Add updates.warn_cache_age config option (#492)

This is useful when the cache is managed externally, e.g. provisioned
from a Nix store path or by a package manager, where the directory's
mtime doesn't reflect the cache's real age, causing a spurious warning
on every invocation. Today the only way to silence it is `--quiet`,
which must be passed every call and hides all other output too.

src/config.rs

@@ -228,6 +228,17 @@ fn default_archive_source() -> String {
     "https://github.com/tldr-pages/tldr/releases/latest/download".to_owned()
 }
 
+/// Controls when a warning about an outdated cache is printed.
+///
+/// Currently, the only nameable option is `"never"`. In the future, this may
+/// be extended to also accept a duration (e.g. `"60d"`), after which the
+/// warning should be shown.
+#[derive(Copy, Clone, Debug, Serialize, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "lowercase")]
+enum RawWarnCacheAge {
+    Never,
+}
+
 #[derive(Debug, Serialize, Deserialize, PartialEq, Eq)]
 struct RawUpdatesConfig {
     #[serde(default)]
@@ -240,6 +251,8 @@ struct RawUpdatesConfig {
     pub tls_backend: RawTlsBackend,
     #[serde(default)]
     pub download_languages: Option<Vec<String>>,
+    #[serde(default)]
+    pub warn_cache_age: Option<RawWarnCacheAge>,
 }
 
 impl Default for RawUpdatesConfig {
@@ -250,6 +263,7 @@ impl Default for RawUpdatesConfig {
             archive_source: default_archive_source(),
             tls_backend: RawTlsBackend::default(),
             download_languages: None,
+            warn_cache_age: None,
         }
     }
 }
@@ -380,6 +394,7 @@ pub struct UpdatesConfig<'a> {
     pub archive_source: &'a str,
     pub tls_backend: TlsBackend,
     pub download_languages: Vec<Language<'a>>,
+    pub warn_cache_age: Option<Duration>,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]
@@ -569,6 +584,10 @@ impl<'a> Config<'a> {
                 || search.languages.clone(),
                 |languages| languages.iter().map(|lang| Language(lang)).collect(),
             ),
+            warn_cache_age: match raw_config.updates.warn_cache_age {
+                None => Some(MAX_CACHE_AGE),
+                Some(RawWarnCacheAge::Never) => None,
+            },
         };
 
         let relative_path_root = config_file_path

src/main.rs

@@ -376,16 +376,18 @@ fn try_main(args: Cli, enable_styles: bool) -> Result<ExitCode> {
             return Ok(ExitCode::FAILURE);
         };
 
-        let age = cache.age()?;
-        if age > config::MAX_CACHE_AGE && !args.quiet {
-            print_warning(
-                enable_styles,
-                &format!(
-                    "The cache hasn't been updated for {} days.\n\
-                     You should probably run `tldr --update` soon.",
-                    age.as_secs() / 24 / 3600
-                ),
-            );
+        if let Some(max_cache_age) = config.updates.warn_cache_age {
+            let age = cache.age()?;
+            if age > max_cache_age && !args.quiet {
+                print_warning(
+                    enable_styles,
+                    &format!(
+                        "The cache hasn't been updated for {} days.\n\
+                         You should probably run `tldr --update` soon.",
+                        age.as_secs() / 24 / 3600
+                    ),
+                );
+            }
         }
 
         cache