Module: support defining const and use const variables inside of function (#9773)

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# Description
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Relative: #8248 

After this pr, user can define const variable inside a module.

![image](https://github.com/nushell/nushell/assets/22256154/e3e03e56-c4b5-4144-a944-d1b20bec1cbd)

And user can export const variables, the following screenshot shows how
it works (it follows
https://github.com/nushell/nushell/issues/8248#issuecomment-1637442612):

![image](https://github.com/nushell/nushell/assets/22256154/b2c14760-3f27-41cc-af77-af70a4367f2a)

## About the change
1. To make module support const, we need to change `parse_module_block`
to support `const` keyword.
2. To suport export `const`, we need to make module tracking variables,
so we add `variables` attribute to `Module`
3. During eval, the const variable may not exists in `stack`, because we
don't eval `const` when we define a module, so we need to find variables
which are already registered in `engine_state`

## One more thing to note about the const value.
Consider the following code
```
module foo { const b = 3; export def bar [] { $b } }
use foo bar
const b = 4;
bar
```
The result will be 3 (which is defined in module) rather than 4. I think
it's expected behavior.

It's something like [dynamic
binding](https://www.gnu.org/software/emacs/manual/html_node/elisp/Dynamic-Binding-Tips.html)
vs [lexical
binding](https://www.gnu.org/software/emacs/manual/html_node/elisp/Lexical-Binding.html)
in lisp like language, and lexical binding should be right behavior
which generates more predicable result, and it doesn't introduce really
subtle bugs in nushell code.

What if user want dynamic-binding?(For example: the example code returns
`4`)
There is no way to do this, user should consider passing the value as
argument to custom command rather than const.

## TODO
- [X] adding tests for the feature.
- [X] support export const out of module to use.

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

@@ -0,0 +1,66 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{
+    Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Type, Value,
+};
+
+#[derive(Clone)]
+pub struct ExportConst;
+
+impl Command for ExportConst {
+    fn name(&self) -> &str {
+        "export const"
+    }
+
+    fn usage(&self) -> &str {
+        "Use parse-time constant from a module and export them from this module."
+    }
+
+    fn signature(&self) -> nu_protocol::Signature {
+        Signature::build("export const")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
+            .allow_variants_without_examples(true)
+            .required("const_name", SyntaxShape::VarWithOptType, "constant name")
+            .required(
+                "initial_value",
+                SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::MathExpression)),
+                "equals sign followed by constant value",
+            )
+            .category(Category::Core)
+    }
+
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nu.html"#
+    }
+
+    fn is_parser_keyword(&self) -> bool {
+        true
+    }
+
+    fn run(
+        &self,
+        _engine_state: &EngineState,
+        _stack: &mut Stack,
+        _call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        Ok(PipelineData::empty())
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Re-export a command from another module",
+            example: r#"module spam { export const foo = 3; }
+    module eggs { export use spam foo }
+    use eggs foo
+    foo
+            "#,
+            result: Some(Value::test_int(3)),
+        }]
+    }
+
+    fn search_terms(&self) -> Vec<&str> {
+        vec!["reexport", "import", "module"]
+    }
+}

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

@@ -11,6 +11,7 @@ mod echo;
 mod error_make;
 mod export;
 mod export_alias;
+mod export_const;
 mod export_def;
 mod export_def_env;
 mod export_extern;
@@ -50,6 +51,7 @@ pub use echo::Echo;
 pub use error_make::ErrorMake;
 pub use export::ExportCommand;
 pub use export_alias::ExportAlias;
+pub use export_const::ExportConst;
 pub use export_def::ExportDef;
 pub use export_def_env::ExportDefEnv;
 pub use export_extern::ExportExtern;

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

@@ -1,8 +1,8 @@
 use nu_engine::{eval_block, find_in_dirs_env, get_dirs_var_from_call, redirect_env};
-use nu_protocol::ast::{Call, Expr, Expression};
+use nu_protocol::ast::{Call, Expr, Expression, ImportPattern, ImportPatternMember};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Type, Value,
+    Category, Example, Module, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -110,6 +110,14 @@ This command is a parser keyword. For details, check:
                 // Merge the block's environment to the current stack
                 redirect_env(engine_state, caller_stack, &callee_stack);
             }
+
+            use_variables(
+                engine_state,
+                import_pattern,
+                module,
+                caller_stack,
+                call.head,
+            );
         } else {
             return Err(ShellError::GenericError(
                 format!(
@@ -162,6 +170,76 @@ This command is a parser keyword. For details, check:
     }
 }
 
+fn use_variables(
+    engine_state: &EngineState,
+    import_pattern: &ImportPattern,
+    module: &Module,
+    caller_stack: &mut Stack,
+    head_span: Span,
+) {
+    if !module.variables.is_empty() {
+        if import_pattern.members.is_empty() {
+            // add a record variable.
+            if let Some(var_id) = import_pattern.module_name_var_id {
+                let mut cols = vec![];
+                let mut vals = vec![];
+                for (var_name, var_id) in module.variables.iter() {
+                    if let Some(val) = engine_state.get_var(*var_id).clone().const_val {
+                        cols.push(String::from_utf8_lossy(var_name).to_string());
+                        vals.push(val)
+                    }
+                }
+                caller_stack.add_var(
+                    var_id,
+                    Value::record(cols, vals, module.span.unwrap_or(head_span)),
+                )
+            }
+        } else {
+            let mut have_glob = false;
+            for m in &import_pattern.members {
+                if matches!(m, ImportPatternMember::Glob { .. }) {
+                    have_glob = true;
+                    break;
+                }
+            }
+            if have_glob {
+                // bring all variables into scope directly.
+                for (_, var_id) in module.variables.iter() {
+                    if let Some(val) = engine_state.get_var(*var_id).clone().const_val {
+                        caller_stack.add_var(*var_id, val);
+                    }
+                }
+            } else {
+                let mut members = vec![];
+                for m in &import_pattern.members {
+                    match m {
+                        ImportPatternMember::List { names, .. } => {
+                            for (n, _) in names {
+                                if module.variables.contains_key(n) {
+                                    members.push(n);
+                                }
+                            }
+                        }
+                        ImportPatternMember::Name { name, .. } => {
+                            if module.variables.contains_key(name) {
+                                members.push(name)
+                            }
+                        }
+                        ImportPatternMember::Glob { .. } => continue,
+                    }
+                }
+                for m in members {
+                    if let Some(var_id) = module.variables.get(m) {
+                        if let Some(val) = engine_state.get_var(*var_id).clone().const_val {
+                            caller_stack.add_var(*var_id, val);
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     #[test]