Detect infinite recursion for #[error("{}", self)]

impl/src/fmt.rs

@@ -53,6 +53,14 @@ impl Display<'_> {
                 None => return Ok(()),
             };
             let member = match next {
+                '}' => {
+                    // `{}` displays extra argument 0. `#[error("{}", self)]`
+                    // recurses through Display the same way `{self}` does.
+                    if first_unnamed.as_ref().is_some_and(extra_arg_is_self) {
+                        infinite_recursive = true;
+                    }
+                    continue;
+                }
                 '0'..='9' => {
                     let int = take_int(&mut read);
                     if !extra_positional_arguments_allowed {
@@ -108,6 +116,9 @@ impl Display<'_> {
                 }
             };
             infinite_recursive |= member == *"self" && bound == Trait::Display;
+            infinite_recursive |= bound == Trait::Display
+                && matches!(&member, MemberUnraw::Unnamed(index) if index.index == 0)
+                && first_unnamed.as_ref().is_some_and(extra_arg_is_self);
             let field = match member_index.get(&member) {
                 Some(&field) => field,
                 None => {
@@ -270,6 +281,14 @@ fn is_syn_full() -> bool {
     }
 }
 
+fn extra_arg_is_self(tokens: &TokenStream) -> bool {
+    let mut iter = tokens.clone().into_iter();
+    match (iter.next(), iter.next()) {
+        (Some(TokenTree::Ident(ident)), None) => ident == "self",
+        _ => false,
+    }
+}
+
 fn take_int<'a>(read: &mut &'a str) -> &'a str {
     let mut int_len = 0;
     for ch in read.chars() {

tests/ui/unconditional-recursion-2.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error("{}", self)]
+pub struct Error;
+
+fn main() {
+    __FAIL__;
+}

tests/ui/unconditional-recursion-2.stderr

@@ -0,0 +1,21 @@
+error[E0425]: cannot find value `__FAIL__` in this scope
+ --> tests/ui/unconditional-recursion-2.rs:8:5
+  |
+8 |     __FAIL__;
+  |     ^^^^^^^^ not found in this scope
+
+warning: function cannot return without recursing
+ --> tests/ui/unconditional-recursion-2.rs:4:9
+  |
+4 | #[error("{}", self)]
+  |         ^^^^
+  |         |
+  |         cannot return without recursing
+  |         recursive call site
+  |
+  = help: a `loop` may express intention better if this is on purpose
+note: the lint level is defined here
+ --> tests/ui/unconditional-recursion-2.rs:4:9
+  |
+4 | #[error("{}", self)]
+  |         ^^^^