Auto merge of #110579 - nnethercote:restrict-From-for-Diagnostics, r=davidtwco

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.

r? `@davidtwco`

2 files changed, 4 insertions, 5 deletions

diff --git a/compiler/rustc_mir_transform/src/elaborate_drops.rs b/compiler/rustc_mir_transform/src/elaborate_drops.rs
index 443f469ce52..98e7a519c20 100644
--- a/compiler/rustc_mir_transform/src/elaborate_drops.rs
+++ b/compiler/rustc_mir_transform/src/elaborate_drops.rs
@@ -366,7 +366,7 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
                     if maybe_dead {
                         self.tcx.sess.delay_span_bug(
                             terminator.source_info.span,
-                            &format!(
+                            format!(
                                 "drop of untracked, uninitialized value {:?}, place {:?} ({:?})",
                                 bb, place, path
                             ),
@@ -440,7 +440,7 @@ impl<'b, 'tcx> ElaborateDropsCtxt<'b, 'tcx> {
                             ) {
                                 self.tcx.sess.delay_span_bug(
                                     terminator.source_info.span,
-                                    &format!("drop of untracked value {:?}", bb),
+                                    format!("drop of untracked value {:?}", bb),
                                 );
                             }
                             // A drop and replace behind a pointer/array/whatever.
diff --git a/compiler/rustc_mir_transform/src/generator.rs b/compiler/rustc_mir_transform/src/generator.rs
index ff1745300da..9e16c400f14 100644
--- a/compiler/rustc_mir_transform/src/generator.rs
+++ b/compiler/rustc_mir_transform/src/generator.rs
@@ -865,7 +865,7 @@ fn sanitize_witness<'tcx>(
         _ => {
             tcx.sess.delay_span_bug(
                 body.span,
-                &format!("unexpected generator witness type {:?}", witness.kind()),
+                format!("unexpected generator witness type {:?}", witness.kind()),
             );
             return;
         }
@@ -1451,8 +1451,7 @@ impl<'tcx> MirPass<'tcx> for StateTransform {
                 )
             }
             _ => {
-                tcx.sess
-                    .delay_span_bug(body.span, &format!("unexpected generator type {}", gen_ty));
+                tcx.sess.delay_span_bug(body.span, format!("unexpected generator type {}", gen_ty));
                 return;
             }
         };