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: atuinsh/atuin#1080)
- ❌
[zoxide](https://github.com/ajeetdsouza/zoxide/blob/main/templates/nushell.txt)
(PR: ajeetdsouza/zoxide#587)
- ✔️
[oh-my-posh](https://github.com/JanDeDobbeleer/oh-my-posh/blob/main/src/shell/scripts/omp.nu)
(pr: JanDeDobbeleer/oh-my-posh#4011)

.github/workflows/release-pkg.nu

@@ -15,23 +15,23 @@
 # unset CARGO_TARGET_DIR if set (I have to do this in the parent shell to get it to work)
 # 2. $env:CARGO_TARGET_DIR = ""
 # 2. hide-env CARGO_TARGET_DIR
-# 3. let-env TARGET = 'x86_64-pc-windows-msvc'
-# 4. let-env TARGET_RUSTFLAGS = ''
-# 5. let-env GITHUB_WORKSPACE = 'C:\Users\dschroeder\source\repos\forks\nushell'
-# 6. let-env GITHUB_OUTPUT = 'C:\Users\dschroeder\source\repos\forks\nushell\output\out.txt'
-# 7. let-env OS = 'windows-latest'
+# 3. $env.TARGET = 'x86_64-pc-windows-msvc'
+# 4. $env.TARGET_RUSTFLAGS = ''
+# 5. $env.GITHUB_WORKSPACE = 'C:\Users\dschroeder\source\repos\forks\nushell'
+# 6. $env.GITHUB_OUTPUT = 'C:\Users\dschroeder\source\repos\forks\nushell\output\out.txt'
+# 7. $env.OS = 'windows-latest'
 # make sure 7z.exe is in your path https://www.7-zip.org/download.html
-# 8. let-env Path = ($env.Path | append 'c:\apps\7-zip')
+# 8. $env.Path = ($env.Path | append 'c:\apps\7-zip')
 # make sure aria2c.exe is in your path https://github.com/aria2/aria2
-# 9. let-env Path = ($env.Path | append 'c:\path\to\aria2c')
+# 9. $env.Path = ($env.Path | append 'c:\path\to\aria2c')
 # make sure you have the wixtools installed https://wixtoolset.org/
-# 10. let-env Path = ($env.Path | append 'C:\Users\dschroeder\AppData\Local\tauri\WixTools')
+# 10. $env.Path = ($env.Path | append 'C:\Users\dschroeder\AppData\Local\tauri\WixTools')
 # You need to run the release-pkg twice. The first pass, with _EXTRA_ as 'bin', makes the output
 # folder and builds everything. The second pass, that generates the msi file, with _EXTRA_ as 'msi'
-# 11. let-env _EXTRA_ = 'bin'
+# 11. $env._EXTRA_ = 'bin'
 # 12. source .github\workflows\release-pkg.nu
 # 13. cd ..
-# 14. let-env _EXTRA_ = 'msi'
+# 14. $env._EXTRA_ = 'msi'
 # 15. source .github\workflows\release-pkg.nu
 # After msi is generated, you have to update winget-pkgs repo, you'll need to patch the release
 # by deleting the existing msi and uploading this new msi. Then you'll need to update the hash
@@ -73,17 +73,17 @@ if $os in [$USE_UBUNTU, 'macos-latest'] {
     match $target {
         'aarch64-unknown-linux-gnu' => {
             sudo apt-get install gcc-aarch64-linux-gnu -y
-            let-env CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER = 'aarch64-linux-gnu-gcc'
+            $env.CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER = 'aarch64-linux-gnu-gcc'
             cargo-build-nu $flags
         }
         'riscv64gc-unknown-linux-gnu' => {
             sudo apt-get install gcc-riscv64-linux-gnu -y
-            let-env CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_GNU_LINKER = 'riscv64-linux-gnu-gcc'
+            $env.CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_GNU_LINKER = 'riscv64-linux-gnu-gcc'
             cargo-build-nu $flags
         }
         'armv7-unknown-linux-gnueabihf' => {
             sudo apt-get install pkg-config gcc-arm-linux-gnueabihf -y
-            let-env CARGO_TARGET_ARMV7_UNKNOWN_LINUX_GNUEABIHF_LINKER = 'arm-linux-gnueabihf-gcc'
+            $env.CARGO_TARGET_ARMV7_UNKNOWN_LINUX_GNUEABIHF_LINKER = 'arm-linux-gnueabihf-gcc'
             cargo-build-nu $flags
         }
         _ => {

crates/nu-cmd-lang/src/core_commands/def_env.rs

@@ -48,7 +48,7 @@ impl Command for DefEnv {
     fn examples(&self) -> Vec<Example> {
         vec![Example {
             description: "Set environment variable by call a custom command",
-            example: r#"def-env foo [] { let-env BAR = "BAZ" }; foo; $env.BAR"#,
+            example: r#"def-env foo [] { $env.BAR = "BAZ" }; foo; $env.BAR"#,
             result: Some(Value::test_string("BAZ")),
         }]
     }

