Use Cow in {D,Subd}iagnosticMessage.

Each of `{D,Subd}iagnosticMessage::{Str,Eager}` has a comment:
```
// FIXME(davidtwco): can a `Cow<'static, str>` be used here?
```
This commit answers that question in the affirmative. It's not the most
compelling change ever, but it might be worth merging.

This requires changing the `impl<'a> From<&'a str>` impls to `impl
From<&'static str>`, which involves a bunch of knock-on changes that
require/result in call sites being a little more precise about exactly
what kind of string they use to create errors, and not just `&str`. This
will result in fewer unnecessary allocations, though this will not have
any notable perf effects given that these are error paths.

Note that I was lazy within Clippy, using `to_string` in a few places to
preserve the existing string imprecision. I could have used `impl
Into<{D,Subd}iagnosticMessage>` in various places as is done in the
compiler, but that would have required changes to *many* call sites
(mostly changing `&format("...")` to `format!("...")`) which didn't seem
worthwhile.

clippy_lints/src/missing_const_for_fn.rs

@@ -154,7 +154,7 @@ fn check_fn(
 
         if let Err((span, err)) = is_min_const_fn(cx.tcx, mir, &self.msrv) {
             if cx.tcx.is_const_fn_raw(def_id.to_def_id()) {
-                cx.tcx.sess.span_err(span, err.as_ref());
+                cx.tcx.sess.span_err(span, err);
             }
         } else {
             span_lint(cx, MISSING_CONST_FOR_FN, span, "this could be a `const fn`");

clippy_lints/src/needless_pass_by_value.rs

@@ -26,7 +26,6 @@
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits;
 use rustc_trait_selection::traits::misc::type_allowed_to_implement_copy;
-use std::borrow::Cow;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -240,9 +239,8 @@ fn check_fn(
                                         snippet_opt(cx, span)
                                             .map_or(
                                                 "change the call to".into(),
-                                                |x| Cow::from(format!("change `{x}` to")),
+                                                |x| format!("change `{x}` to"),
-                                            )
+                                            ),
-                                            .as_ref(),
                                         suggestion,
                                         Applicability::Unspecified,
                                     );
@@ -270,9 +268,8 @@ fn check_fn(
                                         snippet_opt(cx, span)
                                             .map_or(
                                                 "change the call to".into(),
-                                                |x| Cow::from(format!("change `{x}` to"))
+                                                |x| format!("change `{x}` to")
-                                            )
+                                            ),
-                                            .as_ref(),
                                         suggestion,
                                         Applicability::Unspecified,
                                     );

clippy_lints/src/unnecessary_wraps.rs

@@ -163,7 +163,7 @@ fn check_fn(
             span_lint_and_then(cx, UNNECESSARY_WRAPS, span, lint_msg.as_str(), |diag| {
                 diag.span_suggestion(
                     fn_decl.output.span(),
-                    return_type_sugg_msg.as_str(),
+                    return_type_sugg_msg,
                     return_type_sugg,
                     Applicability::MaybeIncorrect,
                 );

