Adds pest2ion command (#44)

Adds a basic CLI driver over the `pest-ion` crate.  This driver supports
setting the input file (defaulting to STDIN), an output file (defaulting
to STDOUT), and flags around the Ion data format.

An explicit TODO is to figure out a good way to test this probably using
integration tests and spawning the command. (See #42)

Resolves #36.

pest-ion/Cargo.toml

@@ -25,5 +25,14 @@ pest = "~2.1.3"
 pest_meta = "~2.1.3"
 ion-rs = "~0.6.0"
 
+# for pest2ion binary--not sure if this can be modeled separately
+clap = "~2.33.3"
+anyhow = "~1.0.40"
+
 [dev-dependencies]
 rstest = "~0.10.0"
+
+[[bin]]
+name = "pest2ion"
+test = false
+bench = false

pest-ion/src/bin/pest2ion/main.rs

@@ -0,0 +1,82 @@
+// Copyright Amazon.com, Inc. or its affiliates.
+
+use anyhow::Result;
+use clap::{crate_authors, crate_description, crate_version, App, Arg, ArgGroup};
+use ion_rs::value::writer::{ElementWriter, Format, TextKind};
+use pest_ion::{from_read, TryPestToElement};
+use std::fs::File;
+use std::io::{stdin, stdout, Write};
+use std::path::Path;
+
+fn main() -> Result<()> {
+    let matches = App::new("Pest to Ion Converter")
+        .version(crate_version!())
+        .author(crate_authors!())
+        .about(crate_description!())
+        .arg(
+            Arg::with_name("INPUT_FILE")
+                .help("The input file to parse (defaults to STDIN)")
+                .index(1),
+        )
+        .arg(
+            Arg::with_name("OUTPUT_FILE")
+                .long("output")
+                .short("o")
+                .takes_value(true)
+                .help("Writes output to the given file (defaults to STDOUT)"),
+        )
+        .arg(
+            Arg::with_name("text")
+                .long("text")
+                .short("t")
+                .help("Generate Ion text (default)"),
+        )
+        .arg(
+            Arg::with_name("binary")
+                .long("binary")
+                .short("b")
+                .help("Generate Ion binary"),
+        )
+        .arg(
+            Arg::with_name("pretty")
+                .long("pretty")
+                .short("p")
+                .help("Generate Ion pretty printed text"),
+        )
+        .group(ArgGroup::with_name("format").args(&["text", "binary", "pretty"]))
+        .get_matches();
+
+    let elem = if let Some(file_name) = matches.value_of("INPUT_FILE") {
+        Path::new(file_name).try_pest_to_element()?
+    } else {
+        // no file argument means read from stdin
+        from_read(stdin()).try_pest_to_element()?
+    };
+
+    // currently Ion element requires a fixed buffer to serialize to, let's choose something
+    // relatively big until this limitation is lifted
+    const BUFFER_SIZE: usize = 32 * 1024 * 1024;
+    let mut out_buf = vec![0u8; BUFFER_SIZE];
+
+    let format = if matches.is_present("binary") {
+        Format::Binary
+    } else if matches.is_present("pretty") {
+        Format::Text(TextKind::Pretty)
+    } else {
+        Format::Text(TextKind::Compact)
+    };
+
+    let mut writer = format.element_writer_for_slice(&mut out_buf)?;
+    writer.write(&elem)?;
+    let out_slice = writer.finish()?;
+
+    // TODO make output (file) configurable
+    let mut out: Box<dyn Write> = if let Some(out_file_name) = matches.value_of("OUTPUT_FILE") {
+        Box::new(File::create(out_file_name)?)
+    } else {
+        Box::new(stdout())
+    };
+    out.write_all(&out_slice)?;
+
+    Ok(())
+}