crates/nu-cmd-lang/src/core_commands/export_def_env.rs

@@ -73,7 +73,7 @@ export def-env cd_with_fallback [arg = ""] {
     fn examples(&self) -> Vec<Example> {
         vec![Example {
             description: "Define a custom command that participates in the environment in a module and call it",
-            example: r#"module foo { export def-env bar [] { let-env FOO_BAR = "BAZ" } }; use foo bar; bar; $env.FOO_BAR"#,
+            example: r#"module foo { export def-env bar [] { $env.FOO_BAR = "BAZ" } }; use foo bar; bar; $env.FOO_BAR"#,
             result: Some(Value::test_string("BAZ")),
         }]
     }

crates/nu-cmd-lang/src/core_commands/hide_env.rs

@@ -72,7 +72,7 @@ impl Command for HideEnv {
     fn examples(&self) -> Vec<Example> {
         vec![Example {
             description: "Hide an environment variable",
-            example: r#"let-env HZ_ENV_ABC = 1; hide-env HZ_ENV_ABC; 'HZ_ENV_ABC' in (env).name"#,
+            example: r#"$env.HZ_ENV_ABC = 1; hide-env HZ_ENV_ABC; 'HZ_ENV_ABC' in (env).name"#,
             result: Some(Value::test_bool(false)),
         }]
     }

crates/nu-cmd-lang/src/core_commands/module.rs

@@ -57,12 +57,12 @@ impl Command for Module {
             },
             Example {
                 description: "Define an environment variable in a module",
-                example: r#"module foo { export-env { let-env FOO = "BAZ" } }; use foo; $env.FOO"#,
+                example: r#"module foo { export-env { $env.FOO = "BAZ" } }; use foo; $env.FOO"#,
                 result: Some(Value::test_string("BAZ")),
             },
             Example {
                 description: "Define a custom command that participates in the environment in a module and call it",
-                example: r#"module foo { export def-env bar [] { let-env FOO_BAR = "BAZ" } }; use foo bar; bar; $env.FOO_BAR"#,
+                example: r#"module foo { export def-env bar [] { $env.FOO_BAR = "BAZ" } }; use foo bar; bar; $env.FOO_BAR"#,
                 result: Some(Value::test_string("BAZ")),
             },
         ]

crates/nu-cmd-lang/src/core_commands/overlay/hide.rs

@@ -120,7 +120,7 @@ impl Command for OverlayHide {
             },
             Example {
                 description: "Hide the last activated overlay",
-                example: r#"module spam { export-env { let-env FOO = "foo" } }
+                example: r#"module spam { export-env { $env.FOO = "foo" } }
     overlay use spam
     overlay hide"#,
                 result: None,

crates/nu-cmd-lang/src/core_commands/overlay/use_.rs

@@ -188,7 +188,7 @@ impl Command for OverlayUse {
             },
             Example {
                 description: "Create an overlay from a file",
-                example: r#"'export-env { let-env FOO = "foo" }' | save spam.nu
+                example: r#"'export-env { $env.FOO = "foo" }' | save spam.nu
     overlay use spam.nu
     $env.FOO"#,
                 result: None,

crates/nu-color-config/src/style_computer.rs

@@ -238,7 +238,7 @@ fn test_computable_style_closure_basic() {
     use nu_test_support::{nu, nu_repl_code, playground::Playground};
     Playground::setup("computable_style_closure_basic", |dirs, _| {
         let inp = [
-            r#"let-env config = {
+            r#"$env.config = {
                 color_config: {
                     string: {|e| touch ($e + '.obj'); 'red' }
                 }
@@ -256,7 +256,7 @@ fn test_computable_style_closure_basic() {
 fn test_computable_style_closure_errors() {
     use nu_test_support::{nu, nu_repl_code};
     let inp = [
-        r#"let-env config = {
+        r#"$env.config = {
             color_config: {
                 string: {|e| $e + 2 }
             }

crates/nu-command/src/debug/view_source.rs

@@ -163,8 +163,8 @@ impl Command for ViewSource {
             },
             Example {
                 description: "View the source of a custom command, which participates in the caller environment",
-                example: r#"def-env foo [] { let-env BAR = 'BAZ' }; view source foo"#,
-                result: Some(Value::test_string("def foo [] { let-env BAR = 'BAZ' }")),
+                example: r#"def-env foo [] { $env.BAR = 'BAZ' }; view source foo"#,
+                result: Some(Value::test_string("def foo [] { $env.BAR = 'BAZ' }")),
             },
             Example {
                 description: "View the source of a custom command with flags and arguments",
@@ -173,8 +173,8 @@ impl Command for ViewSource {
             },
             Example {
                 description: "View the source of a module",
-                example: r#"module mod-foo { export-env { let-env FOO_ENV = 'BAZ' } }; view source mod-foo"#,
-                result: Some(Value::test_string(" export-env { let-env FOO_ENV = 'BAZ' }")),
+                example: r#"module mod-foo { export-env { $env.FOO_ENV = 'BAZ' } }; view source mod-foo"#,
+                result: Some(Value::test_string(" export-env { $env.FOO_ENV = 'BAZ' }")),
             },
             Example {
                 description: "View the source of an alias",

crates/nu-command/src/default_context.rs

@@ -315,8 +315,8 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
 
         // Env
         bind_command! {
+            LetEnvDeprecated,
             ExportEnv,
-            LetEnv,
             LoadEnv,
             SourceEnv,
             WithEnv,

