New upstream version 1.54.0+dfsg1

[rustc.git] / compiler / rustc_error_codes / src / error_codes / E0316.md

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+A `where` clause contains a nested quantification over lifetimes.
+
+Erroneous code example:
+
+```compile_fail,E0316
+trait Tr<'a, 'b> {}
+
+fn foo<T>(t: T)
+where
+    for<'a> &'a T: for<'b> Tr<'a, 'b>, // error: nested quantification
+{
+}
+```
+
+Rust syntax allows lifetime quantifications in two places within
+`where` clauses: Quantifying over the trait bound only (as in
+`Ty: for<'l> Trait<'l>`) and quantifying over the whole clause
+(as in `for<'l> &'l Ty: Trait<'l>`). Using both in the same clause
+leads to a nested lifetime quantification, which is not supported.
+
+The following example compiles, because the clause with the nested
+quantification has been rewritten to use only one `for<>`:
+
+```
+trait Tr<'a, 'b> {}
+
+fn foo<T>(t: T)
+where
+    for<'a, 'b> &'a T: Tr<'a, 'b>, // ok
+{
+}
+```