refactor: Use regular functions rather than FromStr impls

Implementing `FromStr` and then calling `parse()` creates an
indirection, which is hard to follow for people who are not familiar
with Rust. r10s recently had this problem.

src/aheader.rs

@@ -4,9 +4,8 @@
 
 use std::collections::BTreeMap;
 use std::fmt;
-use std::str::FromStr;
 
-use anyhow::{Context as _, Error, Result, bail};
+use anyhow::{Context as _, Result, bail};
 
 use crate::key::{DcKey, SignedPublicKey};
 
@@ -28,10 +27,8 @@ impl fmt::Display for EncryptPreference {
     }
 }
 
-impl FromStr for EncryptPreference {
-    type Err = Error;
-
-    fn from_str(s: &str) -> Result<Self> {
+impl EncryptPreference {
+    fn new(s: &str) -> Result<Self> {
         match s {
             "mutual" => Ok(EncryptPreference::Mutual),
             "nopreference" => Ok(EncryptPreference::NoPreference),
@@ -85,10 +82,8 @@ impl fmt::Display for Aheader {
     }
 }
 
-impl FromStr for Aheader {
-    type Err = Error;
-
-    fn from_str(s: &str) -> Result<Self> {
+impl Aheader {
+    pub(crate) fn from_str(s: &str) -> Result<Self> {
         let mut attributes: BTreeMap<String, String> = s
             .split(';')
             .filter_map(|a| {
@@ -116,7 +111,7 @@ impl FromStr for Aheader {
 
         let prefer_encrypt = attributes
             .remove("prefer-encrypt")
-            .and_then(|raw| raw.parse().ok())
+            .and_then(|raw| EncryptPreference::new(&raw).ok())
             .unwrap_or_default();
 
         let verified = attributes.remove("_verified").is_some();
@@ -144,8 +139,9 @@ mod tests {
 
     #[test]
     fn test_from_str() -> Result<()> {
-        let h: Aheader =
-            format!("addr=me@mail.com; prefer-encrypt=mutual; keydata={RAWKEY}").parse()?;
+        let h = Aheader::from_str(&format!(
+            "addr=me@mail.com; prefer-encrypt=mutual; keydata={RAWKEY}"
+        ))?;
 
         assert_eq!(h.addr, "me@mail.com");
         assert_eq!(h.prefer_encrypt, EncryptPreference::Mutual);
@@ -157,7 +153,7 @@ mod tests {
     #[test]
     fn test_from_str_reset() -> Result<()> {
         let raw = format!("addr=reset@example.com; prefer-encrypt=reset; keydata={RAWKEY}");
-        let h: Aheader = raw.parse()?;
+        let h = Aheader::from_str(&raw)?;
 
         assert_eq!(h.addr, "reset@example.com");
         assert_eq!(h.prefer_encrypt, EncryptPreference::NoPreference);
@@ -167,7 +163,7 @@ mod tests {
     #[test]
     fn test_from_str_non_critical() -> Result<()> {
         let raw = format!("addr=me@mail.com; _foo=one; _bar=two; keydata={RAWKEY}");
-        let h: Aheader = raw.parse()?;
+        let h = Aheader::from_str(&raw)?;
 
         assert_eq!(h.addr, "me@mail.com");
         assert_eq!(h.prefer_encrypt, EncryptPreference::NoPreference);
@@ -177,7 +173,7 @@ mod tests {
     #[test]
     fn test_from_str_superflous_critical() {
         let raw = format!("addr=me@mail.com; _foo=one; _bar=two; other=me; keydata={RAWKEY}");
-        assert!(raw.parse::<Aheader>().is_err());
+        assert!(Aheader::from_str(&raw).is_err());
     }
 
     #[test]

src/ephemeral.rs

@@ -66,7 +66,6 @@ use std::cmp::max;
 use std::collections::BTreeSet;
 use std::fmt;
 use std::num::ParseIntError;
-use std::str::FromStr;
 use std::time::{Duration, UNIX_EPOCH};
 
 use anyhow::{Context as _, Result, ensure};
@@ -124,6 +123,12 @@ impl Timer {
             Self::Enabled { duration }
         }
     }
+
+    /// Tries to parse a string as an integer,
+    /// and converts it to an ephemeral timer value.
+    pub fn from_str(input: &str) -> Result<Timer, ParseIntError> {
+        input.parse::<u32>().map(Self::from_u32)
+    }
 }
 
 impl fmt::Display for Timer {
@@ -132,14 +137,6 @@ impl fmt::Display for Timer {
     }
 }
 
-impl FromStr for Timer {
-    type Err = ParseIntError;
-
-    fn from_str(input: &str) -> Result<Timer, ParseIntError> {
-        input.parse::<u32>().map(Self::from_u32)
-    }
-}
-
 impl rusqlite::types::ToSql for Timer {
     fn to_sql(&self) -> rusqlite::Result<rusqlite::types::ToSqlOutput<'_>> {
         let val = rusqlite::types::Value::Integer(match self {

src/mimeparser.rs

@@ -2127,7 +2127,7 @@ async fn parse_gossip_headers(
     let mut gossiped_keys: BTreeMap<String, GossipedKey> = Default::default();
 
     for value in &gossip_headers {
-        let header = match value.parse::<Aheader>() {
+        let header = match Aheader::from_str(value) {
             Ok(header) => header,
             Err(err) => {
                 warn!(context, "Failed parsing Autocrypt-Gossip header: {}", err);

src/receive_imf.rs

@@ -1873,7 +1873,7 @@ async fn add_parts(
     // Extract ephemeral timer from the message
     let mut ephemeral_timer = if let Some(value) = mime_parser.get_header(HeaderDef::EphemeralTimer)
     {
-        match value.parse::<EphemeralTimer>() {
+        match EphemeralTimer::from_str(value) {
             Ok(timer) => timer,
             Err(err) => {
                 warn!(context, "Can't parse ephemeral timer \"{value}\": {err:#}.");