support env and mut assignment with if block and match guard (#9650)

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Fixes: https://github.com/nushell/nushell/issues/9595

So we can do the following in nushell:
```nushell
mut a = 3
$a = if 4 == 3 { 10 } else {20}
```
or
```nushell
$env.BUILD_EXT = match 3 { 1 => { 'yes!' }, _ => { 'no!' } }
```

# User-Facing Changes
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helps us keep track of breaking changes. -->

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---------

Co-authored-by: WindSoilder <windsoilder@DESKTOP-R8GRJ1D.localdomain>

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

@@ -177,3 +177,15 @@ fn mut_records_update_properly() {
     let actual = nu!(pipeline("mut a = {}; $a.b.c = 100; $a.b.c"));
     assert_eq!(actual.out, "100");
 }
+
+#[test]
+fn mut_value_with_if() {
+    let actual = nu!("mut a = 3; $a = if 3 == 3 { 10 }; echo $a");
+    assert_eq!(actual.out, "10");
+}
+
+#[test]
+fn mut_value_with_match() {
+    let actual = nu!("mut a = 3; $a = match 3 { 1 => { 'yes!' }, _ => { 'no!' } }; echo $a");
+    assert_eq!(actual.out, "no!");
+}

crates/nu-parser/src/parser.rs

@@ -4870,6 +4870,14 @@ pub fn parse_math_expression(
             break;
         }
 
+        let content = working_set.get_span_contents(spans[idx]);
+        // allow `if` to be a special value for assignment.
+        if content == b"if" || content == b"match" {
+            let rhs = parse_call(working_set, &spans[idx..], spans[0], false);
+            expr_stack.push(op);
+            expr_stack.push(rhs);
+            break;
+        }
         let rhs = parse_value(working_set, spans[idx], &SyntaxShape::Any);
 
         while op_prec <= last_prec && expr_stack.len() > 1 {

tests/shell/environment/env.rs

@@ -85,6 +85,18 @@ fn env_assignment() {
     assert_eq!(actual.out, "barbaz");
 }
 
+#[test]
+fn env_assignment_with_if() {
+    let actual = nu!(r#"$env.FOOBAR = if 3 == 4 { "bar" } else { "baz" }; $env.FOOBAR"#);
+    assert_eq!(actual.out, "baz");
+}
+
+#[test]
+fn env_assignment_with_match() {
+    let actual = nu!(r#"$env.FOOBAR = match 1 { 1 => { 'yes!' }, _ => { 'no!' } }; $env.FOOBAR"#);
+    assert_eq!(actual.out, "yes!");
+}
+
 #[test]
 fn mutate_env_file_pwd_env_var_fails() {
     let actual = nu!("$env.FILE_PWD = 'foo'");