Initial support for parse-time constants (#7436)

crates/nu-command/src/core_commands/const_.rs

@@ -0,0 +1,104 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Type};
+
+#[derive(Clone)]
+pub struct Const;
+
+impl Command for Const {
+    fn name(&self) -> &str {
+        "const"
+    }
+
+    fn usage(&self) -> &str {
+        "Create a parse-time constant."
+    }
+
+    fn signature(&self) -> nu_protocol::Signature {
+        Signature::build("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::Expression)),
+                "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 search_terms(&self) -> Vec<&str> {
+        vec!["set", "let"]
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<nu_protocol::PipelineData, nu_protocol::ShellError> {
+        let var_id = call
+            .positional_nth(0)
+            .expect("checked through parser")
+            .as_var()
+            .expect("internal error: missing variable");
+
+        if let Some(constval) = engine_state.find_constant(var_id, &[]) {
+            // Instead of creating a second copy of the value in the stack, we could change
+            // stack.get_var() to check engine_state.find_constant().
+            stack.add_var(var_id, constval.clone());
+
+            Ok(PipelineData::empty())
+        } else {
+            Err(ShellError::NushellFailedSpanned(
+                "Missing Constant".to_string(),
+                "constant not added by the parser".to_string(),
+                call.head,
+            ))
+        }
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Create a new parse-time constant.",
+                example: "let x = 10",
+                result: None,
+            },
+            Example {
+                description: "Create a composite constant value",
+                example: "let x = { a: 10, b: 20 }",
+                result: None,
+            },
+        ]
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use nu_protocol::engine::CommandType;
+
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(Const {})
+    }
+
+    #[test]
+    fn test_command_type() {
+        assert!(matches!(Const.command_type(), CommandType::Keyword));
+    }
+}

crates/nu-command/src/core_commands/let_.rs

@@ -70,9 +70,8 @@ impl Command for Let {
         )?
         .0;
 
-        //println!("Adding: {:?} to {}", rhs, var_id);
-
         stack.add_var(var_id, rhs.into_value(call.head));
+
         Ok(PipelineData::empty())
     }
 

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

@@ -2,6 +2,7 @@ mod alias;
 mod ast;
 mod break_;
 mod commandline;
+mod const_;
 mod continue_;
 mod debug;
 mod def;
@@ -40,6 +41,7 @@ pub use alias::Alias;
 pub use ast::Ast;
 pub use break_::Break;
 pub use commandline::Commandline;
+pub use const_::Const;
 pub use continue_::Continue;
 pub use debug::Debug;
 pub use def::Def;

crates/nu-command/src/core_commands/overlay/use_.rs

@@ -84,25 +84,8 @@ impl Command for OverlayUse {
             ));
         };
 
-        let overlay_name = if let Some(kw_expression) = call.positional_nth(1) {
-            // If renamed via the 'as' keyword, use the new name as the overlay name
-            if let Some(new_name_expression) = kw_expression.as_keyword() {
-                if let Some(new_name) = new_name_expression.as_string() {
-                    new_name
-                } else {
-                    return Err(ShellError::NushellFailedSpanned(
-                        "Wrong keyword type".to_string(),
-                        "keyword argument not a string".to_string(),
-                        new_name_expression.span,
-                    ));
-                }
-            } else {
-                return Err(ShellError::NushellFailedSpanned(
-                    "Wrong keyword type".to_string(),
-                    "keyword argument not a keyword".to_string(),
-                    kw_expression.span,
-                ));
-            }
+        let overlay_name = if let Some(name) = call.opt(engine_state, caller_stack, 1)? {
+            name
         } else if engine_state
             .find_overlay(name_arg.item.as_bytes())
             .is_some()

crates/nu-command/src/default_context.rs

@@ -32,6 +32,7 @@ pub fn create_default_context() -> EngineState {
             Ast,
             Break,
             Commandline,
+            Const,
             Continue,
             Debug,
             Def,

