Enable conditional source and use patterns by allowing null as a no-op module (#14773)

Related:
- #14329
- #13872
- #8214

# Description & User-Facing Changes

This PR allows enables the following uses, which are all no-op.
```nushell
source null
source-env null
use null
overlay use null
```

The motivation for this change is conditional sourcing of files. For
example, with this change `login.nu` may be deprecated and replaced with
the following code in `config.nu`
```nushell
const login_module = if $nu.is-login { "login.nu" } else { null }
source $login_module
```

# Tests + Formatting
I'm hoping for CI to pass 😄

# After Submitting
Add a part about the conditional sourcing pattern to the website.

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

@@ -24,8 +24,8 @@ impl Command for OverlayUse {
             .allow_variants_without_examples(true)
             .required(
                 "name",
-                SyntaxShape::String,
-                "Module name to use overlay for.",
+                SyntaxShape::OneOf(vec![SyntaxShape::String, SyntaxShape::Nothing]),
+                "Module name to use overlay for (`null` for no-op).",
             )
             .optional(
                 "as",
@@ -61,6 +61,11 @@ impl Command for OverlayUse {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        let noop = call.get_parser_info(caller_stack, "noop");
+        if noop.is_some() {
+            return Ok(PipelineData::empty());
+        }
+
         let mut name_arg: Spanned<String> = call.req(engine_state, caller_stack, 0)?;
         name_arg.item = trim_quotes_str(&name_arg.item).to_string();
 

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

@@ -22,7 +22,11 @@ impl Command for Use {
         Signature::build("use")
             .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .allow_variants_without_examples(true)
-            .required("module", SyntaxShape::String, "Module or module file.")
+            .required(
+                "module",
+                SyntaxShape::OneOf(vec![SyntaxShape::String, SyntaxShape::Nothing]),
+                "Module or module file (`null` for no-op).",
+            )
             .rest(
                 "members",
                 SyntaxShape::Any,
@@ -54,6 +58,9 @@ This command is a parser keyword. For details, check:
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        if call.get_parser_info(caller_stack, "noop").is_some() {
+            return Ok(PipelineData::empty());
+        }
         let Some(Expression {
             expr: Expr::ImportPattern(import_pattern),
             ..

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

@@ -19,8 +19,8 @@ impl Command for SourceEnv {
             .input_output_types(vec![(Type::Any, Type::Any)])
             .required(
                 "filename",
-                SyntaxShape::String, // type is string to avoid automatically canonicalizing the path
-                "The filepath to the script file to source the environment from.",
+                SyntaxShape::OneOf(vec![SyntaxShape::String, SyntaxShape::Nothing]), // type is string to avoid automatically canonicalizing the path
+                "The filepath to the script file to source the environment from (`null` for no-op).",
             )
             .category(Category::Core)
     }
@@ -45,6 +45,10 @@ impl Command for SourceEnv {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        if call.get_parser_info(caller_stack, "noop").is_some() {
+            return Ok(PipelineData::empty());
+        }
+
         let source_filename: Spanned<String> = call.req(engine_state, caller_stack, 0)?;
 
         // Note: this hidden positional is the block_id that corresponded to the 0th position
@@ -99,10 +103,17 @@ impl Command for SourceEnv {
     }
 
     fn examples(&self) -> Vec<Example> {
-        vec![Example {
+        vec![
+            Example {
                 description: "Sources the environment from foo.nu in the current context",
                 example: r#"source-env foo.nu"#,
                 result: None,
-        }]
+            },
+            Example {
+                description: "Sourcing `null` is a no-op.",
+                example: r#"source-env null"#,
+                result: None,
+            },
+        ]
     }
 }

crates/nu-command/src/misc/source.rs

@@ -16,8 +16,8 @@ impl Command for Source {
             .input_output_types(vec![(Type::Any, Type::Any)])
             .required(
                 "filename",
-                SyntaxShape::Filepath,
-                "The filepath to the script file to source.",
+                SyntaxShape::OneOf(vec![SyntaxShape::Filepath, SyntaxShape::Nothing]),
+                "The filepath to the script file to source (`null` for no-op).",
             )
             .category(Category::Core)
     }
@@ -42,6 +42,9 @@ impl Command for Source {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        if call.get_parser_info(stack, "noop").is_some() {
+            return Ok(PipelineData::empty());
+        }
         // Note: two hidden positionals are used here that are injected by the parser:
         // 1. The block_id that corresponded to the 0th position
         // 2. The block_id_name that corresponded to the file name at the 0th position
@@ -107,6 +110,16 @@ impl Command for Source {
                 example: r#"source ./foo.nu; say-hi"#,
                 result: None,
             },
+            Example {
+                description: "Sourcing `null` is a no-op.",
+                example: r#"source null"#,
+                result: None,
+            },
+            Example {
+                description: "Source can be used with const variables.",
+                example: r#"const file = if $nu.is-interactive { "interactive.nu" } else { null }; source $file"#,
+                result: None,
+            }
         ]
     }
 }

crates/nu-parser/src/parse_keywords.rs

@@ -2367,18 +2367,37 @@ pub fn parse_use(
 
     let import_pattern_expr = parse_import_pattern(working_set, args_spans);
 
