Restrict From<S> for {D,Subd}iagnosticMessage.

Currently a `{D,Subd}iagnosticMessage` can be created from any type that
impls `Into<String>`. That includes `&str`, `String`, and `Cow<'static,
str>`, which are reasonable. It also includes `&String`, which is pretty
weird, and results in many places making unnecessary allocations for
patterns like this:
```
self.fatal(&format!(...))
```
This creates a string with `format!`, takes a reference, passes the
reference to `fatal`, which does an `into()`, which clones the
reference, doing a second allocation. Two allocations for a single
string, bleh.

This commit changes the `From` impls so that you can only create a
`{D,Subd}iagnosticMessage` from `&str`, `String`, or `Cow<'static,
str>`. This requires changing all the places that currently create one
from a `&String`. Most of these are of the `&format!(...)` form
described above; each one removes an unnecessary static `&`, plus an
allocation when executed. There are also a few places where the existing
use of `&String` was more reasonable; these now just use `clone()` at
the call site.

As well as making the code nicer and more efficient, this is a step
towards possibly using `Cow<'static, str>` in
`{D,Subd}iagnosticMessage::{Str,Eager}`. That would require changing
the `From<&'a str>` impls to `From<&'static str>`, which is doable, but
I'm not yet sure if it's worthwhile.

Author: nnethercote

clippy_lints/src/future_not_send.rs

@@ -96,7 +96,7 @@ impl<'tcx> LateLintPass<'tcx> for FutureNotSend {
                                 if let PredicateKind::Clause(Clause::Trait(trait_pred)) =
                                     obligation.predicate.kind().skip_binder()
                                 {
-                                    db.note(&format!(
+                                    db.note(format!(
                                         "`{}` doesn't implement `{}`",
                                         trait_pred.self_ty(),
                                         trait_pred.trait_ref.print_only_trait_path(),

clippy_lints/src/lib.rs

@@ -353,7 +353,7 @@ pub fn register_pre_expansion_lints(store: &mut rustc_lint::LintStore, sess: &Se
 pub fn read_conf(sess: &Session, path: &io::Result<(Option<PathBuf>, Vec<String>)>) -> Conf {
     if let Ok((_, warnings)) = path {
         for warning in warnings {
-            sess.warn(warning);
+            sess.warn(warning.clone());
         }
     }
     let file_name = match path {

clippy_lints/src/methods/str_splitn.rs

@@ -175,13 +175,13 @@ fn check_manual_split_once_indirect(
             let remove_msg = format!("remove the `{iter_ident}` usages");
             diag.span_suggestion(
                 first.span,
-                &remove_msg,
+                remove_msg.clone(),
                 "",
                 app,
             );
             diag.span_suggestion(
                 second.span,
-                &remove_msg,
+                remove_msg,
                 "",
                 app,
             );

clippy_lints/src/non_send_fields_in_send_ty.rs

@@ -131,13 +131,13 @@ impl<'tcx> LateLintPass<'tcx> for NonSendFieldInSendTy {
                             for field in non_send_fields {
                                 diag.span_note(
                                     field.def.span,
-                                    &format!("it is not safe to send field `{}` to another thread", field.def.ident.name),
+                                    format!("it is not safe to send field `{}` to another thread", field.def.ident.name),
                                 );
 
                                 match field.generic_params.len() {
                                     0 => diag.help("use a thread-safe type that implements `Send`"),
-                                    1 if is_ty_param(field.ty) => diag.help(&format!("add `{}: Send` bound in `Send` impl", field.ty)),
-                                    _ => diag.help(&format!(
+                                    1 if is_ty_param(field.ty) => diag.help(format!("add `{}: Send` bound in `Send` impl", field.ty)),
+                                    _ => diag.help(format!(
                                         "add bounds on type parameter{} `{}` that satisfy `{}: Send`",
                                         if field.generic_params.len() > 1 { "s" } else { "" },
                                         field.generic_params_string(),

clippy_utils/src/attrs.rs

@@ -133,7 +133,7 @@ pub fn get_unique_attr<'a>(
     let mut unique_attr: Option<&ast::Attribute> = None;
     for attr in get_attr(sess, attrs, name) {
         if let Some(duplicate) = unique_attr {
-            sess.struct_span_err(attr.span, &format!("`{name}` is defined multiple times"))
+            sess.struct_span_err(attr.span, format!("`{name}` is defined multiple times"))
                 .span_note(duplicate.span, "first definition found here")
                 .emit();
         } else {