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

@@ -293,3 +293,25 @@ fn source_env_is_scoped() {
         assert!(actual.err.contains("executable was not found"));
     })
 }
+
+#[test]
+fn source_env_const_file() {
+    Playground::setup("source_env_const_file", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "spam.nu",
+            r#"
+                let-env FOO = 'foo'
+            "#,
+        )]);
+
+        let inp = &[
+            r#"const file = 'spam.nu'"#,
+            r#"source-env $file"#,
+            r#"$env.FOO"#,
+        ];
+
+        let actual = nu!(cwd: dirs.test(), pipeline(&inp.join("; ")));
+
+        assert_eq!(actual.out, "foo");
+    })
+}

crates/nu-parser/src/errors.rs

@@ -378,6 +378,19 @@ pub enum ParseError {
     #[diagnostic(code(nu::shell::error_reading_file), url(docsrs))]
     ReadingFile(String, #[label("{0}")] Span),
 
+    /// Tried assigning non-constant value to a constant
+    ///
+    /// ## Resolution
+    ///
+    /// Only a subset of expressions are allowed to be assigned as a constant during parsing.
+    #[error("Not a constant.")]
+    #[diagnostic(
+        code(nu::parser::not_a_constant),
+        url(docsrs),
+        help("Only a subset of expressions are allowed constants during parsing. Try using the 'let' command or typing the value literally.")
+    )]
+    NotAConstant(#[label = "Value is not a parse-time constant"] Span),
+
     #[error("{0}")]
     #[diagnostic()]
     LabeledError(String, String, #[label("{1}")] Span),
@@ -450,6 +463,7 @@ impl ParseError {
             ParseError::ShellErrRedirect(s) => *s,
             ParseError::ShellOutErrRedirect(s) => *s,
             ParseError::UnknownOperator(_, _, s) => *s,
+            ParseError::NotAConstant(s) => *s,
         }
     }
 }

crates/nu-parser/src/eval.rs

@@ -0,0 +1,124 @@
+use crate::ParseError;
+use nu_protocol::{
+    ast::{Expr, Expression},
+    engine::StateWorkingSet,
+    Span, Value,
+};
+
+/// Evaluate a constant value at parse time
+///
+/// Based off eval_expression() in the engine
+pub fn eval_constant(
+    working_set: &StateWorkingSet,
+    expr: &Expression,
+) -> Result<Value, ParseError> {
+    match &expr.expr {
+        Expr::Bool(b) => Ok(Value::boolean(*b, expr.span)),
+        Expr::Int(i) => Ok(Value::int(*i, expr.span)),
+        Expr::Float(f) => Ok(Value::float(*f, expr.span)),
+        Expr::Binary(b) => Ok(Value::Binary {
+            val: b.clone(),
+            span: expr.span,
+        }),
+        Expr::Var(var_id) => match working_set.find_constant(*var_id) {
+            Some(val) => Ok(val.clone()),
+            None => Err(ParseError::NotAConstant(expr.span)),
+        },
+        Expr::CellPath(cell_path) => Ok(Value::CellPath {
+            val: cell_path.clone(),
+            span: expr.span,
+        }),
+        Expr::FullCellPath(cell_path) => {
+            let value = eval_constant(working_set, &cell_path.head)?;
+
+            match value.follow_cell_path(&cell_path.tail, false) {
+                Ok(val) => Ok(val),
+                // TODO: Better error conversion
+                Err(shell_error) => Err(ParseError::LabeledError(
+                    "Error when following cell path".to_string(),
+                    format!("{:?}", shell_error),
+                    expr.span,
+                )),
+            }
+        }
+        Expr::DateTime(dt) => Ok(Value::Date {
+            val: *dt,
+            span: expr.span,
+        }),
+        Expr::List(x) => {
+            let mut output = vec![];
+            for expr in x {
+                output.push(eval_constant(working_set, expr)?);
+            }
+            Ok(Value::List {
+                vals: output,
+                span: expr.span,
+            })
+        }
+        Expr::Record(fields) => {
+            let mut cols = vec![];
+            let mut vals = vec![];
+            for (col, val) in fields {
+                // avoid duplicate cols.
+                let col_name = value_as_string(eval_constant(working_set, col)?, expr.span)?;
+                let pos = cols.iter().position(|c| c == &col_name);
+                match pos {
+                    Some(index) => {
+                        vals[index] = eval_constant(working_set, val)?;
+                    }
+                    None => {
+                        cols.push(col_name);
+                        vals.push(eval_constant(working_set, val)?);
+                    }
+                }
+            }
+
+            Ok(Value::Record {
+                cols,
+                vals,
+                span: expr.span,
+            })
+        }
+        Expr::Table(headers, vals) => {
+            let mut output_headers = vec![];
+            for expr in headers {
+                output_headers.push(value_as_string(
+                    eval_constant(working_set, expr)?,
+                    expr.span,
+                )?);
+            }
+
+            let mut output_rows = vec![];
+            for val in vals {
+                let mut row = vec![];
+                for expr in val {
+                    row.push(eval_constant(working_set, expr)?);
+                }
+                output_rows.push(Value::Record {
+                    cols: output_headers.clone(),
+                    vals: row,
+                    span: expr.span,
+                });
+            }
+            Ok(Value::List {
+                vals: output_rows,
+                span: expr.span,
+            })
+        }
+        Expr::Keyword(_, _, expr) => eval_constant(working_set, expr),
+        Expr::String(s) => Ok(Value::String {
+            val: s.clone(),
+            span: expr.span,
+        }),
+        Expr::Nothing => Ok(Value::Nothing { span: expr.span }),
+        _ => Err(ParseError::NotAConstant(expr.span)),
+    }
+}
+
+/// Get the value as a string
+pub fn value_as_string(value: Value, span: Span) -> Result<String, ParseError> {
+    match value {
+        Value::String { val, .. } => Ok(val),
+        _ => Err(ParseError::NotAConstant(span)),
+    }
+}

