Only point at the two macro invocations that diverged

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -2168,39 +2168,51 @@ impl<'a> Parser<'a> {
         // Walk the chain of macro expansions for the current token to point at how the original
         // code was interpreted. This helps the user realize when a macro argument of one type is
         // later reinterpreted as a different type, like `$x:expr` being reinterpreted as `$x:pat`
-        // in a subsequent macro invokation (#71039).
+        // in a subsequent macro invocation (#71039).
         let mut tok = self.token.clone();
         let mut labels = vec![];
-        let mut pos = 0;
         while let TokenKind::Interpolated(node) = &tok.kind {
             // FIXME: figure out how to point at `$x:pat` when `tok`
             // is a `NtPat` corresponding to `$x`.
             let tokens = node.tokens();
-            labels.push((
-                pos,
-                node.span(),
-                format!("this is interpreted as {} {}", node.descr(), super::token_descr(&tok)),
-            ));
+            labels.push((node.span(), node.descr()));
             if let Some(tokens) = tokens
                 && let tokens = tokens.to_attr_token_stream()
                 && let tokens = tokens.0.deref()
                 && let [AttrTokenTree::Token(token, _)] = &tokens[..]
             {
-                info!(?token);
                 tok = token.clone();
-                pos += 1;
             } else {
                 break;
             }
         }
-        for (i, span, label) in labels {
-            let post = if pos > 0 {
-                // When there are more than one macro expansions at play, clarify the order.
-                format!(" (in expansion #{})", pos + 1 - i)
-            } else {
-                String::new()
-            };
-            err.span_label(span, format!("{label}{post}"));
+        let mut iter = labels.into_iter().peekable();
+        let mut show_link = false;
+        while let Some((span, descr)) = iter.next() {
+            if let Some((next_span, next_descr)) = iter.peek()
+                && next_descr != &descr
+            {
+                err.span_label(
+                    *next_span,
+                    format!("this is interpreted as {next_descr}"),
+                );
+                err.span_label(
+                    span,
+                    format!(
+                        "this is interpreted as {}, but it was previously interpreted as {}",
+                        descr,
+                        next_descr,
+                    ),
+                );
+                show_link = true;
+            }
+        }
+        if show_link {
+            err.note(
+                "when forwarding a matched fragment to another macro-by-example, matchers in the \
+                 second macro will see an opaque AST of the fragment type, not the underlying \
+                 tokens",
+            );
         }
         err
     }

compiler/rustc_parse/src/parser/mod.rs

@@ -365,12 +365,14 @@ impl TokenDescription {
 pub(super) fn token_descr(token: &Token) -> String {
     let name = pprust::token_to_string(token).to_string();
 
-    let kind = TokenDescription::from_token(token).map(|kind| match kind {
-        TokenDescription::ReservedIdentifier => "reserved identifier",
-        TokenDescription::Keyword => "keyword",
-        TokenDescription::ReservedKeyword => "reserved keyword",
-        TokenDescription::DocComment => "doc comment",
-    });
+    let kind = match (TokenDescription::from_token(token), &token.kind) {
+        (Some(TokenDescription::ReservedIdentifier), _) => Some("reserved identifier"),
+        (Some(TokenDescription::Keyword), _) => Some("keyword"),
+        (Some(TokenDescription::ReservedKeyword), _) => Some("reserved keyword"),
+        (Some(TokenDescription::DocComment), _) => Some("doc comment"),
+        (None, TokenKind::Interpolated(node)) => Some(node.descr()),
+        (None, _) => None,
+    };
 
     if let Some(kind) = kind { format!("{kind} `{name}`") } else { format!("`{name}`") }
 }

tests/ui/issues/issue-39848.rs

@@ -1,6 +1,6 @@
 macro_rules! get_opt {
     ($tgt:expr, $field:ident) => {
-        if $tgt.has_$field() {} //~ ERROR expected `{`, found `foo`
+        if $tgt.has_$field() {} //~ ERROR expected `{`, found identifier `foo`
     }
 }
 

tests/ui/issues/issue-39848.stderr

@@ -1,4 +1,4 @@
-error: expected `{`, found `foo`
+error: expected `{`, found identifier `foo`
   --> $DIR/issue-39848.rs:3:21
    |
 LL |         if $tgt.has_$field() {}

tests/ui/macros/syntax-error-recovery.rs

@@ -9,7 +9,7 @@ macro_rules! values {
         }
     };
 }
-//~^^^^^ ERROR expected one of `(`, `,`, `=`, `{`, or `}`, found `(String)`
+//~^^^^^ ERROR expected one of `(`, `,`, `=`, `{`, or `}`, found type `(String)`
 //~| ERROR macro expansion ignores token `(String)` and any following
 
