coverage: Remove next_id methods from counter/expression IDs

When these methods were originally written, I wasn't aware that
`newtype_index!` already supports addition with ordinary numbers, without
needing to unwrap and re-wrap.

compiler/rustc_middle/src/mir/coverage.rs

@@ -18,11 +18,6 @@ rustc_index::newtype_index! {
 
 impl CounterId {
     pub const START: Self = Self::from_u32(0);
-
-    #[inline(always)]
-    pub fn next_id(self) -> Self {
-        Self::from_u32(self.as_u32() + 1)
-    }
 }
 
 rustc_index::newtype_index! {
@@ -38,11 +33,6 @@ rustc_index::newtype_index! {
 
 impl ExpressionId {
     pub const START: Self = Self::from_u32(0);
-
-    #[inline(always)]
-    pub fn next_id(self) -> Self {
-        Self::from_u32(self.as_u32() + 1)
-    }
 }
 
 /// Operand of a coverage-counter expression.

compiler/rustc_mir_transform/src/coverage/counters.rs

@@ -114,15 +114,15 @@ impl CoverageCounters {
     /// Counter IDs start from one and go up.
     fn next_counter(&mut self) -> CounterId {
         let next = self.next_counter_id;
-        self.next_counter_id = next.next_id();
+        self.next_counter_id = self.next_counter_id + 1;
         next
     }
 
     /// Expression IDs start from 0 and go up.
     /// (Counter IDs and Expression IDs are distinguished by the `Operand` enum.)
     fn next_expression(&mut self) -> ExpressionId {
         let next = self.next_expression_id;
-        self.next_expression_id = next.next_id();
+        self.next_expression_id = self.next_expression_id + 1;
         next
     }