Add explain_reason: false in future_incompatible.

This allows supressing the default warning message for future
incompatible ints, for lints that already provide a more detailed
warning.

compiler/rustc_lint_defs/src/lib.rs

@@ -145,6 +145,11 @@ pub struct FutureIncompatibleInfo {
     /// The reason for the lint used by diagnostics to provide
     /// the right help message
     pub reason: FutureIncompatibilityReason,
+    /// Whether to explain the reason to the user.
+    ///
+    /// Set to false for lints that already include a more detailed
+    /// explanation.
+    pub explain_reason: bool,
     /// Information about a future breakage, which will
     /// be emitted in JSON messages to be displayed by Cargo
     /// for upstream deps
@@ -185,6 +190,7 @@ impl FutureIncompatibleInfo {
         FutureIncompatibleInfo {
             reference: "",
             reason: FutureIncompatibilityReason::FutureReleaseError,
+            explain_reason: true,
             future_breakage: None,
         }
     }

compiler/rustc_middle/src/lint.rs

@@ -398,10 +398,15 @@ pub fn struct_lint_level<'s, 'd>(
                  it will become a hard error in a future release!"
                     .to_owned()
             };
-            let citation = format!("for more information, see {}", future_incompatible.reference);
+            if future_incompatible.explain_reason {
                 err.warn(&explanation);
+            }
+            if !future_incompatible.reference.is_empty() {
+                let citation =
+                    format!("for more information, see {}", future_incompatible.reference);
                 err.note(&citation);
             }
+        }
 
         // Finally, run `decorate`. This function is also responsible for emitting the diagnostic.
         decorate(LintDiagnosticBuilder::new(err));