clippy_utils/src/diagnostics.rs

@@ -46,7 +46,7 @@ fn docs_link(diag: &mut Diagnostic, lint: &'static Lint) {
 ///    |     ^^^^^^^^^^^^^^^^^^^^^^^
 /// ```
 pub fn span_lint<T: LintContext>(cx: &T, lint: &'static Lint, sp: impl Into<MultiSpan>, msg: &str) {
-    cx.struct_span_lint(lint, sp, msg, |diag| {
+    cx.struct_span_lint(lint, sp, msg.to_string(), |diag| {
         docs_link(diag, lint);
         diag
     });
@@ -80,11 +80,12 @@ pub fn span_lint_and_help<T: LintContext>(
     help_span: Option<Span>,
     help: &str,
 ) {
-    cx.struct_span_lint(lint, span, msg, |diag| {
+    cx.struct_span_lint(lint, span, msg.to_string(), |diag| {
+        let help = help.to_string();
         if let Some(help_span) = help_span {
-            diag.span_help(help_span, help);
+            diag.span_help(help_span, help.to_string());
         } else {
-            diag.help(help);
+            diag.help(help.to_string());
         }
         docs_link(diag, lint);
         diag
@@ -122,7 +123,8 @@ pub fn span_lint_and_note<T: LintContext>(
     note_span: Option<Span>,
     note: &str,
 ) {
-    cx.struct_span_lint(lint, span, msg, |diag| {
+    cx.struct_span_lint(lint, span, msg.to_string(), |diag| {
+        let note = note.to_string();
         if let Some(note_span) = note_span {
             diag.span_note(note_span, note);
         } else {
@@ -143,7 +145,7 @@ pub fn span_lint_and_then<C, S, F>(cx: &C, lint: &'static Lint, sp: S, msg: &str
     S: Into<MultiSpan>,
     F: FnOnce(&mut Diagnostic),
 {
-    cx.struct_span_lint(lint, sp, msg, |diag| {
+    cx.struct_span_lint(lint, sp, msg.to_string(), |diag| {
         f(diag);
         docs_link(diag, lint);
         diag
@@ -151,7 +153,7 @@ pub fn span_lint_and_then<C, S, F>(cx: &C, lint: &'static Lint, sp: S, msg: &str
 }
 
 pub fn span_lint_hir(cx: &LateContext<'_>, lint: &'static Lint, hir_id: HirId, sp: Span, msg: &str) {
-    cx.tcx.struct_span_lint_hir(lint, hir_id, sp, msg, |diag| {
+    cx.tcx.struct_span_lint_hir(lint, hir_id, sp, msg.to_string(), |diag| {
         docs_link(diag, lint);
         diag
     });
@@ -165,7 +167,7 @@ pub fn span_lint_hir_and_then(
     msg: &str,
     f: impl FnOnce(&mut Diagnostic),
 ) {
-    cx.tcx.struct_span_lint_hir(lint, hir_id, sp, msg, |diag| {
+    cx.tcx.struct_span_lint_hir(lint, hir_id, sp, msg.to_string(), |diag| {
         f(diag);
         docs_link(diag, lint);
         diag
@@ -202,7 +204,7 @@ pub fn span_lint_and_sugg<T: LintContext>(
     applicability: Applicability,
 ) {
     span_lint_and_then(cx, lint, sp, msg, |diag| {
-        diag.span_suggestion(sp, help, sugg, applicability);
+        diag.span_suggestion(sp, help.to_string(), sugg, applicability);
     });
 }
 
@@ -232,5 +234,5 @@ pub fn multispan_sugg_with_applicability<I>(
 ) where
     I: IntoIterator<Item = (Span, String)>,
 {
-    diag.multipart_suggestion(help_msg, sugg.into_iter().collect(), applicability);
+    diag.multipart_suggestion(help_msg.to_string(), sugg.into_iter().collect(), applicability);
 }

clippy_utils/src/sugg.rs

@@ -741,7 +741,7 @@ fn suggest_item_with_attr<D: Display + ?Sized>(
         if let Some(indent) = indentation(cx, item) {
             let span = item.with_hi(item.lo());
 
-            self.span_suggestion(span, msg, format!("{attr}\n{indent}"), applicability);
+            self.span_suggestion(span, msg.to_string(), format!("{attr}\n{indent}"), applicability);
         }
     }
 
@@ -762,7 +762,7 @@ fn suggest_prepend_item(&mut self, cx: &T, item: Span, msg: &str, new_item: &str
                 })
                 .collect::<String>();
 
-            self.span_suggestion(span, msg, format!("{new_item}\n{indent}"), applicability);
+            self.span_suggestion(span, msg.to_string(), format!("{new_item}\n{indent}"), applicability);
         }
     }
 
@@ -779,7 +779,7 @@ fn suggest_remove_item(&mut self, cx: &T, item: Span, msg: &str, applicability:
             }
         }
 
-        self.span_suggestion(remove_span, msg, "", applicability);
+        self.span_suggestion(remove_span, msg.to_string(), "", applicability);
     }
 }