 values!(STRING(1) as (String) => cfg(test),);

tests/ui/macros/syntax-error-recovery.stderr

@@ -1,4 +1,4 @@
-error: expected one of `(`, `,`, `=`, `{`, or `}`, found `(String)`
+error: expected one of `(`, `,`, `=`, `{`, or `}`, found type `(String)`
   --> $DIR/syntax-error-recovery.rs:7:26
    |
 LL |                 $token $($inner)? = $value,

tests/ui/macros/trace_faulty_macros.rs

@@ -46,9 +46,9 @@ macro_rules! test {
     (let $p:pat = $e:expr) => {test!(($p,$e))};
     // this should be expr
     //           vvv
-    (($p:pat, $e:pat)) => {let $p = $e;}; //~ ERROR expected expression, found `1 + 1`
+    (($p:pat, $e:pat)) => {let $p = $e;}; //~ ERROR expected expression, found pattern `1 + 1`
 }
 
 fn foo() {
     test!(let x = 1+1);
-}
+}

tests/ui/macros/trace_faulty_macros.stderr

@@ -50,12 +50,9 @@ LL |     my_recursive_macro!();
    = note: expanding `my_recursive_macro! {  }`
    = note: to `my_recursive_macro! () ;`
 
-error: expected expression, found `A { a: a, b: 0, c: _, .. }`
+error: expected expression, found pattern `A { a: a, b: 0, c: _, .. }`
   --> $DIR/trace_faulty_macros.rs:16:9
    |
-LL |         pat_macro!(A{a:a, b:0, c:_, ..});
-   |                    -------------------- this is interpreted as pattern `A { a: a, b: 0, c: _, .. }`
-...
 LL |         $a
    |         ^^ expected expression
 ...
@@ -72,21 +69,22 @@ LL | #[derive(Debug)]
 LL | fn use_derive_macro_as_attr() {}
    | -------------------------------- not a `struct`, `enum` or `union`
 
-error: expected expression, found `1 + 1`
+error: expected expression, found pattern `1 + 1`
   --> $DIR/trace_faulty_macros.rs:49:37
    |
 LL |     (let $p:pat = $e:expr) => {test!(($p,$e))};
-   |                                          -- this is interpreted as pattern `1 + 1` (in expansion #2)
+   |                                          -- this is interpreted as pattern, but it was previously interpreted as expression
 ...
 LL |     (($p:pat, $e:pat)) => {let $p = $e;};
    |                                     ^^ expected expression
 ...
 LL |     test!(let x = 1+1);
    |     ------------------
    |     |             |
-   |     |             this is interpreted as expression `1 + 1` (in expansion #1)
+   |     |             this is interpreted as expression
    |     in this macro invocation
    |
+   = note: when forwarding a matched fragment to another macro-by-example, matchers in the second macro will see an opaque AST of the fragment type, not the underlying tokens
    = note: this error originates in the macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: trace_macro

tests/ui/parser/float-field-interpolated.rs

@@ -6,9 +6,9 @@ macro_rules! generate_field_accesses {
 
         s.$a; // OK
         { s.$b; } //~ ERROR unexpected token: `1.1`
-                  //~| ERROR expected one of `.`, `;`, `?`, `}`, or an operator, found `1.1`
+                  //~| ERROR expected one of `.`, `;`, `?`, `}`, or an operator, found literal `1.1`
         { s.$c; } //~ ERROR unexpected token: `1.1`
-                  //~| ERROR expected one of `.`, `;`, `?`, `}`, or an operator, found `1.1`
+                  //~| ERROR expected one of `.`, `;`, `?`, `}`, or an operator, found expression `1.1`
     };
 }
 

tests/ui/parser/float-field-interpolated.stderr

@@ -9,7 +9,7 @@ LL |     generate_field_accesses!(1.1, 1.1, 1.1);
    |
    = note: this error originates in the macro `generate_field_accesses` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: expected one of `.`, `;`, `?`, `}`, or an operator, found `1.1`
+error: expected one of `.`, `;`, `?`, `}`, or an operator, found literal `1.1`
   --> $DIR/float-field-interpolated.rs:8:13
    |
 LL |         { s.$b; }
@@ -31,7 +31,7 @@ LL |     generate_field_accesses!(1.1, 1.1, 1.1);
    |
    = note: this error originates in the macro `generate_field_accesses` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: expected one of `.`, `;`, `?`, `}`, or an operator, found `1.1`
+error: expected one of `.`, `;`, `?`, `}`, or an operator, found expression `1.1`
   --> $DIR/float-field-interpolated.rs:10:13
    |
 LL |         { s.$c; }