Apply suggested wording changes from code review.

Co-authored-by: Teymour Aldridge <42674621+teymour-aldridge@users.noreply.github.com>

Author: jumbatm

src/librustc_lint/builtin.rs

@@ -2215,7 +2215,7 @@ impl ClashingExternDeclarations {
 
                 // Disjoint kinds.
                 (_, _) => {
-                    // First, check if the conversion is FFI-safe. This can be so if the type is an
+                    // First, check if the conversion is FFI-safe. This can happen if the type is an
                     // enum with a non-null field (see improper_ctypes).
                     let is_primitive_or_pointer =
                         |ty: Ty<'tcx>| ty.is_primitive() || matches!(ty.kind, RawPtr(..));

src/librustc_lint/types.rs

@@ -563,7 +563,7 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
     }
 
     /// Check if this enum can be safely exported based on the "nullable pointer optimization". If
-    /// the type is it is, return the known non-null field type, else None.  Currently restricted
+    /// it can, return the known non-null field type, otherwise return `None`. Currently restricted
     /// to function pointers, boxes, references, `core::num::NonZero*`, `core::ptr::NonNull`, and
     /// `#[repr(transparent)]` newtypes.
     crate fn is_repr_nullable_ptr(

src/test/ui/lint/clashing-extern-fn.rs

@@ -202,9 +202,9 @@ mod missing_return_type {
 
     mod b {
         extern "C" {
-            // This should warn, because we can't assume that the first declaration is the correct
-            // one -- if this one is the correct one, then calling the usize-returning version
-            // would allow reads into uninitialised memory.
+            // This should output a warning because we can't assume that the first declaration is
+            // the correct one -- if this one is the correct one, then calling the usize-returning
+            // version would allow reads into uninitialised memory.
             fn missing_return_type();
             //~^ WARN `missing_return_type` redeclared with a different signature
         }
@@ -220,7 +220,7 @@ mod non_zero_and_non_null {
     }
     mod b {
         extern "C" {
-            // For both of these cases, if there's a clash, you're either gaining an incorrect
+            // If there's a clash in either of these cases you're either gaining an incorrect
             // invariant that the value is non-zero, or you're missing out on that invariant. Both
             // cases are warning for, from both a caller-convenience and optimisation perspective.
             fn non_zero_usize() -> usize;