crates/nu-parser/src/lib.rs

@@ -1,5 +1,6 @@
 mod deparse;
 mod errors;
+mod eval;
 mod flatten;
 mod known_external;
 mod lex;

crates/nu-parser/src/parse_keywords.rs

@@ -1,3 +1,4 @@
+use crate::eval::{eval_constant, value_as_string};
 use log::trace;
 use nu_path::canonicalize_with;
 use nu_protocol::{
@@ -20,8 +21,8 @@ use crate::{
     lex,
     parser::{
         check_call, check_name, garbage, garbage_pipeline, lite_parse, parse, parse_internal_call,
-        parse_multispan_value, parse_signature, parse_string, parse_var_with_opt_type, trim_quotes,
-        LiteCommand, LiteElement, ParsedInternalCall,
+        parse_multispan_value, parse_signature, parse_string, parse_value, parse_var_with_opt_type,
+        trim_quotes, LiteCommand, LiteElement, ParsedInternalCall,
     },
     unescape_unquote_string, ParseError,
 };
@@ -1636,8 +1637,7 @@ pub fn parse_use(
 
     // TODO: Add checking for importing too long import patterns, e.g.:
     // > use spam foo non existent names here do not throw error
-    let (import_pattern, module) =
-        if let Some(module_id) = working_set.find_module(&import_pattern.head.name) {
+    let (import_pattern, module) = if let Some(module_id) = import_pattern.head.id {
         (import_pattern, working_set.get_module(module_id).clone())
     } else {
         // It could be a file
@@ -2386,16 +2386,16 @@ pub fn parse_overlay_use(
     };
 
     let (overlay_name, overlay_name_span) = if let Some(expr) = call.positional_nth(0) {
-        if let Some(s) = expr.as_string() {
-            (s, expr.span)
-        } else {
-            return (
-                garbage_pipeline(spans),
-                Some(ParseError::UnknownState(
-                    "internal error: Module name not a string".into(),
-                    expr.span,
-                )),
-            );
+        match eval_constant(working_set, expr) {
+            Ok(val) => match value_as_string(val, expr.span) {
+                Ok(s) => (s, expr.span),
+                Err(err) => {
+                    return (garbage_pipeline(spans), Some(err));
+                }
+            },
+            Err(err) => {
+                return (garbage_pipeline(spans), Some(err));
+            }
         }
     } else {
         return (
@@ -2409,20 +2409,15 @@ pub fn parse_overlay_use(
 
