Remove `DelayDm`

With the removal of `LintDiagnostic::msg` / the `msg` param from
lint diag APIs, primary messages for lint diags are always constructed
lazily inside decorator fns rendering this wrapper type unused / useless.

compiler/rustc_error_messages/src/lib.rs

@@ -367,17 +367,6 @@ impl From<Cow<'static, str>> for DiagMessage {
     }
 }
 
-/// A workaround for must_produce_diag ICEs when formatting types in disabled lints.
-///
-/// Delays formatting until `.into(): DiagMessage` is used.
-pub struct DelayDm<F>(pub F);
-
-impl<F: FnOnce() -> String> From<DelayDm<F>> for DiagMessage {
-    fn from(DelayDm(f): DelayDm<F>) -> Self {
-        DiagMessage::from(f())
-    }
-}
-
 /// Translating *into* a subdiagnostic message from a diagnostic message is a little strange - but
 /// the subdiagnostic functions (e.g. `span_label`) take a `SubdiagMessage` and the
 /// subdiagnostic derive refers to typed identifiers that are `DiagMessage`s, so need to be

compiler/rustc_errors/src/lib.rs

@@ -39,7 +39,7 @@ pub use diagnostic_impls::{
 };
 pub use emitter::ColorConfig;
 pub use rustc_error_messages::{
-    fallback_fluent_bundle, fluent_bundle, DelayDm, DiagMessage, FluentBundle, LanguageIdentifier,
+    fallback_fluent_bundle, fluent_bundle, DiagMessage, FluentBundle, LanguageIdentifier,
     LazyFallbackBundle, MultiSpan, SpanLabel, SubdiagMessage,
 };
 pub use rustc_lint_defs::{pluralize, Applicability};
@@ -572,8 +572,8 @@ impl Drop for DiagCtxtInner {
             if let Some(backtrace) = &self.must_produce_diag {
                 panic!(
                     "must_produce_diag: `trimmed_def_paths` called but no diagnostics emitted; \
-                       use `DelayDm` for lints or `with_no_trimmed_paths` for debugging. \
-                       called at: {backtrace}"
+                     `with_no_trimmed_paths` for debugging. \
+                     called at: {backtrace}"
                 );
             }
         }

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -3327,7 +3327,7 @@ fn for_each_def(tcx: TyCtxt<'_>, mut collect_fn: impl for<'b> FnMut(&'b Ident, N
 ///
 /// The implementation uses similar import discovery logic to that of 'use' suggestions.
 ///
-/// See also [`DelayDm`](rustc_error_messages::DelayDm) and [`with_no_trimmed_paths!`].
+/// See also [`with_no_trimmed_paths!`].
 // this is pub to be able to intra-doc-link it
 pub fn trimmed_def_paths(tcx: TyCtxt<'_>, (): ()) -> DefIdMap<Symbol> {
     // Trimming paths is expensive and not optimized, since we expect it to only be used for error