More detail when expecting expression but encountering bad macro argument

Partially address #71039.

Author: estebank

compiler/rustc_ast/src/token.rs

@@ -911,6 +911,23 @@ impl Nonterminal {
             NtVis(vis) => vis.span,
         }
     }
+
+    pub fn descr(&self) -> &'static str {
+        match self {
+            NtItem(_) => "item",
+            NtBlock(_) => "block",
+            NtStmt(_) => "statement",
+            NtPat(_) => "pattern",
+            NtExpr(_) => "expression",
+            NtLiteral(_) => "literal",
+            NtTy(_) => "type",
+            NtIdent(..) => "identifier",
+            NtLifetime(_) => "lifetime",
+            NtMeta(_) => "attribute",
+            NtPath(_) => "path",
+            NtVis(_) => "visibility",
+        }
+    }
 }
 
 impl PartialEq for Nonterminal {

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -23,11 +23,12 @@ use crate::parser;
 use rustc_ast as ast;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, Lit, LitKind, TokenKind};
+use rustc_ast::tokenstream::AttrTokenTree;
 use rustc_ast::util::parser::AssocOp;
 use rustc_ast::{
     AngleBracketedArg, AngleBracketedArgs, AnonConst, AttrVec, BinOpKind, BindingAnnotation, Block,
-    BlockCheckMode, Expr, ExprKind, GenericArg, Generics, Item, ItemKind, Param, Pat, PatKind,
-    Path, PathSegment, QSelf, Ty, TyKind,
+    BlockCheckMode, Expr, ExprKind, GenericArg, Generics, HasTokens, Item, ItemKind, Param, Pat,
+    PatKind, Path, PathSegment, QSelf, Ty, TyKind,
 };
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashSet;
@@ -2163,6 +2164,44 @@ impl<'a> Parser<'a> {
             err.subdiagnostic(ExprParenthesesNeeded::surrounding(*sp));
         }
         err.span_label(span, "expected expression");
+
+        // 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).
+        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)),
+            ));
+            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}"));
+        }
         err
     }
 

tests/ui/macros/trace_faulty_macros.rs

@@ -41,3 +41,14 @@ fn use_bang_macro_as_attr() {}
 
 #[derive(Debug)] //~ ERROR `derive` may only be applied to `struct`s
 fn use_derive_macro_as_attr() {}
+
+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`
+}
+
+fn foo() {
+    test!(let x = 1+1);
+}

tests/ui/macros/trace_faulty_macros.stderr

@@ -53,6 +53,9 @@ LL |     my_recursive_macro!();
 error: expected expression, found `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
 ...
@@ -69,6 +72,23 @@ LL | #[derive(Debug)]
 LL | fn use_derive_macro_as_attr() {}
    | -------------------------------- not a `struct`, `enum` or `union`
 
+error: expected expression, found `1 + 1`
+  --> $DIR/trace_faulty_macros.rs:49:37
+   |
+LL |     (let $p:pat = $e:expr) => {test!(($p,$e))};
+   |                                          -- this is interpreted as pattern `1 + 1` (in expansion #2)
+...
+LL |     (($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)
+   |     in this macro invocation
+   |
+   = note: this error originates in the macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)
+
 note: trace_macro
   --> $DIR/trace_faulty_macros.rs:36:13
    |
@@ -80,6 +100,17 @@ LL |     let a = pat_macro!();
    = note: expanding `pat_macro! { A { a : a, b : 0, c : _, .. } }`
    = note: to `A { a: a, b: 0, c: _, .. }`
 
-error: aborting due to 4 previous errors
+note: trace_macro
+  --> $DIR/trace_faulty_macros.rs:53:5
+   |
+LL |     test!(let x = 1+1);
+   |     ^^^^^^^^^^^^^^^^^^
+   |
+   = note: expanding `test! { let x = 1 + 1 }`
+   = note: to `test! ((x, 1 + 1))`
+   = note: expanding `test! { (x, 1 + 1) }`
+   = note: to `let x = 1 + 1 ;`
+
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0774`.