPattern {
+                                pattern: Pattern::Rest(var_id),
+                                span: command.parts[spans_idx],
+                            });
+                            break;
+                        } else {
+                            args.push(garbage(command.parts[spans_idx]));
+                            error = error.or(Some(ParseError::Expected(
+                                "valid variable name".into(),
+                                command.parts[spans_idx],
+                            )));
+                        }
+                    } else {
+                        let (arg, err) = parse_pattern(working_set, command.parts[spans_idx]);
+                        error = error.or(err);
 
-                    args.push(arg);
+                        args.push(arg);
+                    };
 
                     spans_idx += 1;
                 }

crates/nu-parser/src/parser.rs

@@ -6237,7 +6237,8 @@ pub fn discover_captures_in_pattern(pattern: &MatchPattern, seen: &mut Vec<VarId
                 discover_captures_in_pattern(pattern, seen)
             }
         }
-        Pattern::Value(_) | Pattern::IgnoreValue | Pattern::Garbage => {}
+        Pattern::Rest(var_id) => seen.push(*var_id),
+        Pattern::Value(_) | Pattern::IgnoreValue | Pattern::IgnoreRest | Pattern::Garbage => {}
     }
 }
 

crates/nu-protocol/src/ast/match_pattern.rs

@@ -23,6 +23,8 @@ pub enum Pattern {
     Value(Expression),
     Variable(VarId),
     Or(Vec<MatchPattern>),
+    Rest(VarId), // the ..$foo pattern
+    IgnoreRest,  // the .. pattern
     IgnoreValue, // the _ pattern
     Garbage,
 }
@@ -41,15 +43,14 @@ impl Pattern {
                     output.append(&mut item.variables());
                 }
             }
-            Pattern::Value(_) => {}
             Pattern::Variable(var_id) => output.push(*var_id),
             Pattern::Or(patterns) => {
                 for pattern in patterns {
                     output.append(&mut pattern.variables());
                 }
             }
-            Pattern::IgnoreValue => {}
-            Pattern::Garbage => {}
+            Pattern::Rest(var_id) => output.push(*var_id),
+            Pattern::Value(_) | Pattern::IgnoreValue | Pattern::Garbage | Pattern::IgnoreRest => {}
         }
 
         output

crates/nu-protocol/src/engine/pattern_match.rs

@@ -18,6 +18,8 @@ impl Matcher for Pattern {
         match self {
             Pattern::Garbage => false,
             Pattern::IgnoreValue => true,
+            Pattern::IgnoreRest => false, // `..` and `..$foo` only match in specific contexts
+            Pattern::Rest(_) => false,    // so we return false here and handle them elsewhere
             Pattern::Record(field_patterns) => match value {
                 Value::Record { cols, vals, .. } => {
                     'top: for field_pattern in field_patterns {
@@ -46,17 +48,47 @@ impl Matcher for Pattern {
             Pattern::List(items) => match &value {
                 Value::List { vals, .. } => {
                     if items.len() > vals.len() {
-                        // We need more items in our pattern than are available in the Value
-                        return false;
+                        // The only we we allow this is to have a rest pattern in the n+1 position
+                        if items.len() == (vals.len() + 1) {
+                            match &items[vals.len()].pattern {
+                                Pattern::IgnoreRest => {}
+                                Pattern::Rest(var_id) => {
+                                    matches.push((*var_id, Value::nothing(items[vals.len()].span)))
+                                }
+                                _ => {
+                                    // There is a pattern which can't skip missing values, so we fail
+                                    return false;
+                                }
+                            }
+                        } else {
+                            // There are patterns that can't be matches, so we fail
+                            return false;
+                        }
                     }
-
                     for (val_idx, val) in vals.iter().enumerate() {
                         // We require that the pattern and the value have the same number of items, or the pattern does not match
                         // The only exception is if the pattern includes a `..` pattern
-
                         if let Some(pattern) = items.get(val_idx) {
-                            if !pattern.match_value(val, matches) {
-                                return false;
+                            match &pattern.pattern {
+                                Pattern::IgnoreRest => {
+                                    break;
+                                }
+                                Pattern::Rest(var_id) => {
+                                    let rest_vals = vals[val_idx..].to_vec();
+                                    matches.push((
+                                        *var_id,
+                                        Value::List {
+                                            vals: rest_vals,
+                                            span: pattern.span,
+                                        },
+                                    ));
+                                    break;
+                                }
+                                _ => {
+                                    if !pattern.match_value(val, matches) {
+                                        return false;
+                                    }
+                                }
                             }
                         } else {
                             return false;