remove let-env, focus on mutating $env (#9574)

# Description

For years, Nushell has used `let-env` to set a single environment
variable. As our work on scoping continued, we refined what it meant for
a variable to be in scope using `let` but never updated how `let-env`
would work. Instead, `let-env` confusingly created mutations to the
command's copy of `$env`.

So, to help fix the mental model and point people to the right way of
thinking about what changing the environment means, this PR removes
`let-env` to encourage people to think of it as updating the command's
environment variable via mutation.

Before:

```
let-env FOO = "BAR"
```

Now:

```
$env.FOO = "BAR"
```

It's also a good reminder that the environment owned by the command is
in the `$env` variable rather than global like it is in other shells.

# User-Facing Changes

BREAKING CHANGE BREAKING CHANGE

This completely removes `let-env FOO = "BAR"` so that we can focus on
`$env.FOO = "BAR"`.

# 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
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- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect -A clippy::result_large_err` to check that
you're using the standard code style
- `cargo test --workspace` to check that all tests pass
- `cargo run -- crates/nu-std/tests/run.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 / Before Submitting
integration scripts to update:
- ✔️
[starship](https://github.com/starship/starship/blob/master/src/init/starship.nu)
- ✔️
[virtualenv](https://github.com/pypa/virtualenv/blob/main/src/virtualenv/activation/nushell/activate.nu)
- ✔️
[atuin](https://github.com/ellie/atuin/blob/main/atuin/src/shell/atuin.nu)
(PR: https://github.com/ellie/atuin/pull/1080)
- ❌
[zoxide](https://github.com/ajeetdsouza/zoxide/blob/main/templates/nushell.txt)
(PR: https://github.com/ajeetdsouza/zoxide/pull/587)
- ✔️
[oh-my-posh](https://github.com/JanDeDobbeleer/oh-my-posh/blob/main/src/shell/scripts/omp.nu)
(pr: https://github.com/JanDeDobbeleer/oh-my-posh/pull/4011)

crates/nu-command/src/env/export_env.rs

@@ -57,14 +57,14 @@ impl Command for ExportEnv {
         vec![
             Example {
                 description: "Set an environment variable",
-                example: r#"export-env { let-env SPAM = 'eggs' }"#,
+                example: r#"export-env { $env.SPAM = 'eggs' }"#,
                 result: Some(Value::Nothing {
                     span: Span::test_data(),
                 }),
             },
             Example {
                 description: "Set an environment variable and examine its value",
-                example: r#"export-env { let-env SPAM = 'eggs' }; $env.SPAM"#,
+                example: r#"export-env { $env.SPAM = 'eggs' }; $env.SPAM"#,
                 result: Some(Value::test_string("eggs")),
             },
         ]

crates/nu-command/src/env/let_env.rs

@@ -1,72 +0,0 @@
-use nu_engine::{eval_expression_with_input, CallExt};
-use nu_protocol::ast::Call;
-use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Spanned, SyntaxShape, Type, Value,
-};
-
-#[derive(Clone)]
-pub struct LetEnv;
-
-impl Command for LetEnv {
-    fn name(&self) -> &str {
-        "let-env"
-    }
-
-    fn usage(&self) -> &str {
-        "Create an environment variable and give it a value."
-    }
-
-    fn signature(&self) -> nu_protocol::Signature {
-        Signature::build("let-env")
-            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
-            .allow_variants_without_examples(true)
-            .required("var_name", SyntaxShape::String, "variable name")
-            .required(
-                "initial_value",
-                SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::MathExpression)),
-                "equals sign followed by value",
-            )
-            .category(Category::Env)
-    }
-
-    fn run(
-        &self,
-        engine_state: &EngineState,
-        stack: &mut Stack,
-        call: &Call,
-        input: PipelineData,
-    ) -> Result<PipelineData, ShellError> {
-        // TODO: find and require the crossplatform restrictions on environment names
-        let env_var: Spanned<String> = call.req(engine_state, stack, 0)?;
-
-        let keyword_expr = call
-            .positional_nth(1)
-            .expect("checked through parser")
-            .as_keyword()
-            .expect("internal error: missing keyword");
-
-        let rhs =
-            eval_expression_with_input(engine_state, stack, keyword_expr, input, false, true)?
-                .0
-                .into_value(call.head);
-
-        if env_var.item == "FILE_PWD" || env_var.item == "CURRENT_FILE" || env_var.item == "PWD" {
-            return Err(ShellError::AutomaticEnvVarSetManually {
-                envvar_name: env_var.item,
-                span: env_var.span,
-            });
-        } else {
-            stack.add_env_var(env_var.item, rhs);
-        }
-        Ok(PipelineData::empty())
-    }
-
-    fn examples(&self) -> Vec<Example> {
-        vec![Example {
-            description: "Create an environment variable and display it",
-            example: "let-env MY_ENV_VAR = 1; $env.MY_ENV_VAR",
-            result: Some(Value::test_int(1)),
-        }]
-    }
-}

crates/nu-command/src/env/mod.rs

@@ -1,6 +1,5 @@
 mod config;
 mod export_env;
-mod let_env;
 mod load_env;
 mod source_env;
 mod with_env;
@@ -10,7 +9,6 @@ pub use config::ConfigMeta;
 pub use config::ConfigNu;
 pub use config::ConfigReset;
 pub use export_env::ExportEnv;
-pub use let_env::LetEnv;
 pub use load_env::LoadEnv;
 pub use source_env::SourceEnv;
 pub use with_env::WithEnv;