Add debug env command (#15875)

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# Description
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When calling external commands, we convert our `$env` into a map where
each value is a string. If a value cannot be converted, it will be
skipped or when an `ENV_CONVERSION` is defined, will be converted via
that. This makes this conversion not that trivial. To ease debugging
this behavior or allowing to generate `.env` files from the current
environment did I add `debug env`.

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

New command `debug env`.

# Tests + Formatting
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I did not add extra tests, as I just called the function we also call in
`start`, `exec` or `run-external`.

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I can use this to make my life easier implementing `colcon-nushell` 😉

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

@@ -0,0 +1,48 @@
+use nu_engine::{command_prelude::*, env_to_strings};
+
+#[derive(Clone)]
+pub struct DebugEnv;
+
+impl Command for DebugEnv {
+    fn name(&self) -> &str {
+        "debug env"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::new(self.name())
+            .input_output_type(Type::Nothing, Type::record())
+            .category(Category::Debug)
+    }
+
+    fn description(&self) -> &str {
+        "Show environment variables as external commands would get it."
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        Ok(PipelineData::Value(
+            env_to_strings(engine_state, stack)?.into_value(call.head),
+            None,
+        ))
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Get PATH variable that externals see",
+                example: "debug env | get PATH!",
+                result: None,
+            },
+            Example {
+                description: "Create a .env file",
+                example: r#"debug env | transpose key value | each {$"($in.key)=($in.value | to json)"} | save .env"#,
+                result: None,
+            },
+        ]
+    }
+}

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

@@ -1,5 +1,6 @@
 mod ast;
 mod debug_;
+mod env;
 mod explain;
 mod info;
 mod inspect;
@@ -19,6 +20,7 @@ mod view_span;
 
 pub use ast::Ast;
 pub use debug_::Debug;
+pub use env::DebugEnv;
 pub use explain::Explain;
 pub use info::DebugInfo;
 pub use inspect::Inspect;

crates/nu-command/src/default_context.rs

@@ -152,6 +152,7 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
         bind_command! {
             Ast,
             Debug,
+            DebugEnv,
             DebugInfo,
             DebugProfile,
             Explain,