Custom command attributes (#14906)

# Description
Add custom command attributes.

- Attributes are placed before a command definition and start with a `@`
character.
- Attribute invocations consist of const command call. The command's
name must start with "attr ", but this prefix is not used in the
invocation.
- A command named `attr example` is invoked as an attribute as
`@example`
-   Several built-in attribute commands are provided as part of this PR
    -   `attr example`: Attaches an example to the commands help text
        ```nushell
        # Double numbers
        @example "double an int"  { 5 | double }   --result 10
        @example "double a float" { 0.5 | double } --result 1.0
        def double []: [number -> number] {
            $in * 2
        }
        ```
    -   `attr search-terms`: Adds search terms to a command
    -   ~`attr env`: Equivalent to using `def --env`~
- ~`attr wrapped`: Equivalent to using `def --wrapped`~ shelved for
later discussion
    -   several testing related attributes in `std/testing`
- If an attribute has no internal/special purpose, it's stored as
command metadata that can be obtained with `scope commands`.
- This allows having attributes like `@test` which can be used by test
runners.
-   Used the `@example` attribute for `std` examples.
-   Updated the std tests and test runner to use `@test` attributes
-   Added completions for attributes

# User-Facing Changes
Users can add examples to their own command definitions, and add other
arbitrary attributes.

# Tests + Formatting

- 🟢 toolkit fmt
- 🟢 toolkit clippy
- 🟢 toolkit test
- 🟢 toolkit test stdlib

# After Submitting
- Add documentation about the attribute syntax and built-in attributes
- `help attributes`

---------

Co-authored-by: 132ikl <132@ikl.sh>

crates/nu-cmd-lang/src/core_commands/attr/example.rs

@@ -0,0 +1,159 @@
+use nu_engine::command_prelude::*;
+
+#[derive(Clone)]
+pub struct AttrExample;
+
+impl Command for AttrExample {
+    fn name(&self) -> &str {
+        "attr example"
+    }
+
+    // TODO: When const closure are available, switch to using them for the `example` argument
+    // rather than a block. That should remove the need for `requires_ast_for_arguments` to be true
+    fn signature(&self) -> Signature {
+        Signature::build("attr example")
+            .input_output_types(vec![(
+                Type::Nothing,
+                Type::Record(
+                    [
+                        ("description".into(), Type::String),
+                        ("example".into(), Type::String),
+                    ]
+                    .into(),
+                ),
+            )])
+            .allow_variants_without_examples(true)
+            .required(
+                "description",
+                SyntaxShape::String,
+                "Description of the example.",
+            )
+            .required(
+                "example",
+                SyntaxShape::OneOf(vec![SyntaxShape::Block, SyntaxShape::String]),
+                "Example code snippet.",
+            )
+            .named(
+                "result",
+                SyntaxShape::Any,
+                "Expected output of example.",
+                None,
+            )
+            .category(Category::Core)
+    }
+
+    fn description(&self) -> &str {
+        "Attribute for adding examples to custom commands."
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let description: Spanned<String> = call.req(engine_state, stack, 0)?;
+        let result: Option<Value> = call.get_flag(engine_state, stack, "result")?;
+
+        let example_string: Result<String, _> = call.req(engine_state, stack, 1);
+        let example_expr = call
+            .positional_nth(stack, 1)
+            .ok_or(ShellError::MissingParameter {
+                param_name: "example".into(),
+                span: call.head,
+            })?;
+
+        let working_set = StateWorkingSet::new(engine_state);
+
+        attr_example_impl(
+            example_expr,
+            example_string,
+            &working_set,
+            call,
+            description,
+            result,
+        )
+    }
+
+    fn run_const(
+        &self,
+        working_set: &StateWorkingSet,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let description: Spanned<String> = call.req_const(working_set, 0)?;
+        let result: Option<Value> = call.get_flag_const(working_set, "result")?;
+
+        let example_string: Result<String, _> = call.req_const(working_set, 1);
+        let example_expr =
+            call.assert_ast_call()?
+                .positional_nth(1)
+                .ok_or(ShellError::MissingParameter {
+                    param_name: "example".into(),
+                    span: call.head,
+                })?;
+
+        attr_example_impl(
+            example_expr,
+            example_string,
+            working_set,
+            call,
+            description,
+            result,
+        )
+    }
+
+    fn is_const(&self) -> bool {
+        true
+    }
+
+    fn requires_ast_for_arguments(&self) -> bool {
+        true
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Add examples to custom command",
+            example: r###"# Double numbers
+    @example "double an int" { 2 | double } --result 4
+    @example "double a float" { 0.25 | double } --result 0.5
+    def double []: [number -> number] { $in * 2 }"###,
+            result: None,
+        }]
+    }
+}
+
+fn attr_example_impl(
+    example_expr: &nu_protocol::ast::Expression,
+    example_string: Result<String, ShellError>,
+    working_set: &StateWorkingSet<'_>,
+    call: &Call<'_>,
+    description: Spanned<String>,
+    result: Option<Value>,
+) -> Result<PipelineData, ShellError> {
+    let example_content = match example_expr.as_block() {
+        Some(block_id) => {
+            let block = working_set.get_block(block_id);
+            let contents =
+                working_set.get_span_contents(block.span.expect("a block must have a span"));
+            let contents = contents
+                .strip_prefix(b"{")
+                .and_then(|x| x.strip_suffix(b"}"))
+                .unwrap_or(contents)
+                .trim_ascii();
+            String::from_utf8_lossy(contents).into_owned()
+        }
+        None => example_string?,
+    };
+
+    let mut rec = record! {
+        "description" => Value::string(description.item, description.span),
+        "example" => Value::string(example_content, example_expr.span),
+    };
+    if let Some(result) = result {
+        rec.push("result", result);
+    }
+
+    Ok(Value::record(rec, call.head).into_pipeline_data())
+}

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

@@ -0,0 +1,5 @@
+mod example;
+mod search_terms;
+
+pub use example::AttrExample;
+pub use search_terms::AttrSearchTerms;

crates/nu-cmd-lang/src/core_commands/attr/search_terms.rs

@@ -0,0 +1,57 @@
+use nu_engine::command_prelude::*;
+
+#[derive(Clone)]
+pub struct AttrSearchTerms;
+
+impl Command for AttrSearchTerms {
+    fn name(&self) -> &str {
+        "attr search-terms"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("attr search-terms")
+            .input_output_type(Type::Nothing, Type::list(Type::String))
+            .allow_variants_without_examples(true)
+            .rest("terms", SyntaxShape::String, "Search terms.")
+            .category(Category::Core)
+    }
+
+    fn description(&self) -> &str {
+        "Attribute for adding search terms to custom commands."
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let args = call.rest(engine_state, stack, 0)?;
+        Ok(Value::list(args, call.head).into_pipeline_data())
+    }
+
+    fn run_const(
+        &self,
+        working_set: &StateWorkingSet,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let args = call.rest_const(working_set, 0)?;
+        Ok(Value::list(args, call.head).into_pipeline_data())
+    }
+
+    fn is_const(&self) -> bool {
+        true
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Add search terms to a custom command",
+            example: r###"# Double numbers
+    @search-terms multiply times
+    def double []: [number -> number] { $in * 2 }"###,
+            result: None,
+        }]
+    }
+}

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

@@ -1,4 +1,5 @@
 mod alias;
+mod attr;
 mod break_;
 mod collect;
 mod const_;
@@ -35,6 +36,7 @@ mod version;
 mod while_;
 
 pub use alias::Alias;
+pub use attr::*;
 pub use break_::Break;
 pub use collect::Collect;
 pub use const_::Const;