Commits on May 2, 2023

1. 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.

   

   nnethercote committed May 2, 2023
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