Carry the municipality of a CEP that does not existHEADv0.2.0main

A 404 from the API now brings a "faixa" naming the municipality the CEP
would have belonged to, and NotFound carries it. A CEP that fails is
rarely a random number: it is usually someone who knows the city and
guessed the digits, so naming the city turns a dead end into an answer
the caller can show.

It stays an error. The range it carries is the CEPs that actually exist
there, not the bounds the Correios publish — those start at the very CEP
that just failed — and it is sparse, which is what "ceps" measures. The
field is decoration on a failure, so parsing it is entirely defensive:
absent, null or malformed all yield None rather than costing a clean
not-found.

Breaking: NotFound gained a field, hence 0.2.0.

1 files changed, 24 insertions, 3 deletions

diff --git a/src/client.rs b/src/client.rs
index 8d73f06..4e6afcd 100644
--- a/src/client.rs
+++ b/src/client.rs
@@ -12,7 +12,7 @@ use serde_json::{Map, Value};
 
 use crate::csv;
 use crate::error::{Error, Result};
-use crate::model::{Address, Health};
+use crate::model::{Address, Faixa, Health};
 use crate::retry::{jitter, parse_retry_after};
 
 /// Where the public API lives.
@@ -270,7 +270,9 @@ impl Client {
     /// # Errors
     ///
     /// - [`Error::InvalidCep`] — not 8 digits (no request is made).
-    /// - [`Error::NotFound`] — well-formed, but not in the database.
+    /// - [`Error::NotFound`] — well-formed, but not in the database. It carries
+    ///   a [`Faixa`] naming the municipality the CEP would belong to, whenever
+    ///   the API can place it in one.
     /// - [`Error::RateLimited`] — 429 after every attempt.
     /// - [`Error::Server`] — 5xx after every attempt.
     /// - [`Error::Network`] — the request never completed.
@@ -280,7 +282,10 @@ impl Client {
         let (status, body) = self.request(&format!("/{number}"), None, &[200])?;
 
         if status == 404 {
-            return Err(Error::NotFound { cep: number });
+            return Err(Error::NotFound {
+                cep: number,
+                faixa: faixa(&body),
+            });
         }
         if status == 400 {
             return Err(Error::invalid_cep(error_message(
@@ -548,6 +553,22 @@ fn error_message(body: &[u8], fallback: &str) -> String {
         .unwrap_or_else(|| fallback.to_owned())
 }
 
+/// Pull the `faixa` block out of a 404 body, if there is a readable one.
+///
+/// Every step is allowed to fail into `None`: a body that is not JSON, a
+/// missing or null `faixa`, an object that names no municipality. The caller is
+/// on its way to returning [`Error::NotFound`] either way, and an unparseable
+/// extra field must not turn a correct not-found into an
+/// [`Error::InvalidResponse`].
+fn faixa(body: &[u8]) -> Option<Box<Faixa>> {
+    serde_json::from_slice::<Value>(body)
+        .ok()
+        .as_ref()
+        .and_then(|v| v.get("faixa"))
+        .and_then(Faixa::from_json)
+        .map(Box::new)
+}
+
 fn preview(body: &[u8]) -> String {
     let text = String::from_utf8_lossy(body);
     let cut = text