Added Expect lint level and attribute (RFC-2383)

* Also added the `LintExpectationId` which will be used in future commits

Cargo.lock

@@ -3882,6 +3882,7 @@ version = "0.0.0"
 dependencies = [
  "rustc_ast",
  "rustc_data_structures",
+ "rustc_index",
  "rustc_macros",
  "rustc_serialize",
  "rustc_span",

compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs

@@ -75,6 +75,7 @@ fn annotation_type_for_level(level: Level) -> AnnotationType {
         // FIXME(#59346): Not sure how to map this level
         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

@@ -133,7 +133,11 @@ impl Diagnostic {
             | Level::Error { .. }
             | Level::FailureNote => true,
 
-            Level::Warning | Level::Note | Level::Help | Level::Allow => false,
+            Level::Warning
+            | Level::Note
+            | Level::Help
+            | Level::Allow
+            | Level::Expect(_) => false,
         }
     }
 

compiler/rustc_errors/src/lib.rs

@@ -20,6 +20,7 @@ extern crate tracing;
 
 pub use emitter::ColorConfig;
 
+use rustc_lint_defs::LintExpectationId;
 use Level::*;
 
 use emitter::{is_case_difference, Emitter, EmitterWriter};
@@ -677,6 +678,11 @@ 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, id: LintExpectationId) -> DiagnosticBuilder<'_, ()> {
+        DiagnosticBuilder::new(self, Level::Expect(id), msg)
+    }
+
     /// Construct a builder at the `Error` level at the given `span` and with the `msg`.
     pub fn struct_span_err(
         &self,
@@ -953,7 +959,9 @@ impl HandlerInner {
 
         (*TRACK_DIAGNOSTICS)(diagnostic);
 
-        if diagnostic.level == Allow {
+        if let Level::Expect(_) = diagnostic.level {
+            return;
+        } else if diagnostic.level == Allow {
             return;
         }
 
@@ -1250,6 +1258,7 @@ pub enum Level {
     Help,
     FailureNote,
     Allow,
+    Expect(LintExpectationId),
 }
 
 impl fmt::Display for Level {
@@ -1275,7 +1284,7 @@ impl Level {
                 spec.set_fg(Some(Color::Cyan)).set_intense(true);
             }
             FailureNote => {}
-            Allow => unreachable!(),
+            Allow | Expect(_) => unreachable!(),
         }
         spec
     }
@@ -1289,6 +1298,7 @@ impl Level {
             Help => "help",
             FailureNote => "failure-note",
             Allow => panic!("Shouldn't call on allowed error"),
+            Expect(_) => panic!("Shouldn't call on expected error"),
         }
     }
 

compiler/rustc_feature/src/builtin_attrs.rs

@@ -282,6 +282,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ungated!(
         allow, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#), DuplicatesOk
     ),
+    gated!(
+        expect, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#), DuplicatesOk,
+        lint_reasons, experimental!(expect)
+    ),
     ungated!(
         forbid, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#), DuplicatesOk
     ),

compiler/rustc_lint/src/context.rs

@@ -109,6 +109,7 @@ 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.
@@ -377,6 +378,9 @@ 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/Cargo.toml

@@ -10,3 +10,4 @@ rustc_span = { path = "../rustc_span" }
 rustc_serialize = { path = "../rustc_serialize" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_target = { path = "../rustc_target" }
+rustc_index = { path = "../rustc_index" }

compiler/rustc_lint_defs/src/lib.rs

@@ -1,3 +1,5 @@
+#![feature(min_specialization)]
+
 #[macro_use]
 extern crate rustc_macros;
 
