Added expect lint level and attribute (RFC-2383)

Author: xFrednet

compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs

@@ -73,6 +73,7 @@ fn annotation_type_for_level(level: Level) -> AnnotationType {
         // FIXME(#59346): Not sure how to map these two levels
         Level::Cancelled | Level::FailureNote => AnnotationType::Error,
         Level::Allow => panic!("Should not call with Allow"),
+        Level::Expect => panic!("Should not call with Expect"),
     }
 }
 

compiler/rustc_errors/src/diagnostic.rs

@@ -98,7 +98,12 @@ impl Diagnostic {
         match self.level {
             Level::Bug | Level::Fatal | Level::Error | Level::FailureNote => true,
 
-            Level::Warning | Level::Note | Level::Help | Level::Cancelled | Level::Allow => false,
+            Level::Warning
+            | Level::Note
+            | Level::Help
+            | Level::Cancelled
+            | Level::Allow
+            | Level::Expect => false,
         }
     }
 

compiler/rustc_errors/src/lib.rs

@@ -589,6 +589,11 @@ impl Handler {
         DiagnosticBuilder::new(self, Level::Allow, msg)
     }
 
+    /// Construct a builder at the `Expect` level with the `msg`.
+    pub fn struct_expect(&self, msg: &str) -> DiagnosticBuilder<'_> {
+        DiagnosticBuilder::new(self, Level::Expect, msg)
+    }
+
     /// Construct a builder at the `Error` level at the given `span` and with the `msg`.
     pub fn struct_span_err(&self, span: impl Into<MultiSpan>, msg: &str) -> DiagnosticBuilder<'_> {
         let mut result = self.struct_err(msg);
@@ -1101,6 +1106,7 @@ pub enum Level {
     Cancelled,
     FailureNote,
     Allow,
+    Expect,
 }
 
 impl fmt::Display for Level {
@@ -1126,7 +1132,7 @@ impl Level {
                 spec.set_fg(Some(Color::Cyan)).set_intense(true);
             }
             FailureNote => {}
-            Allow | Cancelled => unreachable!(),
+            Allow | Expect | Cancelled => unreachable!(),
         }
         spec
     }
@@ -1141,6 +1147,7 @@ impl Level {
             FailureNote => "failure-note",
             Cancelled => panic!("Shouldn't call on cancelled error"),
             Allow => panic!("Shouldn't call on allowed error"),
+            Expect => panic!("Shouldn't call on expected error"),
         }
     }
 

compiler/rustc_feature/src/builtin_attrs.rs

@@ -204,6 +204,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     // Lints:
     ungated!(warn, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#)),
     ungated!(allow, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#)),
+    gated!(
+        expect, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#), lint_reasons,
+        experimental!(expect)
+    ),
     ungated!(forbid, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#)),
     ungated!(deny, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#)),
     ungated!(must_use, AssumedUsed, template!(Word, NameValueStr: "reason")),

compiler/rustc_lint/src/context.rs

@@ -125,6 +125,7 @@ struct LintGroup {
     depr: Option<LintAlias>,
 }
 
+#[derive(Debug)]
 pub enum CheckLintNameResult<'a> {
     Ok(&'a [LintId]),
     /// Lint doesn't exist. Potentially contains a suggestion for a correct lint name.
@@ -373,6 +374,9 @@ impl LintStore {
                     Level::ForceWarn => "--force-warn",
                     Level::Deny => "-D",
                     Level::Forbid => "-F",
+                    Level::Expect => {
+                        unreachable!("lints with the level of `expect` should not run this code");
+                    }
                 },
                 lint_name
             );

compiler/rustc_lint_defs/src/lib.rs

@@ -47,12 +47,31 @@ pub enum Applicability {
 }
 
 /// Setting for how to handle a lint.
+///
+/// See: https://doc.rust-lang.org/rustc/lints/levels.html
 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
 pub enum Level {
+    /// The `allow` level will not issue any message.
     Allow,
+    /// The `expect` level will suppress the lint message but intern produce a message
+    /// if the lint wasn't issued in the expected scope. Expect should not be used as
+    /// an initial level for a lint.
+    ///
+    /// Note that this still means that the lint is enabled in this position and should
+    /// be passed onwards to the `LintContext` which will fulfill the expectation and
+    /// suppress the lint.
+    ///
+    /// See RFC 2383.
+    Expect,
+    /// The `warn` level will produce a warning if the lint was violated, however the
+    /// compiler will continue with its execution.
     Warn,
     ForceWarn,
+    /// The `deny` level will produce an error and stop further execution after the lint
+    /// pass is complete.
     Deny,
+    /// `Forbid` is equivalent to the `deny` level but can't be overwritten like the previous
+    /// levels.
     Forbid,
 }
 
