mbe: Shorten MetaVarExpr -> Mve in structural diagnostics

More diagnostic structs related to metavariable expressions will be
introduced. Introduce the abbreviation "mve" which is reasonably
unambiguous (`rg Mve` and `rg '(\b|_|-)mve(\b|_|-)'` return no results
outside of a Thumb target feature) and use it for these diagnostic
types. A new module is also created.

Author: tgross35

compiler/rustc_expand/messages.ftl

@@ -109,9 +109,6 @@ expand_malformed_feature_attribute =
 
 expand_meta_var_dif_seq_matchers = {$msg}
 
-expand_meta_var_expr_unrecognized_var =
-    variable `{$key}` is not recognized in meta-variable expression
-
 expand_missing_fragment_specifier = missing fragment specifier
     .note = fragment specifiers must be provided
     .suggestion_add_fragspec = try adding a specifier here
@@ -136,6 +133,9 @@ expand_module_multiple_candidates =
 expand_must_repeat_once =
     this must repeat at least once
 
+expand_mve_unrecognized_var =
+    variable `{$key}` is not recognized in meta-variable expression
+
 expand_non_inline_modules_in_proc_macro_input_are_unstable =
     non-inline modules in proc macro input are unstable
 

compiler/rustc_expand/src/errors.rs

@@ -27,14 +27,6 @@ pub(crate) struct CountRepetitionMisplaced {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(expand_meta_var_expr_unrecognized_var)]
-pub(crate) struct MetaVarExprUnrecognizedVar {
-    #[primary_span]
-    pub span: Span,
-    pub key: MacroRulesNormalizedIdent,
-}
-
 #[derive(Diagnostic)]
 #[diag(expand_var_still_repeating)]
 pub(crate) struct VarStillRepeating {
@@ -500,3 +492,16 @@ pub(crate) struct ProcMacroBackCompat {
     pub crate_name: String,
     pub fixed_version: String,
 }
+
+pub(crate) use metavar_exprs::*;
+mod metavar_exprs {
+    use super::*;
+
+    #[derive(Diagnostic)]
+    #[diag(expand_mve_unrecognized_var)]
+    pub(crate) struct MveUnrecognizedVar {
+        #[primary_span]
+        pub span: Span,
+        pub key: MacroRulesNormalizedIdent,
+    }
+}

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -17,7 +17,7 @@ use rustc_span::{
 use smallvec::{SmallVec, smallvec};
 
 use crate::errors::{
-    CountRepetitionMisplaced, MetaVarExprUnrecognizedVar, MetaVarsDifSeqMatchers, MustRepeatOnce,
+    CountRepetitionMisplaced, MetaVarsDifSeqMatchers, MustRepeatOnce, MveUnrecognizedVar,
     NoSyntaxVarsExprRepeat, VarStillRepeating,
 };
 use crate::mbe::macro_parser::NamedMatch;
@@ -879,7 +879,7 @@ where
 {
     let span = ident.span;
     let key = MacroRulesNormalizedIdent::new(ident);
-    interp.get(&key).ok_or_else(|| dcx.create_err(MetaVarExprUnrecognizedVar { span, key }))
+    interp.get(&key).ok_or_else(|| dcx.create_err(MveUnrecognizedVar { span, key }))
 }
 
 /// Used by meta-variable expressions when an user input is out of the actual declared bounds. For