macro_metavar_expr_concat: explain why idents are invalid

compiler/rustc_expand/src/errors.rs

@@ -545,6 +545,49 @@ mod metavar_exprs {
         pub span: Span,
         pub key: MacroRulesNormalizedIdent,
     }
+
+    #[derive(Diagnostic)]
+    #[diag(r#"`${"{"}concat(..){"}"}` is not generating a valid identifier"#)]
+    pub(crate) struct ConcatInvalidIdent {
+        #[primary_span]
+        pub span: Span,
+        #[subdiagnostic]
+        pub reason: InvalidIdentReason,
+    }
+
+    #[derive(Subdiagnostic)]
+    pub(crate) enum InvalidIdentReason {
+        #[note(r#"this `${"{"}concat(..){"}"}` invocation generated an empty ident"#)]
+        Empty,
+        #[note(r#"this `${"{"}concat(..){"}"}` invocation generated `{$symbol}`, but {$start} is neither '_' nor XID_Start"#)]
+        #[note(
+            "see <https://doc.rust-lang.org/reference/identifiers.html> for the definition of valid identifiers"
+        )]
+        InvalidStart { symbol: Symbol, start: char },
+        #[note(r#"this `${"{"}concat(..){"}"}` invocation generated `{$symbol}`, but {$not_continue} is not XID_Continue"#)]
+        #[note(
+            "see <https://doc.rust-lang.org/reference/identifiers.html> for the definition of valid identifiers"
+        )]
+        InvalidContinue { symbol: Symbol, not_continue: char },
+    }
+
+    impl InvalidIdentReason {
+        pub(crate) fn new(symbol: Symbol) -> Self {
+            let mut chars = symbol.as_str().chars();
+            if let Some(start) = chars.next() {
+                if rustc_lexer::is_id_start(start) {
+                    let not_continue = chars
+                        .find(|c| !rustc_lexer::is_id_continue(*c))
+                        .expect("InvalidIdentReason: cannot find invalid ident reason");
+                    InvalidIdentReason::InvalidContinue { symbol, not_continue }
+                } else {
+                    InvalidIdentReason::InvalidStart { symbol, start }
+                }
+            } else {
+                InvalidIdentReason::Empty
+            }
+        }
+    }
 }
 
 #[derive(Diagnostic)]

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -18,9 +18,10 @@ use rustc_span::{
 use smallvec::{SmallVec, smallvec};
 
 use crate::errors::{
-    CountRepetitionMisplaced, MacroVarStillRepeating, MetaVarsDifSeqMatchers, MustRepeatOnce,
-    MveUnrecognizedVar, NoRepeatableVar, NoSyntaxVarsExprRepeat, VarNoTypo,
-    VarTypoSuggestionRepeatable, VarTypoSuggestionUnrepeatable, VarTypoSuggestionUnrepeatableLabel,
+    ConcatInvalidIdent, CountRepetitionMisplaced, InvalidIdentReason, MacroVarStillRepeating,
+    MetaVarsDifSeqMatchers, MustRepeatOnce, MveUnrecognizedVar, NoRepeatableVar,
+    NoSyntaxVarsExprRepeat, VarNoTypo, VarTypoSuggestionRepeatable, VarTypoSuggestionUnrepeatable,
+    VarTypoSuggestionUnrepeatableLabel,
 };
 use crate::mbe::macro_parser::NamedMatch;
 use crate::mbe::macro_parser::NamedMatch::*;
@@ -656,10 +657,10 @@ fn metavar_expr_concat<'tx>(
     let symbol = nfc_normalize(&concatenated);
     let concatenated_span = tscx.visited_dspan(dspan);
     if !rustc_lexer::is_ident(symbol.as_str()) {
-        return Err(dcx.struct_span_err(
-            concatenated_span,
-            "`${concat(..)}` is not generating a valid identifier",
-        ));
+        return Err(dcx.create_err(ConcatInvalidIdent {
+            span: concatenated_span,
+            reason: InvalidIdentReason::new(symbol),
+        }));
     }
     tscx.psess.symbol_gallery.insert(symbol, concatenated_span);
 

tests/ui/macros/metavar-expressions/concat-trace-errors.rs

@@ -16,18 +16,28 @@ macro_rules! post_expansion {
     ($a:literal) => {
         const _: () = ${concat("hi", $a, "bye")};
         //~^ ERROR is not generating a valid identifier
+        //~| NOTE this `${concat(..)}` invocation generated `hi!bye`, but '!' is not XID_Continue
+        //~| NOTE see <https://doc.rust-lang.org/reference/identifiers.html> for the definition of valid identifiers
     }
 }
 
 post_expansion!("!");
+//~^ NOTE in this expansion of post_expansion!
+//~| NOTE in this expansion of post_expansion!
+//~| NOTE in this expansion of post_expansion!
 
