Add a quick post on using deserialize_with

_posts/2020-08-03-serde-deserialize-with-string.md

@@ -0,0 +1,45 @@
+---
+layout: post
+title: "Using serde's deserialize_with to handle custom strings"
+tags:
+- rust
+---
+
+I stumbled across a crate which implemented string parsing that I
+wished to incorporate into some of my [serde.rs](https://serde.rs)
+deserialization code. Unfortunately the crate in question,
+[cron](https://github.com/zslayton/cron) does not implement the
+`#[derive(Deserialize)]` macro on its `Schedule`, so I needed to fiddle with
+one of serde's "field attributes" in order to move forward: `deserialize_with`.
+
+Below is an example of my code, which was inspired by
+[this comment](https://github.com/serde-rs/serde/issues/1174#issuecomment-372411280)
+I stumbled across on GitHub:
+
+
+```rust
+use serde::de::{Deserialize, Deserializer};
+use std::str::FromStr;
+
+#[derive(Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Trigger {
+    #[serde(deserialize_with = "deserialize_cron_schedule")]
+    cron: cron::Schedule,
+}
+
+fn deserialize_cron_schedule<'de, D>(deserializer: D) -> Result<cron::Schedule, D::Error>
+where D: Deserializer<'de> {
+    let buf = String::deserialize(deserializer)?;
+
+    cron::Schedule::from_str(&buf).map_err(serde::de::Error::custom)
+}
+```
+
+I won't bother to explain what's going on here, because I don't understand much
+of it anyways. But the gist is that `String::deserialize` gets us a `String` to
+work with, which can then be passed along to the "real" parsing code.
+
+I hope this code snippet helps, I know it will sure make deserializing certain
+types of strings in my serde types _much_ easier!
+