     let new_name = if let Some(kw_expression) = call.positional_nth(1) {
         if let Some(new_name_expression) = kw_expression.as_keyword() {
-            if let Some(new_name) = new_name_expression.as_string() {
-                Some(Spanned {
-                    item: new_name,
+            match eval_constant(working_set, new_name_expression) {
+                Ok(val) => match value_as_string(val, new_name_expression.span) {
+                    Ok(s) => Some(Spanned {
+                        item: s,
                         span: new_name_expression.span,
-                })
-            } else {
-                return (
-                    garbage_pipeline(spans),
-                    Some(ParseError::TypeMismatch(
-                        Type::String,
-                        new_name_expression.ty.clone(),
-                        new_name_expression.span,
-                    )),
-                );
+                    }),
+                    Err(err) => return (garbage_pipeline(spans), Some(err)),
+                },
+                Err(err) => return (garbage_pipeline(spans), Some(err)),
             }
         } else {
             return (
@@ -2751,19 +2746,23 @@ pub fn parse_overlay_hide(
     (pipeline, None)
 }
 
-pub fn parse_let(
+pub fn parse_let_or_const(
     working_set: &mut StateWorkingSet,
     spans: &[Span],
     expand_aliases_denylist: &[usize],
 ) -> (Pipeline, Option<ParseError>) {
     let name = working_set.get_span_contents(spans[0]);
 
-    if name == b"let" {
+    if name == b"let" || name == b"const" {
+        let is_const = &name == b"const";
+
         if let Some((span, err)) = check_name(working_set, spans) {
             return (Pipeline::from_vec(vec![garbage(*span)]), Some(err));
         }
 
-        if let Some(decl_id) = working_set.find_decl(b"let", &Type::Any) {
+        if let Some(decl_id) =
+            working_set.find_decl(if is_const { b"const" } else { b"let" }, &Type::Any)
+        {
             let cmd = working_set.get_decl(decl_id);
             let call_signature = cmd.signature().call_signature();
 
@@ -2815,6 +2814,15 @@ pub fn parse_let(
 
                         if let Some(var_id) = var_id {
                             working_set.set_variable_type(var_id, rhs_type);
+
+                            if is_const {
+                                match eval_constant(working_set, &rvalue) {
+                                    Ok(val) => {
+                                        working_set.add_constant(var_id, val);
+                                    }
+                                    Err(err) => error = error.or(Some(err)),
+                                }
+                            }
                         }
 
                         let call = Box::new(Call {
@@ -2866,7 +2874,7 @@ pub fn parse_let(
     (
         garbage_pipeline(spans),
         Some(ParseError::UnknownState(
-            "internal error: let statement unparseable".into(),
+            "internal error: let or const statement unparseable".into(),
             span(spans),
         )),
     )
@@ -3036,10 +3044,45 @@ pub fn parse_source(
 
             // Command and one file name
             if spans.len() >= 2 {
-                let name_expr = working_set.get_span_contents(spans[1]);
-                let (filename, err) = unescape_unquote_string(name_expr, spans[1]);
+                let (expr, err) = parse_value(
+                    working_set,
+                    spans[1],
+                    &SyntaxShape::Any,
+                    expand_aliases_denylist,
+                );
+
+                error = error.or(err);
+
+                let val = match eval_constant(working_set, &expr) {
+                    Ok(val) => val,
+                    Err(err) => {
+                        return (
+                            Pipeline::from_vec(vec![Expression {
+                                expr: Expr::Call(call),
+                                span: span(&spans[1..]),
+                                ty: Type::Any,
+                                custom_completion: None,
+                            }]),
+                            Some(err),
+                        );
+                    }
+                };
+
+                let filename = match value_as_string(val, spans[1]) {
+                    Ok(s) => s,
+                    Err(err) => {
+                        return (
+                            Pipeline::from_vec(vec![Expression {
+                                expr: Expr::Call(call),
+                                span: span(&spans[1..]),
+                                ty: Type::Any,
+                                custom_completion: None,
+                            }]),
+                            Some(err),
+                        );
+                    }
+                };
 