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

@@ -0,0 +1,43 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{Category, PipelineData, ShellError, Signature, SyntaxShape, Type};
+
+#[derive(Clone)]
+pub struct LetEnvDeprecated;
+
+impl Command for LetEnvDeprecated {
+    fn name(&self) -> &str {
+        "let-env"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .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::Deprecated)
+    }
+
+    fn usage(&self) -> &str {
+        "`let-env FOO = ...` is deprecated, use `$env.FOO = ...` instead."
+    }
+
+    fn run(
+        &self,
+        _: &EngineState,
+        _: &mut Stack,
+        call: &Call,
+        _: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        Err(nu_protocol::ShellError::DeprecatedCommand(
+            self.name().to_string(),
+            "$nu.<environment variable> = ...".to_owned(),
+            call.head,
+        ))
+    }
+}

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

@@ -1,6 +1,7 @@
 mod collect;
 mod deprecated_commands;
 mod hash_base64;
+mod let_env;
 mod lpad;
 mod math_eval;
 mod rpad;
@@ -12,6 +13,7 @@ mod str_int;
 pub use collect::StrCollectDeprecated;
 pub use deprecated_commands::*;
 pub use hash_base64::HashBase64;
+pub use let_env::LetEnvDeprecated;
 pub use lpad::LPadDeprecated;
 pub use math_eval::SubCommand as MathEvalDeprecated;
 pub use rpad::RPadDeprecated;

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/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;

crates/nu-command/src/example_test.rs

@@ -9,9 +9,9 @@ pub fn test_examples(cmd: impl Command + 'static) {
 #[cfg(test)]
 mod test_examples {
     use super::super::{
-        Ansi, Date, Enumerate, Flatten, From, Get, Into, IntoString, LetEnv, Math, MathEuler,
-        MathPi, MathRound, ParEach, Path, PathParse, Random, Sort, SortBy, Split, SplitColumn,
-        SplitRow, Str, StrJoin, StrLength, StrReplace, Update, Url, Values, Wrap,
+        Ansi, Date, Enumerate, Flatten, From, Get, Into, IntoString, Math, MathEuler, MathPi,
+        MathRound, ParEach, Path, PathParse, Random, Sort, SortBy, Split, SplitColumn, SplitRow,
+        Str, StrJoin, StrLength, StrReplace, Update, Url, Values, Wrap,
     };
     use crate::{Each, To};
     use nu_cmd_lang::example_support::{
@@ -78,7 +78,6 @@ mod test_examples {
             working_set.add_decl(Box::new(Into));
             working_set.add_decl(Box::new(IntoString));
             working_set.add_decl(Box::new(Let));
-            working_set.add_decl(Box::new(LetEnv));
             working_set.add_decl(Box::new(Math));
             working_set.add_decl(Box::new(MathEuler));
             working_set.add_decl(Box::new(MathPi));

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

@@ -230,7 +230,7 @@ fn parse_module_success_2() {
             r#"
                 # foo.nu
 
-                export-env { let-env MYNAME = "Arthur, King of the Britons" }
+                export-env { $env.MYNAME = "Arthur, King of the Britons" }
             "#,
         )]);
 
@@ -749,7 +749,7 @@ fn parse_script_with_nested_scripts_success() {
             .with_files(vec![FileWithContentToBeTrimmed(
                 "foo.nu",
                 r#"
-                    let-env FOO = 'foo'
+                    $env.FOO = 'foo'
                 "#,
             )]);
 
@@ -772,7 +772,7 @@ fn nu_check_respects_file_pwd() {
             .with_files(vec![FileWithContentToBeTrimmed(
                 "lol/lol.nu",
                 r#"
-                    let-env RETURN = (nu-check ../foo.nu)
+                    $env.RETURN = (nu-check ../foo.nu)
                 "#,
             )])
             .with_files(vec![FileWithContentToBeTrimmed(

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

@@ -143,7 +143,7 @@ fn same_target_redirection_with_too_much_stderr_not_hang_nushell() {
         nu!(
             cwd: dirs.test(), pipeline(
                 r#"
-                let-env LARGE = (open --raw a_large_file.txt);
+                $env.LARGE = (open --raw a_large_file.txt);
                 nu --testbin echo_env_stderr LARGE out+err> another_large_file.txt
                 "#
             ),

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

@@ -142,7 +142,7 @@ fn errors_if_attempting_to_delete_home() {
     Playground::setup("rm_test_8", |dirs, _| {
         let actual = nu!(
             cwd: dirs.root(),
-            "let-env HOME = myhome ; rm -rf ~"
+            "$env.HOME = myhome ; rm -rf ~"
         );
 
         assert!(actual.err.contains("please use -I or -i"));