Error code E0794 for late-bound lifetime parameter error.

compiler/rustc_error_codes/src/error_codes.rs

@@ -513,6 +513,7 @@ E0790: include_str!("./error_codes/E0790.md"),
 E0791: include_str!("./error_codes/E0791.md"),
 E0792: include_str!("./error_codes/E0792.md"),
 E0793: include_str!("./error_codes/E0793.md"),
+E0794: include_str!("./error_codes/E0794.md"),
 }
 
 // Undocumented removed error codes. Note that many removed error codes are documented.

compiler/rustc_error_codes/src/error_codes/E0794.md

@@ -0,0 +1,64 @@
+A lifetime parameter of a function definition is called *late-bound* if it both:
+
+1. appears in an argument type
+2. does not appear in a generic type constraint
+
+You cannot specify lifetime arguments for late-bound lifetime parameters.
+
+Erroneous code example:
+
+```compile_fail,E0794
+fn foo<'a>(x: &'a str) -> &'a str { x }
+let _ = foo::<'static>;
+```
+
+The type of a concrete instance of a generic function is universally quantified
+over late-bound lifetime parameters. This is because we want the function to
+work for any lifetime substituted for the late-bound lifetime parameter, no
+matter where the function is called. Consequently, it doesn't make sense to
+specify arguments for late-bound lifetime parameters, since they are not
+resolved until the function's call site(s).
+
+To fix the issue, remove the specified lifetime:
+
+```
+fn foo<'a>(x: &'a str) -> &'a str { x }
+let _ = foo;
+```
+
+### Additional information
+
+Lifetime parameters that are not late-bound are called *early-bound*.
+Confusion may arise from the fact that late-bound and early-bound
+lifetime parameters are declared the same way in function definitions.
+When referring to a function pointer type, universal quantification over
+late-bound lifetime parameters can be made explicit:
+
+```
+trait BarTrait<'a> {}
+
+struct Bar<'a> {
+    s: &'a str
+}
+
+impl<'a> BarTrait<'a> for Bar<'a> {}
+
+fn bar<'a, 'b, T>(x: &'a str, _t: T) -> &'a str
+where T: BarTrait<'b>
+{
+    x
+}
+
+let bar_fn: for<'a> fn(&'a str, Bar<'static>) -> &'a str = bar; // OK
+let bar_fn2 = bar::<'static, Bar>; // Not allowed
+let bar_fn3 = bar::<Bar>; // OK
+```
+
+In the definition of `bar`, the lifetime parameter `'a` is late-bound, while
+`'b` is early-bound. This is reflected in the type annotation for `bar_fn`,
+where `'a` is universally quantified and `'b` is substituted by a specific
+lifetime. It is not allowed to explicitly specify early-bound lifetime
+arguments when late-bound lifetime parameters are present (as for `bar_fn2`,
+see issue #42868: https://github.com/rust-lang/rust/issues/42868), although the
+types that are constrained by early-bound parameters can be specified (as for
+`bar_fn3`).

compiler/rustc_hir_analysis/src/astconv/generics.rs

@@ -618,7 +618,7 @@ pub(crate) fn prohibit_explicit_late_bound_lifetimes(
         if position == GenericArgPosition::Value
             && args.num_lifetime_params() != param_counts.lifetimes
         {
-            let mut err = tcx.sess.struct_span_err(span, msg);
+            let mut err = struct_span_err!(tcx.sess, span, E0794, "{}", msg);
             err.span_note(span_late, note);
             err.emit();
         } else {

tests/ui/const-generics/const-arg-in-const-arg.full.stderr

@@ -1,4 +1,4 @@
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:18:23
    |
 LL |     let _: [u8; faz::<'a>(&())];
@@ -10,7 +10,7 @@ note: the late bound lifetime parameter is introduced here
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:21:23
    |
 LL |     let _: [u8; faz::<'b>(&())];
@@ -22,7 +22,7 @@ note: the late bound lifetime parameter is introduced here
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:41:24
    |
 LL |     let _: Foo<{ faz::<'a>(&()) }>;
@@ -34,7 +34,7 @@ note: the late bound lifetime parameter is introduced here
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:44:24
    |
 LL |     let _: Foo<{ faz::<'b>(&()) }>;
@@ -94,7 +94,7 @@ LL |     let _ = [0; bar::<N>()];
    |
    = help: try adding a `where` bound using this expression: `where [(); bar::<N>()]:`
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:30:23
    |
 LL |     let _ = [0; faz::<'a>(&())];
@@ -106,7 +106,7 @@ note: the late bound lifetime parameter is introduced here
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:33:23
    |
 LL |     let _ = [0; faz::<'b>(&())];
@@ -134,7 +134,7 @@ LL |     let _ = Foo::<{ bar::<N>() }>;
    |
    = help: try adding a `where` bound using this expression: `where [(); { bar::<N>() }]:`
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:52:27
    |
 LL |     let _ = Foo::<{ faz::<'a>(&()) }>;
@@ -146,7 +146,7 @@ note: the late bound lifetime parameter is introduced here
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:55:27
    |
 LL |     let _ = Foo::<{ faz::<'b>(&()) }>;
@@ -160,3 +160,4 @@ LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
 
 error: aborting due to 16 previous errors
 
+For more information about this error, try `rustc --explain E0794`.