-                if err.is_none() {
                 if let Some(path) = find_in_dirs(&filename, working_set, &cwd, LIB_DIRS_ENV) {
                     if let Ok(contents) = std::fs::read(&path) {
                         // Change currently parsed directory
@@ -3107,9 +3150,6 @@ pub fn parse_source(
                 } else {
                     error = error.or(Some(ParseError::SourcedFileNotFound(filename, spans[1])));
                 }
-                } else {
-                    return (garbage_pipeline(spans), Some(ParseError::NonUtf8(spans[1])));
-                }
             }
             return (
                 Pipeline::from_vec(vec![Expression {

crates/nu-parser/src/parser.rs

@@ -1,4 +1,5 @@
 use crate::{
+    eval::{eval_constant, value_as_string},
     lex, parse_mut,
     type_check::{math_result_type, type_compatible},
     ParseError, Token, TokenContents,
@@ -17,8 +18,8 @@ use nu_protocol::{
 
 use crate::parse_keywords::{
     parse_alias, parse_def, parse_def_predecl, parse_export_in_block, parse_extern, parse_for,
-    parse_hide, parse_let, parse_module, parse_overlay, parse_source, parse_use, parse_where,
-    parse_where_expr,
+    parse_hide, parse_let_or_const, parse_module, parse_overlay, parse_source, parse_use,
+    parse_where, parse_where_expr,
 };
 
 use itertools::Itertools;
@@ -2832,11 +2833,8 @@ pub fn parse_import_pattern(
 ) -> (Expression, Option<ParseError>) {
     let mut error = None;
 
-    let (head, head_span) = if let Some(head_span) = spans.get(0) {
-        (
-            working_set.get_span_contents(*head_span).to_vec(),
-            head_span,
-        )
+    let head_span = if let Some(head_span) = spans.get(0) {
+        head_span
     } else {
         return (
             garbage(span(spans)),
@@ -2844,7 +2842,25 @@ pub fn parse_import_pattern(
         );
     };
 
-    let maybe_module_id = working_set.find_module(&head);
+    let (head_expr, err) = parse_value(
+        working_set,
+        *head_span,
+        &SyntaxShape::Any,
+        expand_aliases_denylist,
+    );
+    error = error.or(err);
+
+    let (maybe_module_id, head_name) = match eval_constant(working_set, &head_expr) {
+        Ok(val) => match value_as_string(val, head_expr.span) {
+            Ok(s) => (working_set.find_module(s.as_bytes()), s.into_bytes()),
+            Err(err) => {
+                return (garbage(span(spans)), error.or(Some(err)));
+            }
+        },
+        Err(err) => {
+            return (garbage(span(spans)), error.or(Some(err)));
+        }
+    };
 
