tamer: xir: Format tokens without tt quotes

Whether or not quoting is appropriate depends on context, and that parent
context is already performing the quoting.  For example:

  error: expected `</rater>`, but found `<import>`
    --> /home/[...]/foo.xml:2:1
     |
   2 | <rater xmlns="http://www.lovullo.com/rater"
     | ------ note: element starts here

    --> /home/[...]/foo.xml:7:3
     |
   7 |   <import package="/rater/core/base" />
     |   ^^^^^^^ error: expected `</rater>`

In these cases (obviously I'm still working on the parser, since this is
nonsense), the parser is responsible for quoting the token "<import>".

DEV-7145

tamer/src/xir.rs

@@ -33,6 +33,7 @@
 //!   as opposed to "start" and "end" as used in the XML specification.
 //! TAMER uses a uniform terminology for all delimited data.
 
+use crate::fmt::DisplayWrapper;
 use crate::span::{Span, SpanLenSize};
 use crate::sym::{
     st_as_sym, GlobalSymbolIntern, GlobalSymbolInternBytes, SymbolId,
@@ -51,6 +52,8 @@ pub use escape::{DefaultEscaper, Escaper};
 use error::SpanlessError;
 use st::qname::QNameCompatibleStaticSymbolId;
 
+use self::fmt::{CloseXmlEle, OpenXmlEle, XmlAttr, XmlAttrValueQuote};
+
 pub mod attr;
 pub mod flat;
 pub mod fmt;
@@ -597,21 +600,21 @@ impl Display for Token {
         //     but the diagnostic system also quote source lines to provide
         //     the necessary context.
         match self {
-            Self::Open(qname, _) => write!(f, "`<{}>`", qname),
-            Self::Close(Some(qname), _) => write!(f, "`</{}>`", qname),
+            Self::Open(qname, _) => OpenXmlEle::fmt(qname, f),
+            Self::Close(Some(qname), _) => CloseXmlEle::fmt(qname, f),
             // Its context is contained within the Open,
             //   and hopefully any user-visible errors will display that instead.
             Self::Close(None, _) => {
-                write!(f, "`/>`")
-            }
-            Self::AttrName(qname, _) => {
-                write!(f, "`@{}`", qname)
-            }
-            Self::AttrValue(attr_val, _) => {
-                write!(f, "attribute value `{}`", attr_val)
+                write!(f, "/>")
             }
+            Self::AttrName(qname, _) => XmlAttr::fmt(qname, f),
+            Self::AttrValue(attr_val, _) => XmlAttrValueQuote::fmt(attr_val, f),
             Self::AttrValueFragment(attr_val, _) => {
-                write!(f, "attribute value fragment `{}`", attr_val)
+                write!(
+                    f,
+                    "value fragment {}",
+                    XmlAttrValueQuote::wrap(attr_val)
+                )
             }
             Self::Comment(..) => write!(f, "comment"),
             Self::Text(..) => write!(f, "text"),

tamer/src/xir/fmt.rs

@@ -44,3 +44,6 @@ pub type TtCloseXmlEle = Tt<CloseXmlEle>;
 /// A choice of a list of opening XML tags.
 pub type OpenEleSumList =
     OrQualConjList<"element", "one of elements", TtOpenXmlEle>;
+
+/// Quote an attribute value using double quotes.
+pub type XmlAttrValueQuote = Delim<"\"", "\"", Raw>;