String interpolation (#1849)

* Add string interpolation

* fix coloring

* A few more fixups + tests

* merge master again

crates/nu-cli/src/commands/build_string.rs

@@ -0,0 +1,65 @@
+use crate::prelude::*;
+use nu_errors::ShellError;
+
+use crate::commands::WholeStreamCommand;
+use crate::data::value::format_leaf;
+use nu_protocol::{ReturnSuccess, Signature, SyntaxShape, UntaggedValue, Value};
+
+#[derive(Deserialize)]
+pub struct BuildStringArgs {
+    rest: Vec<Value>,
+}
+
+pub struct BuildString;
+
+impl WholeStreamCommand for BuildString {
+    fn name(&self) -> &str {
+        "build-string"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("build-string")
+            .rest(SyntaxShape::Any, "all values to form into the string")
+    }
+
+    fn usage(&self) -> &str {
+        "Builds a string from the arguments"
+    }
+
+    fn run(
+        &self,
+        args: CommandArgs,
+        registry: &CommandRegistry,
+    ) -> Result<OutputStream, ShellError> {
+        build_string(args, registry)
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Builds a string from a string and a number, without spaces between them",
+            example: "build-string 'foo' 3",
+            result: None,
+        }]
+    }
+}
+
+pub fn build_string(
+    args: CommandArgs,
+    registry: &CommandRegistry,
+) -> Result<OutputStream, ShellError> {
+    let registry = registry.clone();
+    let tag = args.call_info.name_tag.clone();
+    let stream = async_stream! {
+        let (BuildStringArgs { rest }, mut input) = args.process(&registry).await?;
+
+        let mut output_string = String::new();
+
+        for r in rest {
+            output_string.push_str(&format_leaf(&r).plain_string(100_000))
+        }
+
+        yield Ok(ReturnSuccess::Value(UntaggedValue::string(&output_string).into_value(tag)));
+    };
+
+    Ok(stream.to_output_stream())
+}