     let (import_pattern, err) = if let Some(tail_span) = spans.get(1) {
         // FIXME: expand this to handle deeper imports once we support module imports
@@ -2853,7 +2869,7 @@ pub fn parse_import_pattern(
             (
                 ImportPattern {
                     head: ImportPatternHead {
-                        name: head,
+                        name: head_name,
                         id: maybe_module_id,
                         span: *head_span,
                     },
@@ -2886,7 +2902,7 @@ pub fn parse_import_pattern(
                     (
                         ImportPattern {
                             head: ImportPatternHead {
-                                name: head,
+                                name: head_name,
                                 id: maybe_module_id,
                                 span: *head_span,
                             },
@@ -2899,7 +2915,7 @@ pub fn parse_import_pattern(
                 _ => (
                     ImportPattern {
                         head: ImportPatternHead {
-                            name: head,
+                            name: head_name,
                             id: maybe_module_id,
                             span: *head_span,
                         },
@@ -2914,7 +2930,7 @@ pub fn parse_import_pattern(
             (
                 ImportPattern {
                     head: ImportPatternHead {
-                        name: head,
+                        name: head_name,
                         id: maybe_module_id,
                         span: *head_span,
                     },
@@ -2931,7 +2947,7 @@ pub fn parse_import_pattern(
         (
             ImportPattern {
                 head: ImportPatternHead {
-                    name: head,
+                    name: head_name,
                     id: maybe_module_id,
                     span: *head_span,
                 },
@@ -4898,7 +4914,7 @@ pub fn parse_expression(
                 .0,
                 Some(ParseError::BuiltinCommandInPipeline("for".into(), spans[0])),
             ),
-            b"let" => (
+            b"let" | b"const" => (
                 parse_call(
                     working_set,
                     &spans[pos..],
@@ -5166,7 +5182,9 @@ pub fn parse_builtin_commands(
     match name {
         b"def" | b"def-env" => parse_def(working_set, lite_command, expand_aliases_denylist),
         b"extern" => parse_extern(working_set, lite_command, expand_aliases_denylist),
-        b"let" => parse_let(working_set, &lite_command.parts, expand_aliases_denylist),
+        b"let" | b"const" => {
+            parse_let_or_const(working_set, &lite_command.parts, expand_aliases_denylist)
+        }
         b"mut" => parse_mut(working_set, &lite_command.parts, expand_aliases_denylist),
         b"for" => {
             let (expr, err) = parse_for(working_set, &lite_command.parts, expand_aliases_denylist);

crates/nu-protocol/src/engine/engine_state.rs

@@ -173,6 +173,9 @@ impl EngineState {
                 for item in delta_overlay.vars.into_iter() {
                     existing_overlay.vars.insert(item.0, item.1);
                 }
+                for item in delta_overlay.constants.into_iter() {
+                    existing_overlay.constants.insert(item.0, item.1);
+                }
                 for item in delta_overlay.aliases.into_iter() {
                     existing_overlay.aliases.insert(item.0, item.1);
                 }
@@ -626,6 +629,16 @@ impl EngineState {
         output
     }
 
+    pub fn find_constant(&self, var_id: VarId, removed_overlays: &[Vec<u8>]) -> Option<&Value> {
+        for overlay_frame in self.active_overlays(removed_overlays).iter().rev() {
+            if let Some(val) = overlay_frame.constants.get(&var_id) {
+                return Some(val);
+            }
+        }
+
+        None
+    }
+
     pub fn get_span_contents(&self, span: &Span) -> &[u8] {
         for (contents, start, finish) in &self.file_contents {
             if span.start >= *start && span.end <= *finish {
@@ -1654,6 +1667,29 @@ impl<'a> StateWorkingSet<'a> {
         }
     }
 
+    pub fn add_constant(&mut self, var_id: VarId, val: Value) {
+        self.last_overlay_mut().constants.insert(var_id, val);
+    }
+
+    pub fn find_constant(&self, var_id: VarId) -> Option<&Value> {
+        let mut removed_overlays = vec![];
+
+        for scope_frame in self.delta.scope.iter().rev() {
+            for overlay_frame in scope_frame
+                .active_overlays(&mut removed_overlays)
+                .iter()
+                .rev()
+            {
+                if let Some(val) = overlay_frame.constants.get(&var_id) {
+                    return Some(val);
+                }
+            }
+        }
+
+        self.permanent_state
+            .find_constant(var_id, &removed_overlays)
+    }
+
     pub fn get_variable(&self, var_id: VarId) -> &Variable {
         let num_permanent_vars = self.permanent_state.num_vars();
         if var_id < num_permanent_vars {

