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.

1 files changed, 12 insertions, 10 deletions

diff --git a/clippy_utils/src/diagnostics.rs b/clippy_utils/src/diagnostics.rs
index 812f6fe71a0..edd87546a5f 100644
--- a/clippy_utils/src/diagnostics.rs
+++ b/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 @@ where
     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 @@ where
 }
 
 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);
 }