@@ -63,6 +82,7 @@ impl Level {
     pub fn as_str(self) -> &'static str {
         match self {
             Level::Allow => "allow",
+            Level::Expect => "expect",
             Level::Warn => "warn",
             Level::ForceWarn => "force-warn",
             Level::Deny => "deny",
@@ -74,6 +94,7 @@ impl Level {
     pub fn from_str(x: &str) -> Option<Level> {
         match x {
             "allow" => Some(Level::Allow),
+            "expect" => Some(Level::Expect),
             "warn" => Some(Level::Warn),
             "deny" => Some(Level::Deny),
             "forbid" => Some(Level::Forbid),
@@ -85,6 +106,7 @@ impl Level {
     pub fn from_symbol(x: Symbol) -> Option<Level> {
         match x {
             sym::allow => Some(Level::Allow),
+            sym::expect => Some(Level::Expect),
             sym::warn => Some(Level::Warn),
             sym::deny => Some(Level::Deny),
             sym::forbid => Some(Level::Forbid),

compiler/rustc_middle/src/lint.rs

@@ -243,6 +243,16 @@ pub fn struct_lint_level<'s, 'd>(
                     return;
                 }
             }
+            (Level::Expect, _) => {
+                // This case is special as we actually allow the lint itself in this context, but
+                // we can't return early like in the case for `Level::Allow` because we still
+                // need the lint diagnostic to be emitted to `rustc_error::HanderInner`.
+                //
+                // We can also not save the diagnostic here right away as it could for instance
+                // still be cancelled in the decorate closure. All of this means that we simply
+                // create a `DiagnosticBuilder` and continue as we would for warnings.
+                sess.struct_expect("")
+            }
             (Level::Warn, Some(span)) => sess.struct_span_warn(span, ""),
             (Level::Warn, None) => sess.struct_warn(""),
             (Level::ForceWarn, Some(span)) => sess.struct_span_force_warn(span, ""),
@@ -274,6 +284,17 @@ pub fn struct_lint_level<'s, 'd>(
         }
 
         let name = lint.name_lower();
+
+        // Lint diagnostics that are covered by the expect level will not be emitted outside
+        // the compiler. It is therefore not necessary to add any information for the user.
+        // This will therefore directly call the decorate function which will intern emit
+        // the `Diagnostic`.
+        if level == Level::Expect {
+            err.code(DiagnosticId::Lint { name, has_future_breakage, is_force_warn: false });
+            decorate(LintDiagnosticBuilder::new(err));
+            return;
+        }
+
         match src {
             LintLevelSource::Default => {
                 sess.diag_note_once(
@@ -289,6 +310,9 @@ pub fn struct_lint_level<'s, 'd>(
                     Level::Forbid => "-F",
                     Level::Allow => "-A",
                     Level::ForceWarn => "--force-warn",
+                    Level::Expect => {
+                        unreachable!("lints with the level of `expect` should not run this code");
+                    }
                 };
                 let hyphen_case_lint_name = name.replace("_", "-");
                 if lint_flag_val.as_str() == name {

compiler/rustc_session/src/session.rs

@@ -360,6 +360,9 @@ impl Session {
     pub fn struct_allow(&self, msg: &str) -> DiagnosticBuilder<'_> {
         self.diagnostic().struct_allow(msg)
     }
+    pub fn struct_expect(&self, msg: &str) -> DiagnosticBuilder<'_> {
+        self.diagnostic().struct_expect(msg)
+    }
     pub fn struct_span_err<S: Into<MultiSpan>>(&self, sp: S, msg: &str) -> DiagnosticBuilder<'_> {
         self.diagnostic().struct_span_err(sp, msg)
     }

src/test/ui/lint/rfc-2383-lint-reason/expect_enabled_feature_gate.rs

@@ -0,0 +1,7 @@
+// check-pass
+
+#![feature(lint_reasons)]
+
+// should be fine due to the enabled feature gate
+#[expect(unused_variables)]
+fn main() {}

src/test/ui/lint/rfc-2383-lint-reason/expect_missing_feature_gate.rs

@@ -0,0 +1,7 @@
+// should error due to missing feature gate.
+
+#[expect(unused)]
+//~^ ERROR: the `#[expect]` attribute is an experimental feature [E0658]
+fn main() {
+    let x = 1;
+}