Remove LintDiagnostic::msg

* instead simply set the primary message inside the lint decorator functions
* it used to be this way before [#]101986 which introduced `msg` to prevent
  good path delayed bugs (which no longer exist) from firing under certain
  circumstances when lints were suppressed / silenced
* this is no longer necessary for various reasons I presume
* it shaves off complexity and makes further changes easier to implement

compiler/rustc_middle/src/lint.rs

@@ -2,7 +2,7 @@ use std::cmp;
 
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::sorted_map::SortedMap;
-use rustc_errors::{Diag, DiagMessage, MultiSpan};
+use rustc_errors::{Diag, MultiSpan};
 use rustc_hir::{HirId, ItemLocalId};
 use rustc_macros::HashStable;
 use rustc_session::lint::{
@@ -269,7 +269,6 @@ pub fn lint_level(
     level: Level,
     src: LintLevelSource,
     span: Option<MultiSpan>,
-    msg: impl Into<DiagMessage>,
     decorate: impl for<'a, 'b> FnOnce(&'b mut Diag<'a, ()>),
 ) {
     // Avoid codegen bloat from monomorphization by immediately doing dyn dispatch of `decorate` to
@@ -281,7 +280,6 @@ pub fn lint_level(
         level: Level,
         src: LintLevelSource,
         span: Option<MultiSpan>,
-        msg: impl Into<DiagMessage>,
         decorate: Box<dyn '_ + for<'a, 'b> FnOnce(&'b mut Diag<'a, ()>)>,
     ) {
         // Check for future incompatibility lints and issue a stronger warning.
@@ -350,10 +348,6 @@ pub fn lint_level(
             }
         }
 
-        // Delay evaluating and setting the primary message until after we've
-        // suppressed the lint due to macros.
-        err.primary_message(msg);
-
         err.is_lint(lint.name_lower(), has_future_breakage);
 
         // Lint diagnostics that are covered by the expect level will not be emitted outside
@@ -418,7 +412,7 @@ pub fn lint_level(
         explain_lint_level_source(lint, level, src, &mut err);
         err.emit()
     }
-    lint_level_impl(sess, lint, level, src, span, msg, Box::new(decorate))
+    lint_level_impl(sess, lint, level, src, span, Box::new(decorate))
 }
 
 /// Returns whether `span` originates in a foreign crate's external macro.