crates/nu-protocol/src/engine/overlay.rs

@@ -1,4 +1,4 @@
-use crate::{AliasId, DeclId, ModuleId, OverlayId, Type, VarId};
+use crate::{AliasId, DeclId, ModuleId, OverlayId, Type, Value, VarId};
 use std::borrow::Borrow;
 use std::collections::HashMap;
 use std::hash::{Hash, Hasher};
@@ -199,6 +199,7 @@ impl ScopeFrame {
 #[derive(Debug, Clone)]
 pub struct OverlayFrame {
     pub vars: HashMap<Vec<u8>, VarId>,
+    pub constants: HashMap<VarId, Value>,
     pub predecls: HashMap<Vec<u8>, DeclId>, // temporary storage for predeclarations
     pub decls: HashMap<(Vec<u8>, Type), DeclId>,
     pub aliases: HashMap<Vec<u8>, AliasId>,
@@ -212,6 +213,7 @@ impl OverlayFrame {
     pub fn from_origin(origin: ModuleId, prefixed: bool) -> Self {
         Self {
             vars: HashMap::new(),
+            constants: HashMap::new(),
             predecls: HashMap::new(),
             decls: HashMap::new(),
             aliases: HashMap::new(),

tests/const_/mod.rs

@@ -0,0 +1,103 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn const_bool() {
+    let inp = &[r#"const x = false"#, r#"$x"#];
+
+    let actual = nu!(cwd: "tests/const_", pipeline(&inp.join("; ")));
+
+    assert_eq!(actual.out, "false");
+}
+
+#[test]
+fn const_int() {
+    let inp = &[r#"const x = 10"#, r#"$x"#];
+
+    let actual = nu!(cwd: "tests/const_", pipeline(&inp.join("; ")));
+
+    assert_eq!(actual.out, "10");
+}
+
+#[test]
+fn const_float() {
+    let inp = &[r#"const x = 1.234"#, r#"$x"#];
+
+    let actual = nu!(cwd: "tests/const_", pipeline(&inp.join("; ")));
+
+    assert_eq!(actual.out, "1.234");
+}
+
+#[test]
+fn const_binary() {
+    let inp = &[r#"const x = 0x[12]"#, r#"$x"#];
+
+    let actual = nu!(cwd: "tests/const_", pipeline(&inp.join("; ")));
+
+    assert!(actual.out.contains("12"));
+}
+
+#[test]
+fn const_datetime() {
+    let inp = &[r#"const x = 2021-02-27T13:55:40+00:00"#, r#"$x"#];
+
+    let actual = nu!(cwd: "tests/const_", pipeline(&inp.join("; ")));
+
+    assert!(actual.out.contains("Sat, 27 Feb 2021 13:55:40"));
+}
+
+#[test]
+fn const_list() {
+    let inp = &[r#"const x = [ a b c ]"#, r#"$x | describe"#];
+
+    let actual = nu!(cwd: "tests/const_", pipeline(&inp.join("; ")));
+
+    assert_eq!(actual.out, "list<string>");
+}
+
+#[test]
+fn const_record() {
+    let inp = &[r#"const x = { a: 10, b: 20, c: 30 }"#, r#"$x | describe"#];
+
+    let actual = nu!(cwd: "tests/const_", pipeline(&inp.join("; ")));
+
+    assert_eq!(actual.out, "record<a: int, b: int, c: int>");
+}
+
+#[test]
+fn const_table() {
+    let inp = &[
+        r#"const x = [[a b c]; [10 20 30] [100 200 300]]"#,
+        r#"$x | describe"#,
+    ];
+
+    let actual = nu!(cwd: "tests/const_", pipeline(&inp.join("; ")));
+
+    assert_eq!(actual.out, "table<a: int, b: int, c: int>");
+}
+
+#[test]
+fn const_string() {
+    let inp = &[r#"const x = "abc""#, r#"$x"#];
+
+    let actual = nu!(cwd: "tests/const_", pipeline(&inp.join("; ")));
+
+    assert_eq!(actual.out, "abc");
+}
+
+#[test]
+fn const_nothing() {
+    let inp = &[r#"const x = $nothing"#, r#"$x | describe"#];
+
+    let actual = nu!(cwd: "tests/const_", pipeline(&inp.join("; ")));
+
+    assert_eq!(actual.out, "nothing");
+}
+
+#[test]
+fn const_unsupported() {
+    let inp = &[r#"const x = ('abc' | str length)"#];
+
+    let actual = nu!(cwd: "tests/const_", pipeline(&inp.join("; ")));
+
+    assert!(actual.err.contains("not_a_constant"));
+}

tests/main.rs

@@ -1,5 +1,6 @@
 extern crate nu_test_support;
 
