Auto merge of #41745 - oli-obk:diagnostics, r=jonathandturner

Remove need for &format!(...) or &&"" dances in `span_label` calls

These were always a thorn in my eye. Note that this will monomorphize to two impls, one for `String` and one for `&str`. But I think that cost is worth the ergonomics at the call sites that can be seen throughout this PR.

2 files changed, 6 insertions, 5 deletions

diff --git a/src/librustc_errors/diagnostic.rs b/src/librustc_errors/diagnostic.rs
index 38fa35ecb12..0822f713499 100644
--- a/src/librustc_errors/diagnostic.rs
+++ b/src/librustc_errors/diagnostic.rs
@@ -114,9 +114,8 @@ impl Diagnostic {
     /// all, and you just supplied a `Span` to create the diagnostic,
     /// then the snippet will just include that `Span`, which is
     /// called the primary span.
-    pub fn span_label(&mut self, span: Span, label: &fmt::Display)
-                      -> &mut Self {
-        self.span.push_span_label(span, format!("{}", label));
+    pub fn span_label<T: Into<String>>(&mut self, span: Span, label: T) -> &mut Self {
+        self.span.push_span_label(span, label.into());
         self
     }
 
diff --git a/src/librustc_errors/diagnostic_builder.rs b/src/librustc_errors/diagnostic_builder.rs
index 9dfd47b8464..a9c2bbeba2a 100644
--- a/src/librustc_errors/diagnostic_builder.rs
+++ b/src/librustc_errors/diagnostic_builder.rs
@@ -112,8 +112,10 @@ impl<'a> DiagnosticBuilder<'a> {
     /// all, and you just supplied a `Span` to create the diagnostic,
     /// then the snippet will just include that `Span`, which is
     /// called the primary span.
-    forward!(pub fn span_label(&mut self, span: Span, label: &fmt::Display)
-                               -> &mut Self);
+    pub fn span_label<T: Into<String>>(&mut self, span: Span, label: T) -> &mut Self {
+        self.diagnostic.span_label(span, label);
+        self
+    }
 
     forward!(pub fn note_expected_found(&mut self,
                                         label: &fmt::Display,