-    let import_pattern = if let Expression {
+    let import_pattern = match &import_pattern_expr {
+        Expression {
+            expr: Expr::Nothing,
+            ..
+        } => {
+            let mut call = call;
+            call.set_parser_info(
+                "noop".to_string(),
+                Expression::new_unknown(Expr::Nothing, Span::unknown(), Type::Nothing),
+            );
+            return (
+                Pipeline::from_vec(vec![Expression::new(
+                    working_set,
+                    Expr::Call(call),
+                    Span::concat(spans),
+                    Type::Any,
+                )]),
+                vec![],
+            );
+        }
+        Expression {
             expr: Expr::ImportPattern(import_pattern),
             ..
-    } = &import_pattern_expr
-    {
-        import_pattern.clone()
-    } else {
+        } => import_pattern.clone(),
+        _ => {
             working_set.error(ParseError::UnknownState(
                 "internal error: Import pattern positional is not import pattern".into(),
                 import_pattern_expr.span,
             ));
             return (garbage_pipeline(working_set, spans), vec![]);
+        }
     };
 
     let (mut import_pattern, module, module_id) = if let Some(module_id) = import_pattern.head.id {
@@ -2755,6 +2774,19 @@ pub fn parse_overlay_use(working_set: &mut StateWorkingSet, call: Box<Call>) ->
 
     let (overlay_name, overlay_name_span) = if let Some(expr) = call.positional_nth(0) {
         match eval_constant(working_set, expr) {
+            Ok(Value::Nothing { .. }) => {
+                let mut call = call;
+                call.set_parser_info(
+                    "noop".to_string(),
+                    Expression::new_unknown(Expr::Bool(true), Span::unknown(), Type::Bool),
+                );
+                return Pipeline::from_vec(vec![Expression::new(
+                    working_set,
+                    Expr::Call(call),
+                    call_span,
+                    Type::Any,
+                )]);
+            }
             Ok(val) => match val.coerce_into_string() {
                 Ok(s) => (s, expr.span),
                 Err(err) => {
@@ -3494,6 +3526,20 @@ pub fn parse_source(working_set: &mut StateWorkingSet, lite_command: &LiteComman
                     }
                 };
 
+                if val.is_nothing() {
+                    let mut call = call;
+                    call.set_parser_info(
+                        "noop".to_string(),
+                        Expression::new_unknown(Expr::Nothing, Span::unknown(), Type::Nothing),
+                    );
+                    return Pipeline::from_vec(vec![Expression::new(
+                        working_set,
+                        Expr::Call(call),
+                        Span::concat(spans),
+                        Type::Any,
+                    )]);
+                }
+
                 let filename = match val.coerce_into_string() {
                     Ok(s) => s,
                     Err(err) => {

crates/nu-parser/src/parser.rs

@@ -15,7 +15,7 @@ use nu_engine::DIR_VAR_PARSER_INFO;
 use nu_protocol::{
     ast::*, engine::StateWorkingSet, eval_const::eval_constant, BlockId, DeclId, DidYouMean,
     FilesizeUnit, Flag, ParseError, PositionalArg, Signature, Span, Spanned, SyntaxShape, Type,
-    VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
+    Value, VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
 };
 use std::{
     collections::{HashMap, HashSet},
@@ -3054,6 +3054,14 @@ pub fn parse_import_pattern(working_set: &mut StateWorkingSet, spans: &[Span]) -
     let head_expr = parse_value(working_set, *head_span, &SyntaxShape::Any);
 
     let (maybe_module_id, head_name) = match eval_constant(working_set, &head_expr) {
+        Ok(Value::Nothing { .. }) => {
+            return Expression::new(
+                working_set,
+                Expr::Nothing,
+                Span::concat(spans),
+                Type::Nothing,
+            );
+        }
         Ok(val) => match val.coerce_into_string() {
             Ok(s) => (working_set.find_module(s.as_bytes()), s.into_bytes()),
             Err(err) => {

tests/shell/mod.rs

@@ -335,6 +335,51 @@ fn source_empty_file() {
     })
 }
 
+#[test]
+fn source_use_null() {
+    let actual = nu!(r#"source null"#);
+    assert!(actual.out.is_empty());
+    assert!(actual.err.is_empty());
+
+    let actual = nu!(r#"source-env null"#);
+    assert!(actual.out.is_empty());
+    assert!(actual.err.is_empty());
+
+    let actual = nu!(r#"use null"#);
+    assert!(actual.out.is_empty());
+    assert!(actual.err.is_empty());
+
+    let actual = nu!(r#"overlay use null"#);
+    assert!(actual.out.is_empty());
+    assert!(actual.err.is_empty());
+}
+
+#[test]
+fn source_use_file_named_null() {
+    Playground::setup("source_file_named_null", |dirs, sandbox| {
+        sandbox.with_files(&[FileWithContent(
+            "null",
+            r#"export-env { print "hello world" }"#,
+        )]);
+
+        let actual = nu!(cwd: dirs.test(), r#"source "null""#);
+        assert!(actual.out.contains("hello world"));
+        assert!(actual.err.is_empty());
+
+        let actual = nu!(cwd: dirs.test(), r#"source-env "null""#);
+        assert!(actual.out.contains("hello world"));
+        assert!(actual.err.is_empty());
+
+        let actual = nu!(cwd: dirs.test(), r#"use "null""#);
+        assert!(actual.out.contains("hello world"));
+        assert!(actual.err.is_empty());
+
+        let actual = nu!(cwd: dirs.test(), r#"overlay use "null""#);
+        assert!(actual.out.contains("hello world"));
+        assert!(actual.err.is_empty());
+    })
+}
+
 #[test]
 fn main_script_help_uses_script_name1() {
     // Note: this test is somewhat fragile and might need to be adapted if the usage help message changes