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.

2 files changed, 3 insertions, 3 deletions

diff --git a/compiler/rustc_passes/src/check_const.rs b/compiler/rustc_passes/src/check_const.rs
index 30dd3e4d016..6742722ce52 100644
--- a/compiler/rustc_passes/src/check_const.rs
+++ b/compiler/rustc_passes/src/check_const.rs
@@ -148,7 +148,7 @@ impl<'tcx> CheckConstVisitor<'tcx> {
             [missing_primary, ref missing_secondary @ ..] => {
                 let msg =
                     format!("{} is not allowed in a `{}`", expr.name(), const_kind.keyword_name());
-                let mut err = feature_err(&tcx.sess.parse_sess, *missing_primary, span, &msg);
+                let mut err = feature_err(&tcx.sess.parse_sess, *missing_primary, span, msg);
 
                 // If multiple feature gates would be required to enable this expression, include
                 // them as help messages. Don't emit a separate error for each missing feature gate.
@@ -161,7 +161,7 @@ impl<'tcx> CheckConstVisitor<'tcx> {
                             "add `#![feature({})]` to the crate attributes to enable",
                             gate,
                         );
-                        err.help(&note);
+                        err.help(note);
                     }
                 }
 
diff --git a/compiler/rustc_passes/src/hir_id_validator.rs b/compiler/rustc_passes/src/hir_id_validator.rs
index 3942a73befd..363e1743677 100644
--- a/compiler/rustc_passes/src/hir_id_validator.rs
+++ b/compiler/rustc_passes/src/hir_id_validator.rs
@@ -31,7 +31,7 @@ pub fn check_crate(tcx: TyCtxt<'_>) {
 
         if !errors.is_empty() {
             let message = errors.iter().fold(String::new(), |s1, s2| s1 + "\n" + s2);
-            tcx.sess.delay_span_bug(rustc_span::DUMMY_SP, &message);
+            tcx.sess.delay_span_bug(rustc_span::DUMMY_SP, message);
         }
     }
 }