 macro_rules! post_expansion_many {
     ($a:ident, $b:ident, $c:ident, $d:literal, $e:ident) => {
         const _: () = ${concat($a, $b, $c, $d, $e)};
         //~^ ERROR is not generating a valid identifier
+        //~| NOTE this `${concat(..)}` invocation generated `abc.de`, but '.' is not XID_Continue
+        //~| NOTE see <https://doc.rust-lang.org/reference/identifiers.html> for the definition of valid identifiers
     }
 }
 
 post_expansion_many!(a, b, c, ".d", e);
+//~^ NOTE in this expansion of post_expansion_many!
+//~| NOTE in this expansion of post_expansion_many!
+//~| NOTE in this expansion of post_expansion_many!
 
 fn main() {}

tests/ui/macros/metavar-expressions/concat-trace-errors.stderr

@@ -7,17 +7,21 @@ LL |         const _: () = ${concat("hi", $a, "bye")};
 LL | post_expansion!("!");
    | -------------------- in this macro invocation
    |
+   = note: this `${concat(..)}` invocation generated `hi!bye`, but '!' is not XID_Continue
+   = note: see <https://doc.rust-lang.org/reference/identifiers.html> for the definition of valid identifiers
    = note: this error originates in the macro `post_expansion` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: `${concat(..)}` is not generating a valid identifier
-  --> $DIR/concat-trace-errors.rs:26:24
+  --> $DIR/concat-trace-errors.rs:31:24
    |
 LL |         const _: () = ${concat($a, $b, $c, $d, $e)};
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 ...
 LL | post_expansion_many!(a, b, c, ".d", e);
    | -------------------------------------- in this macro invocation
    |
+   = note: this `${concat(..)}` invocation generated `abc.de`, but '.' is not XID_Continue
+   = note: see <https://doc.rust-lang.org/reference/identifiers.html> for the definition of valid identifiers
    = note: this error originates in the macro `post_expansion_many` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 2 previous errors

tests/ui/macros/metavar-expressions/concat-usage-errors.rs

@@ -1,4 +1,5 @@
 //@ edition: 2021
+//@ dont-require-annotations: NOTE
 
 #![feature(macro_metavar_expr_concat)]
 
@@ -43,6 +44,7 @@ macro_rules! starting_number {
     ($ident:ident) => {{
         let ${concat("1", $ident)}: () = ();
         //~^ ERROR `${concat(..)}` is not generating a valid identifier
+        //~| NOTE this `${concat(..)}` invocation generated `1_abc`, but '1' is neither '_' nor XID_Start
     }};
 }
 
@@ -56,6 +58,8 @@ macro_rules! starting_invalid_unicode {
     ($ident:ident) => {{
         let ${concat("\u{00BD}", $ident)}: () = ();
         //~^ ERROR `${concat(..)}` is not generating a valid identifier
+        //~| NOTE this `${concat(..)}` invocation generated `\u{00BD}_abc`, but '\' is neither '_' nor XID_Start
+        //~| NOTE see <https://doc.rust-lang.org/reference/identifiers.html> for the definition of valid identifiers
     }};
 }
 
@@ -75,13 +79,15 @@ macro_rules! ending_invalid_unicode {
     ($ident:ident) => {{
         let ${concat($ident, "\u{00BD}")}: () = ();
         //~^ ERROR `${concat(..)}` is not generating a valid identifier
+        //~| NOTE this `${concat(..)}` invocation generated `_abc\u{00BD}`, but '\' is not XID_Continue
     }};
 }
 
 macro_rules! empty {
     () => {{
         let ${concat("", "")}: () = ();
         //~^ ERROR `${concat(..)}` is not generating a valid identifier
+        //~| NOTE this `${concat(..)}` invocation generated an empty ident
     }};
 }
 
@@ -130,6 +136,8 @@ macro_rules! bad_literal_string {
         //~| ERROR `${concat(..)}` is not generating a valid identifier
         //~| ERROR `${concat(..)}` is not generating a valid identifier
         //~| ERROR `${concat(..)}` is not generating a valid identifier
+        //~| NOTE this `${concat(..)}` invocation generated `_foo🤷`, but '🤷' is not XID_Continue
+        //~| NOTE this `${concat(..)}` invocation generated `_foo-1`, but '-' is not XID_Continue
     }
 }