refactor: reduce usage of slicing in qr module

src/qr.rs

@@ -367,9 +367,10 @@ pub fn format_backup(qr: &Qr) -> Result<String> {
 /// scheme: `OPENPGP4FPR:FINGERPRINT#a=ADDR&n=NAME&i=INVITENUMBER&s=AUTH`
 ///     or: `OPENPGP4FPR:FINGERPRINT#a=ADDR&g=GROUPNAME&x=GROUPID&i=INVITENUMBER&s=AUTH`
 ///     or: `OPENPGP4FPR:FINGERPRINT#a=ADDR`
-#[allow(clippy::indexing_slicing)]
 async fn decode_openpgp(context: &Context, qr: &str) -> Result<Qr> {
-    let payload = &qr[OPENPGP4FPR_SCHEME.len()..];
+    let payload = qr
+        .strip_prefix(OPENPGP4FPR_SCHEME)
+        .context("Invalid OPENPGP4FPR scheme")?;
 
     // macOS and iOS sometimes replace the # with %23 (uri encode it), we should be able to parse this wrong format too.
     // see issue https://github.com/deltachat/deltachat-core-rust/issues/1969 for more info
@@ -545,8 +546,8 @@ async fn decode_ideltachat(context: &Context, prefix: &str, qr: &str) -> Result<
 /// scheme: `DCACCOUNT:https://example.org/new_email?t=1w_7wDjgjelxeX884x96v3`
 fn decode_account(qr: &str) -> Result<Qr> {
     let payload = qr
-        .get(DCACCOUNT_SCHEME.len()..)
-        .context("invalid DCACCOUNT payload")?;
+        .strip_prefix(DCACCOUNT_SCHEME)
+        .context("Invalid DCACCOUNT payload")?;
     let url = url::Url::parse(payload).context("Invalid account URL")?;
     if url.scheme() == "http" || url.scheme() == "https" {
         Ok(Qr::Account {
@@ -563,8 +564,8 @@ fn decode_account(qr: &str) -> Result<Qr> {
 /// scheme: `DCWEBRTC:https://meet.jit.si/$ROOM`
 fn decode_webrtc_instance(_context: &Context, qr: &str) -> Result<Qr> {
     let payload = qr
-        .get(DCWEBRTC_SCHEME.len()..)
-        .context("invalid DCWEBRTC payload")?;
+        .strip_prefix(DCWEBRTC_SCHEME)
+        .context("Invalid DCWEBRTC payload")?;
 
     let (_type, url) = Message::parse_webrtc_instance(payload);
     let url = url::Url::parse(&url).context("Invalid WebRTC instance")?;
@@ -637,7 +638,7 @@ fn decode_shadowsocks_proxy(qr: &str) -> Result<Qr> {
 fn decode_backup2(qr: &str) -> Result<Qr> {
     let payload = qr
         .strip_prefix(DCBACKUP2_SCHEME)
-        .ok_or_else(|| anyhow!("invalid DCBACKUP scheme"))?;
+        .context("Invalid DCBACKUP2 scheme")?;
     let (auth_token, node_addr) = payload
         .split_once('&')
         .context("Backup QR code has no separator")?;
@@ -668,9 +669,10 @@ struct CreateAccountErrorResponse {
 /// take a qr of the type DC_QR_ACCOUNT, parse it's parameters,
 /// download additional information from the contained url and set the parameters.
 /// on success, a configure::configure() should be able to log in to the account
-#[allow(clippy::indexing_slicing)]
 async fn set_account_from_qr(context: &Context, qr: &str) -> Result<()> {
-    let url_str = &qr[DCACCOUNT_SCHEME.len()..];
+    let url_str = qr
+        .strip_prefix(DCACCOUNT_SCHEME)
+        .context("Invalid DCACCOUNT scheme")?;
 
     if !url_str.starts_with(HTTPS_SCHEME) {
         bail!("DCACCOUNT QR codes must use HTTPS scheme");
@@ -800,9 +802,10 @@ pub async fn set_config_from_qr(context: &Context, qr: &str) -> Result<()> {
 /// Extract address for the mailto scheme.
 ///
 /// Scheme: `mailto:addr...?subject=...&body=..`
-#[allow(clippy::indexing_slicing)]
 async fn decode_mailto(context: &Context, qr: &str) -> Result<Qr> {
-    let payload = &qr[MAILTO_SCHEME.len()..];
+    let payload = qr
+        .strip_prefix(MAILTO_SCHEME)
+        .context("Invalid mailto: scheme")?;
 
     let (addr, query) = if let Some(query_index) = payload.find('?') {
         (&payload[..query_index], &payload[query_index + 1..])
@@ -855,9 +858,10 @@ async fn decode_mailto(context: &Context, qr: &str) -> Result<Qr> {
 /// Extract address for the smtp scheme.
 ///
 /// Scheme: `SMTP:addr...:subject...:body...`
-#[allow(clippy::indexing_slicing)]
 async fn decode_smtp(context: &Context, qr: &str) -> Result<Qr> {
-    let payload = &qr[SMTP_SCHEME.len()..];
+    let payload = qr
+        .strip_prefix(SMTP_SCHEME)
+        .context("Invalid SMTP scheme")?;
 
     let addr = if let Some(query_index) = payload.find(':') {
         &payload[..query_index]