Reduce diagnostic type visibilities.

Most diagnostic types are only used within their own crate, and so have
a `pub(crate)` visibility. We have some diagnostic types that are
unnecessarily `pub`. This is bad because (a) information hiding, and (b)
if a `pub(crate)` type becomes unused the compiler will warn but it
won't warn for a `pub` type.

This commit eliminates unnecessary `pub` visibilities for some
diagnostic types, and also some related things due to knock-on effects.
(I found these types with some ad hoc use of `grep`.)

compiler/rustc_pattern_analysis/src/errors.rs

@@ -48,7 +48,7 @@ pub fn new<'p, 'tcx>(
 #[derive(Diagnostic)]
 #[diag("multiple patterns overlap on their endpoints")]
 #[note("you likely meant to write mutually exclusive ranges")]
-pub struct OverlappingRangeEndpoints {
+pub(crate) struct OverlappingRangeEndpoints {
     #[label("... with this range")]
     pub range: Span,
     #[subdiagnostic]
@@ -57,7 +57,7 @@ pub struct OverlappingRangeEndpoints {
 
 #[derive(Subdiagnostic)]
 #[label("this range overlaps on `{$range}`...")]
-pub struct Overlap {
+pub(crate) struct Overlap {
     #[primary_span]
     pub span: Span,
     pub range: String, // a printed pattern
@@ -65,7 +65,7 @@ pub struct Overlap {
 
 #[derive(Diagnostic)]
 #[diag("exclusive range missing `{$max}`")]
-pub struct ExclusiveRangeMissingMax {
+pub(crate) struct ExclusiveRangeMissingMax {
     #[label("this range doesn't match `{$max}` because `..` is an exclusive range")]
     #[suggestion(
         "use an inclusive range instead",
@@ -81,7 +81,7 @@ pub struct ExclusiveRangeMissingMax {
 
 #[derive(Diagnostic)]
 #[diag("multiple ranges are one apart")]
-pub struct ExclusiveRangeMissingGap {
+pub(crate) struct ExclusiveRangeMissingGap {
     #[label("this range doesn't match `{$gap}` because `..` is an exclusive range")]
     #[suggestion(
         "use an inclusive range instead",
@@ -102,7 +102,7 @@ pub struct ExclusiveRangeMissingGap {
 #[label(
     "this could appear to continue range `{$first_range}`, but `{$gap}` isn't matched by either of them"
 )]
-pub struct GappedRange {
+pub(crate) struct GappedRange {
     #[primary_span]
     pub span: Span,
     pub gap: String,         // a printed pattern