Add note for identifier with attempted hygiene violation

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -1125,6 +1125,8 @@ fn err_code_special_cases(
                         }
                     }
                 }
+
+                self.suggest_ident_hidden_by_hygiene(err, path, span);
             } else if err_code == E0412 {
                 if let Some(correct) = Self::likely_rust_type(path) {
                     err.span_suggestion(
@@ -1138,6 +1140,28 @@ fn err_code_special_cases(
         }
     }
 
+    fn suggest_ident_hidden_by_hygiene(&self, err: &mut Diag<'_>, path: &[Segment], span: Span) {
+        let [segment] = path else { return };
+
+        let ident = segment.ident;
+        let callsite_span = span.source_callsite();
+        for rib in self.ribs[ValueNS].iter().rev() {
+            for (binding_ident, _) in &rib.bindings {
+                if binding_ident.name == ident.name
+                    && !binding_ident.span.eq_ctxt(span)
+                    && !binding_ident.span.from_expansion()
+                    && binding_ident.span.lo() < callsite_span.lo()
+                {
+                    err.span_help(
+                        binding_ident.span,
+                        "an identifier with the same name exists, but is not accessible due to macro hygiene",
+                    );
+                    return;
+                }
+            }
+        }
+    }
+
     /// Emit special messages for unresolved `Self` and `self`.
     fn suggest_self_ty(
         &self,

tests/ui/macros/macro-hygiene-help-issue-148580.rs

@@ -0,0 +1,15 @@
+macro_rules! print_it { {} => { println!("{:?}", it); } }
+//~^ ERROR cannot find value `it` in this scope
+
+fn main() {
+    {
+        let it = "hello";
+    }
+    {
+        let it = "world";
+        {
+            let it = ();
+            print_it!();
+        }
+    }
+}

tests/ui/macros/macro-hygiene-help-issue-148580.stderr

@@ -0,0 +1,19 @@
+error[E0425]: cannot find value `it` in this scope
+  --> $DIR/macro-hygiene-help-issue-148580.rs:1:50
+   |
+LL | macro_rules! print_it { {} => { println!("{:?}", it); } }
+   |                                                  ^^ not found in this scope
+...
+LL |             print_it!();
+   |             ----------- in this macro invocation
+   |
+help: an identifier with the same name exists, but is not accessible due to macro hygiene
+  --> $DIR/macro-hygiene-help-issue-148580.rs:11:17
+   |
+LL |             let it = ();
+   |                 ^^
+   = note: this error originates in the macro `print_it` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0425`.

tests/ui/macros/macro-hygiene-scope-15167.stderr

@@ -7,6 +7,11 @@ LL | macro_rules! f { () => (n) }
 LL |         println!("{}", f!());
    |                        ---- in this macro invocation
    |
+help: an identifier with the same name exists, but is not accessible due to macro hygiene
+  --> $DIR/macro-hygiene-scope-15167.rs:12:9
+   |
+LL |     for n in 0..1 {
+   |         ^
    = note: this error originates in the macro `f` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0425]: cannot find value `n` in this scope
@@ -18,6 +23,11 @@ LL | macro_rules! f { () => (n) }
 LL |         println!("{}", f!());
    |                        ---- in this macro invocation
    |
+help: an identifier with the same name exists, but is not accessible due to macro hygiene
+  --> $DIR/macro-hygiene-scope-15167.rs:16:17
+   |
+LL |     if let Some(n) = None {
+   |                 ^
    = note: this error originates in the macro `f` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0425]: cannot find value `n` in this scope
@@ -29,6 +39,11 @@ LL | macro_rules! f { () => (n) }
 LL |         println!("{}", f!());
    |                        ---- in this macro invocation
    |
+help: an identifier with the same name exists, but is not accessible due to macro hygiene
+  --> $DIR/macro-hygiene-scope-15167.rs:21:24
+   |
+LL |     } else if let Some(n) = None {
+   |                        ^
    = note: this error originates in the macro `f` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0425]: cannot find value `n` in this scope
@@ -40,6 +55,11 @@ LL | macro_rules! f { () => (n) }
 LL |         println!("{}", f!());
    |                        ---- in this macro invocation
    |
+help: an identifier with the same name exists, but is not accessible due to macro hygiene
+  --> $DIR/macro-hygiene-scope-15167.rs:25:20
+   |
+LL |     while let Some(n) = None {
+   |                    ^
    = note: this error originates in the macro `f` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 4 previous errors

tests/ui/macros/metavar-expressions/concat-hygiene.stderr

@@ -7,6 +7,11 @@ LL |         ${concat($lhs, $rhs)}
 LL |     let _another = join!(abc, def);
    |                    --------------- in this macro invocation
    |
+help: an identifier with the same name exists, but is not accessible due to macro hygiene
+  --> $DIR/concat-hygiene.rs:11:9
+   |
+LL |     let abcdef = 1;
+   |         ^^^^^^
    = note: this error originates in the macro `join` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 1 previous error

tests/ui/proc-macro/gen-macro-rules-hygiene.stderr

@@ -18,6 +18,11 @@ LL | gen_macro_rules!();
 LL |         generated!();
    |         ------------ in this macro invocation
    |
+help: an identifier with the same name exists, but is not accessible due to macro hygiene
+  --> $DIR/gen-macro-rules-hygiene.rs:19:13
+   |
+LL |         let local_use = 1;
+   |             ^^^^^^^^^
    = note: this error originates in the macro `generated` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0425]: cannot find value `local_def` in this scope

tests/ui/proc-macro/mixed-site-span.stderr

@@ -594,6 +594,11 @@ error[E0425]: cannot find value `local_use` in this scope
 LL |         proc_macro_rules!();
    |         ^^^^^^^^^^^^^^^^^^^ help: a local variable with a similar name exists: `local_def`
    |
+help: an identifier with the same name exists, but is not accessible due to macro hygiene
+  --> $DIR/mixed-site-span.rs:21:13
+   |
+LL |         let local_use = 1;
+   |             ^^^^^^^^^
    = note: this error originates in the macro `proc_macro_rules` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0425]: cannot find value `local_def` in this scope

tests/ui/proc-macro/weird-hygiene.stderr

@@ -7,6 +7,11 @@ LL |             Value = (stringify!($tokens + hidden_ident), 1).1
 LL |     other!(50);
    |     ---------- in this macro invocation
    |
+help: an identifier with the same name exists, but is not accessible due to macro hygiene
+  --> $DIR/weird-hygiene.rs:44:9
+   |
+LL |     let hidden_ident = "Hello1";
+   |         ^^^^^^^^^^^^
    = note: this error originates in the macro `inner` which comes from the expansion of the macro `other` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0425]: cannot find value `hidden_ident` in this scope
@@ -18,6 +23,11 @@ LL |             hidden_ident
 LL |     invoke_it!(25);
    |     -------------- in this macro invocation
    |
+help: an identifier with the same name exists, but is not accessible due to macro hygiene
+  --> $DIR/weird-hygiene.rs:44:9
+   |
+LL |     let hidden_ident = "Hello1";
+   |         ^^^^^^^^^^^^
    = note: this error originates in the macro `invoke_it` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 2 previous errors