Rollup merge of #116284 - RalfJung:no-nan-match, r=cjgillot

make matching on NaN a hard error, and remove the rest of illegal_floating_point_literal_pattern

These arms would never be hit anyway, so the pattern makes little sense. We have had a future-compat lint against float matches in general for a *long* time, so I hope we can get away with immediately making this a hard error.

This is part of implementing https://github.com/rust-lang/rfcs/pull/3535.

Closes https://github.com/rust-lang/rust/issues/41620 by removing the lint.

https://github.com/rust-lang/reference/pull/1456 updates the reference to match.

compiler/rustc_mir_build/src/errors.rs

@@ -780,9 +780,14 @@ pub struct UnsizedPattern<'tcx> {
     pub non_sm_ty: Ty<'tcx>,
 }
 
-#[derive(LintDiagnostic)]
-#[diag(mir_build_float_pattern)]
-pub struct FloatPattern;
+#[derive(Diagnostic)]
+#[diag(mir_build_nan_pattern)]
+#[note]
+#[help]
+pub struct NaNPattern {
+    #[primary_span]
+    pub span: Span,
+}
 
 #[derive(LintDiagnostic)]
 #[diag(mir_build_pointer_pattern)]