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.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/librustdoc/scrape_examples.rs b/src/librustdoc/scrape_examples.rs
index dfa99ffcb7c..d2fa7769bbd 100644
--- a/src/librustdoc/scrape_examples.rs
+++ b/src/librustdoc/scrape_examples.rs
@@ -331,7 +331,7 @@ pub(crate) fn run(
     };
 
     if let Err(e) = inner() {
-        tcx.sess.fatal(&e);
+        tcx.sess.fatal(e);
     }
 
     Ok(())
@@ -358,7 +358,7 @@ pub(crate) fn load_call_locations(
     };
 
     inner().map_err(|e: String| {
-        diag.err(&format!("failed to load examples: {}", e));
+        diag.err(format!("failed to load examples: {}", e));
         1
     })
 }