Add suggestion to upgrade the compiler

Author: George-lewis

compiler/rustc_session/messages.ftl

@@ -24,6 +24,9 @@ session_feature_diagnostic_for_issue =
 session_feature_diagnostic_help =
     add `#![feature({$feature})]` to the crate attributes to enable
 
+session_feature_suggest_upgrade_compiler =
+    this compiler is version {$version} built on {$date}, consider upgrading?
+
 session_file_is_not_writeable = output file {$file} is not writeable -- check its permissions
 
 session_file_write_fail = failed to write `{$path}` due to error `{$err}`

compiler/rustc_session/src/errors.rs

@@ -32,6 +32,22 @@ pub struct FeatureDiagnosticForIssue {
     pub n: NonZeroU32,
 }
 
+#[derive(Subdiagnostic)]
+#[note(session_feature_suggest_upgrade_compiler)]
+pub struct SuggestUpgradeCompiler {
+    version: &'static str,
+    date: &'static str
+}
+
+impl SuggestUpgradeCompiler {
+    pub fn new() -> Self {
+        let version = option_env!("CFG_VERSION").unwrap_or("unknown");
+        let date = option_env!("CFG_VER_DATE").unwrap_or("unknown");
+
+        Self { version, date }
+    }
+}
+
 #[derive(Subdiagnostic)]
 #[help(session_feature_diagnostic_help)]
 pub struct FeatureDiagnosticHelp {

compiler/rustc_session/src/parse.rs

@@ -3,7 +3,7 @@
 
 use crate::config::{Cfg, CheckCfg};
 use crate::errors::{
-    CliFeatureDiagnosticHelp, FeatureDiagnosticForIssue, FeatureDiagnosticHelp, FeatureGateError,
+    CliFeatureDiagnosticHelp, FeatureDiagnosticForIssue, FeatureDiagnosticHelp, FeatureGateError, SuggestUpgradeCompiler,
 };
 use crate::lint::{
     builtin::UNSTABLE_SYNTAX_PRE_EXPANSION, BufferedEarlyLint, BuiltinLintDiagnostics, Lint, LintId,
@@ -184,6 +184,8 @@ pub fn add_feature_diagnostics_for_issue(
         } else {
             err.subdiagnostic(FeatureDiagnosticHelp { feature });
         }
+
+        err.subdiagnostic(SuggestUpgradeCompiler::new());
     }
 }