@@ -46,13 +48,60 @@ pub enum Applicability {
     Unspecified,
 }
 
+rustc_index::newtype_index! {
+    /// FIXME: The lint expectation ID is currently a simple copy of the `AttrId`
+    /// that the expectation originated from. In the future it should be generated
+    /// by other means. This is for one to keep the IDs independent of each other
+    /// and also to ensure that it is actually stable between compilation sessions.
+    /// (The `AttrId` for instance, is not stable).
+    ///
+    /// Additionally, it would be nice if this generation could be moved into
+    /// [`Level::from_symbol`] to have it all contained in one module and to
+    /// make it simpler to use.
+    pub struct LintExpectationId {
+        DEBUG_FORMAT = "LintExpectationId({})"
+    }
+}
+
+rustc_data_structures::impl_stable_hash_via_hash!(LintExpectationId);
+
+impl<HCX> ToStableHashKey<HCX> for LintExpectationId {
+    type KeyType = u32;
+
+    #[inline]
+    fn to_stable_hash_key(&self, _: &HCX) -> Self::KeyType {
+        self.as_u32()
+    }
+}
+
 /// 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 emitted, this will intern fulfill the expectation and suppress the lint.
+    ///
+    /// See RFC 2383.
+    ///
+    /// The `LintExpectationId` is used to later link a lint emission to the actual
+    /// expectation. It can be ignored in most cases.
+    Expect(LintExpectationId),
+    /// 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 +112,7 @@ 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",
@@ -70,14 +120,15 @@ impl Level {
         }
     }
 
-    /// Converts a lower-case string to a level.
+    /// Converts a lower-case string to a level. This will never construct the expect
+    /// level as that would require a [`LintExpectationId`]
     pub fn from_str(x: &str) -> Option<Level> {
         match x {
             "allow" => Some(Level::Allow),
             "warn" => Some(Level::Warn),
             "deny" => Some(Level::Deny),
             "forbid" => Some(Level::Forbid),
-            _ => None,
+            "expect" | _ => None,
         }
     }
 
@@ -85,6 +136,7 @@ impl Level {
     pub fn from_symbol(x: Symbol) -> Option<Level> {
         match x {
             sym::allow => Some(Level::Allow),
+            sym::expect => Some(Level::Expect(LintExpectationId::from(0u32))),
             sym::warn => Some(Level::Warn),
             sym::deny => Some(Level::Deny),
             sym::forbid => Some(Level::Forbid),

compiler/rustc_middle/src/lint.rs

@@ -225,6 +225,7 @@ pub fn explain_lint_level_source(
                 Level::Forbid => "-F",
                 Level::Allow => "-A",
                 Level::ForceWarn => "--force-warn",
+                Level::Expect(_) => unreachable!("the expect level does not have a commandline flag"),
             };
             let hyphen_case_lint_name = name.replace('_', "-");
             if lint_flag_val.as_str() == name {
@@ -314,6 +315,16 @@ pub fn struct_lint_level<'s, 'd>(
                     return;
                 }
             }
+            (Level::Expect(expect_id), _) => {
+                // 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 mark the lint expectation as fulfilled here right away, as it
+                // can still be cancelled in the decorate function. All of this means that we simply
+                // create a `DiagnosticBuilder` and continue as we would for warnings.
+                sess.struct_expect("", expect_id)
+            }
             (Level::Warn | Level::ForceWarn, Some(span)) => sess.struct_span_warn(span, ""),
             (Level::Warn | Level::ForceWarn, None) => sess.struct_warn(""),
             (Level::Deny | Level::Forbid, Some(span)) => {
@@ -346,6 +357,17 @@ pub fn struct_lint_level<'s, 'd>(
             }
         }
 
+        // 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 let Level::Expect(_) = level {
+            let name = lint.name_lower();
+            err.code(DiagnosticId::Lint { name, has_future_breakage, is_force_warn: false });
+            decorate(LintDiagnosticBuilder::new(err));
+            return;
+        }
+
         explain_lint_level_source(sess, lint, level, src, &mut err);
 
         let name = lint.name_lower();

compiler/rustc_session/src/session.rs

@@ -331,6 +331,9 @@ impl Session {
     pub fn struct_allow(&self, msg: &str) -> DiagnosticBuilder<'_, ()> {
         self.diagnostic().struct_allow(msg)
     }
+    pub fn struct_expect(&self, msg: &str, id: lint::LintExpectationId) -> DiagnosticBuilder<'_, ()> {
+        self.diagnostic().struct_expect(msg, id)
+    }
     pub fn struct_span_err<S: Into<MultiSpan>>(
         &self,
         sp: S,