+mod const_;
 mod hooks;
 mod modules;
 mod overlays;

tests/modules/mod.rs

@@ -433,3 +433,44 @@ fn module_cyclical_imports_3() {
         assert!(actual.err.contains("cyclical"));
     })
 }
+
+#[test]
+fn module_import_const_file() {
+    Playground::setup("module_import_const_file", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "spam.nu",
+            r#"
+                export def foo [] { "foo" }
+            "#,
+        )]);
+
+        let inp = &[r#"const file = 'spam.nu'"#, r#"use $file foo"#, r#"foo"#];
+
+        let actual = nu!(cwd: dirs.test(), pipeline(&inp.join("; ")));
+
+        assert_eq!(actual.out, "foo");
+    })
+}
+
+#[test]
+fn module_import_const_module_name() {
+    Playground::setup("module_import_const_file", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "spam.nu",
+            r#"
+                export def foo [] { "foo" }
+            "#,
+        )]);
+
+        let inp = &[
+            r#"module spam { export def foo [] { "foo" } }"#,
+            r#"const mod = 'spam'"#,
+            r#"use $mod foo"#,
+            r#"foo"#,
+        ];
+
+        let actual = nu!(cwd: dirs.test(), pipeline(&inp.join("; ")));
+
+        assert_eq!(actual.out, "foo");
+    })
+}

tests/overlays/mod.rs

@@ -169,6 +169,33 @@ fn add_overlay_from_file_decl() {
     assert_eq!(actual_repl.out, "foo");
 }
 
+#[test]
+fn add_overlay_from_const_file_decl() {
+    let inp = &[
+        r#"const file = 'samples/spam.nu'"#,
+        r#"overlay use $file"#,
+        r#"foo"#,
+    ];
+
+    let actual = nu!(cwd: "tests/overlays", pipeline(&inp.join("; ")));
+
+    assert_eq!(actual.out, "foo");
+}
+
+#[test]
+fn add_overlay_from_const_module_name_decl() {
+    let inp = &[
+        r#"module spam { export def foo [] { "foo" } }"#,
+        r#"const mod = 'spam'"#,
+        r#"overlay use $mod"#,
+        r#"foo"#,
+    ];
+
+    let actual = nu!(cwd: "tests/overlays", pipeline(&inp.join("; ")));
+
+    assert_eq!(actual.out, "foo");
+}
+
 // This one tests that the `nu_repl()` loop works correctly
 #[test]
 fn add_overlay_from_file_decl_cd() {
@@ -693,6 +720,23 @@ fn overlay_add_renamed() {
     assert_eq!(actual_repl.out, "foo");
 }
 
+#[test]
+fn overlay_add_renamed_const() {
+    let inp = &[
+        r#"module spam { export def foo [] { "foo" } }"#,
+        r#"const name = 'spam'"#,
+        r#"const new_name = 'eggs'"#,
+        r#"overlay use $name as $new_name --prefix"#,
+        r#"eggs foo"#,
+    ];
+
+    let actual = nu!(cwd: "tests/overlays", pipeline(&inp.join("; ")));
+    let actual_repl = nu!(cwd: "tests/overlays", nu_repl_code(inp));
+
+    assert_eq!(actual.out, "foo");
+    assert_eq!(actual_repl.out, "foo");
+}
+
 #[test]
 fn overlay_add_renamed_from_file() {
     let inp = &[

tests/parsing/mod.rs

@@ -11,6 +11,17 @@ fn source_file_relative_to_file() {
     assert_eq!(actual.out, "5");
 }
 
+#[test]
+fn source_const_file() {
+    let actual = nu!(cwd: "tests/parsing/samples",
+    r#"
+        const file = 'single_line.nu'
+        source $file
+    "#);
+
+    assert_eq!(actual.out, "5");
+}
+
 #[test]
 fn run_nu_script_single_line() {
     let actual = nu!(cwd: "tests/parsing/samples", r#"