Remove possibly footgunny is_const method

Author: oli-obk

compiler/rustc_hir/src/hir.rs

@@ -4593,10 +4593,6 @@ impl FnHeader {
         matches!(self.asyncness, IsAsync::Async(_))
     }
 
-    pub fn is_const(&self) -> bool {
-        matches!(self.constness, Constness::Const { .. })
-    }
-
     pub fn is_unsafe(&self) -> bool {
         self.safety().is_unsafe()
     }

compiler/rustc_passes/src/stability.rs

@@ -206,7 +206,7 @@ fn lookup_const_stability(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<ConstSt
             let parent_stab = tcx.lookup_stability(parent)?;
             if parent_stab.is_unstable()
                 && let Some(fn_sig) = tcx.hir_node_by_def_id(def_id).fn_sig()
-                && fn_sig.header.is_const()
+                && matches!(fn_sig.header.constness, Constness::Const { .. })
             {
                 let const_stable_indirect = find_attr!(tcx, def_id, RustcConstStableIndirect);
                 return Some(ConstStability::unmarked(const_stable_indirect, parent_stab));
@@ -228,7 +228,7 @@ fn lookup_const_stability(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<ConstSt
     // If this is a const fn but not annotated with stability markers, see if we can inherit
     // regular stability.
     if let Some(fn_sig) = tcx.hir_node_by_def_id(def_id).fn_sig()
-        && fn_sig.header.is_const()
+        && matches!(fn_sig.header.constness, Constness::Const { .. })
         && const_stab.is_none()
         // We only ever inherit unstable features.
         && let Some(inherit_regular_stab) = tcx.lookup_stability(def_id)
@@ -384,7 +384,7 @@ impl<'tcx> MissingStabilityAnnotations<'tcx> {
         // implied), check if the function/method is const or the parent impl block is const.
         let fn_sig = self.tcx.hir_node_by_def_id(def_id).fn_sig();
         if let Some(fn_sig) = fn_sig
-            && !fn_sig.header.is_const()
+            && !matches!(fn_sig.header.constness, Constness::Const { .. })
             && const_stab.is_some()
             && find_attr_span!(RustcConstStability).is_some()
         {