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

Author: fmease

clippy_utils/src/diagnostics.rs

@@ -61,7 +61,8 @@ fn docs_link(diag: &mut Diag<'_, ()>, lint: &'static Lint) {
 /// ```
 pub fn span_lint<T: LintContext>(cx: &T, lint: &'static Lint, sp: impl Into<MultiSpan>, msg: impl Into<DiagMessage>) {
     #[expect(clippy::disallowed_methods)]
-    cx.span_lint(lint, sp, msg.into(), |diag| {
+    cx.span_lint(lint, sp, |diag| {
+        diag.primary_message(msg);
         docs_link(diag, lint);
     });
 }
@@ -109,7 +110,8 @@ pub fn span_lint_and_help<T: LintContext>(
     help: impl Into<SubdiagMessage>,
 ) {
     #[expect(clippy::disallowed_methods)]
-    cx.span_lint(lint, span, msg.into(), |diag| {
+    cx.span_lint(lint, span, |diag| {
+        diag.primary_message(msg);
         if let Some(help_span) = help_span {
             diag.span_help(help_span, help.into());
         } else {
@@ -165,7 +167,8 @@ pub fn span_lint_and_note<T: LintContext>(
     note: impl Into<SubdiagMessage>,
 ) {
     #[expect(clippy::disallowed_methods)]
-    cx.span_lint(lint, span, msg.into(), |diag| {
+    cx.span_lint(lint, span, |diag| {
+        diag.primary_message(msg);
         if let Some(note_span) = note_span {
             diag.span_note(note_span, note.into());
         } else {
@@ -201,7 +204,8 @@ where
     F: FnOnce(&mut Diag<'_, ()>),
 {
     #[expect(clippy::disallowed_methods)]
-    cx.span_lint(lint, sp, msg, |diag| {
+    cx.span_lint(lint, sp, |diag| {
+        diag.primary_message(msg);
         f(diag);
         docs_link(diag, lint);
     });
@@ -233,7 +237,8 @@ where
 /// the `#[allow]` will work.
 pub fn span_lint_hir(cx: &LateContext<'_>, lint: &'static Lint, hir_id: HirId, sp: Span, msg: impl Into<DiagMessage>) {
     #[expect(clippy::disallowed_methods)]
-    cx.tcx.node_span_lint(lint, hir_id, sp, msg.into(), |diag| {
+    cx.tcx.node_span_lint(lint, hir_id, sp, |diag| {
+        diag.primary_message(msg);
         docs_link(diag, lint);
     });
 }
@@ -271,7 +276,8 @@ pub fn span_lint_hir_and_then(
     f: impl FnOnce(&mut Diag<'_, ()>),
 ) {
     #[expect(clippy::disallowed_methods)]
-    cx.tcx.node_span_lint(lint, hir_id, sp, msg.into(), |diag| {
+    cx.tcx.node_span_lint(lint, hir_id, sp, |diag| {
+        diag.primary_message(msg);
         f(diag);
         docs_link(diag, lint);
     });

tests/ui-internal/disallow_span_lint.rs

@@ -11,11 +11,11 @@ use rustc_lint::{Lint, LintContext};
 use rustc_middle::ty::TyCtxt;
 
 pub fn a(cx: impl LintContext, lint: &'static Lint, span: impl Into<MultiSpan>, msg: impl Into<DiagMessage>) {
-    cx.span_lint(lint, span, msg, |_| {});
+    cx.span_lint(lint, span, |lint| { lint.primary_message(msg); });
 }
 
 pub fn b(tcx: TyCtxt<'_>, lint: &'static Lint, hir_id: HirId, span: impl Into<MultiSpan>, msg: impl Into<DiagMessage>) {
-    tcx.node_span_lint(lint, hir_id, span, msg, |_| {});
+    tcx.node_span_lint(lint, hir_id, span, |lint| { lint.primary_message(msg); });
 }
 
 fn main() {}

tests/ui-internal/disallow_span_lint.stderr

@@ -1,8 +1,8 @@
 error: use of a disallowed method `rustc_lint::context::LintContext::span_lint`
   --> tests/ui-internal/disallow_span_lint.rs:14:5
    |
-LL |     cx.span_lint(lint, span, msg, |_| {});
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     cx.span_lint(lint, span, |lint| { lint.primary_message(msg); });
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: this function does not add a link to our documentation, please use the `clippy_utils::diagnostics::span_lint*` functions instead (from clippy.toml)
    = note: `-D clippy::disallowed-methods` implied by `-D warnings`
@@ -11,8 +11,8 @@ LL |     cx.span_lint(lint, span, msg, |_| {});
 error: use of a disallowed method `rustc_middle::ty::context::TyCtxt::node_span_lint`
   --> tests/ui-internal/disallow_span_lint.rs:18:5
    |
-LL |     tcx.node_span_lint(lint, hir_id, span, msg, |_| {});
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     tcx.node_span_lint(lint, hir_id, span, |lint| { lint.primary_message(msg); });
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: this function does not add a link to our documentation, please use the `clippy_utils::diagnostics::span_lint_hir*` functions instead (from clippy.toml)