tests/ui/const-generics/const-arg-in-const-arg.min.stderr

@@ -216,7 +216,7 @@ help: if this generic argument was intended as a const parameter, surround it wi
 LL |     let _: [u8; bar::<{ N }>()];
    |                       +   +
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:18:23
    |
 LL |     let _: [u8; faz::<'a>(&())];
@@ -228,7 +228,7 @@ note: the late bound lifetime parameter is introduced here
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:21:23
    |
 LL |     let _: [u8; faz::<'b>(&())];
@@ -251,7 +251,7 @@ help: if this generic argument was intended as a const parameter, surround it wi
 LL |     let _: Foo<{ bar::<{ N }>() }>;
    |                        +   +
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:41:24
    |
 LL |     let _: Foo<{ faz::<'a>(&()) }>;
@@ -263,7 +263,7 @@ note: the late bound lifetime parameter is introduced here
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:44:24
    |
 LL |     let _: Foo<{ faz::<'b>(&()) }>;
@@ -294,7 +294,7 @@ help: if this generic argument was intended as a const parameter, surround it wi
 LL |     let _ = [0; bar::<{ N }>()];
    |                       +   +
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:30:23
    |
 LL |     let _ = [0; faz::<'a>(&())];
@@ -306,7 +306,7 @@ note: the late bound lifetime parameter is introduced here
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:33:23
    |
 LL |     let _ = [0; faz::<'b>(&())];
@@ -329,7 +329,7 @@ help: if this generic argument was intended as a const parameter, surround it wi
 LL |     let _ = Foo::<{ bar::<{ N }>() }>;
    |                           +   +
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:52:27
    |
 LL |     let _ = Foo::<{ faz::<'a>(&()) }>;
@@ -341,7 +341,7 @@ note: the late bound lifetime parameter is introduced here
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:55:27
    |
 LL |     let _ = Foo::<{ faz::<'b>(&()) }>;
@@ -355,5 +355,5 @@ LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
 
 error: aborting due to 36 previous errors
 
-Some errors have detailed explanations: E0658, E0747.
+Some errors have detailed explanations: E0658, E0747, E0794.
 For more information about an error, try `rustc --explain E0658`.

tests/ui/late-bound-lifetimes/issue-80618.rs

@@ -0,0 +1,8 @@
+fn foo<'a>(x: &'a str) -> &'a str {
+    x
+}
+
+fn main() {
+    let _ = foo::<'static>;
+//~^ ERROR cannot specify lifetime arguments explicitly if late bound lifetime parameters are present [E0794]
+}

tests/ui/late-bound-lifetimes/issue-80618.stderr

@@ -0,0 +1,15 @@
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+  --> $DIR/issue-80618.rs:6:19
+   |
+LL |     let _ = foo::<'static>;
+   |                   ^^^^^^^
+   |
+note: the late bound lifetime parameter is introduced here
+  --> $DIR/issue-80618.rs:1:8
+   |
+LL | fn foo<'a>(x: &'a str) -> &'a str {
+   |        ^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0794`.

tests/ui/methods/method-call-lifetime-args-fail.stderr

@@ -30,7 +30,7 @@ note: method defined here, with 2 lifetime parameters: `'a`, `'b`
 LL |     fn early<'a, 'b>(self) -> (&'a u8, &'b u8) { loop {} }
    |        ^^^^^ --  --
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:27:15
    |
 LL |     S::late::<'static>(S, &0, &0);
@@ -42,7 +42,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late<'a, 'b>(self, _: &'a u8, _: &'b u8) {}
    |             ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:29:15
    |
 LL |     S::late::<'static, 'static>(S, &0, &0);
@@ -54,7 +54,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late<'a, 'b>(self, _: &'a u8, _: &'b u8) {}
    |             ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:31:15
    |
 LL |     S::late::<'static, 'static, 'static>(S, &0, &0);
@@ -66,7 +66,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late<'a, 'b>(self, _: &'a u8, _: &'b u8) {}
    |             ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:34:21
    |
 LL |     S::late_early::<'static, 'static>(S, &0);
@@ -78,7 +78,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late_early<'a, 'b>(self, _: &'a u8) -> &'b u8 { loop {} }
    |                   ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:36:21
    |
 LL |     S::late_early::<'static, 'static, 'static>(S, &0);
@@ -90,7 +90,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late_early<'a, 'b>(self, _: &'a u8) -> &'b u8 { loop {} }
    |                   ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:40:24
    |
 LL |     S::late_implicit::<'static>(S, &0, &0);
@@ -102,7 +102,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late_implicit(self, _: &u8, _: &u8) {}
    |                               ^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:42:24
    |
 LL |     S::late_implicit::<'static, 'static>(S, &0, &0);
@@ -114,7 +114,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late_implicit(self, _: &u8, _: &u8) {}
    |                               ^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:44:24
    |
 LL |     S::late_implicit::<'static, 'static, 'static>(S, &0, &0);
@@ -126,7 +126,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late_implicit(self, _: &u8, _: &u8) {}
    |                               ^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:47:30
    |
 LL |     S::late_implicit_early::<'static, 'static>(S, &0);
@@ -138,7 +138,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late_implicit_early<'b>(self, _: &u8) -> &'b u8 { loop {} }
    |                                         ^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:49:30
    |
 LL |     S::late_implicit_early::<'static, 'static, 'static>(S, &0);
@@ -150,7 +150,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late_implicit_early<'b>(self, _: &u8) -> &'b u8 { loop {} }
    |                                         ^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:52:35
    |
 LL |     S::late_implicit_self_early::<'static, 'static>(&S);
@@ -162,7 +162,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late_implicit_self_early<'b>(&self) -> &'b u8 { loop {} }
    |                                     ^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:54:35
    |
 LL |     S::late_implicit_self_early::<'static, 'static, 'static>(&S);
@@ -174,7 +174,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late_implicit_self_early<'b>(&self) -> &'b u8 { loop {} }
    |                                     ^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:57:28
    |
 LL |     S::late_unused_early::<'static, 'static>(S);
@@ -186,7 +186,7 @@ note: the late bound lifetime parameter is introduced here
 LL |     fn late_unused_early<'a, 'b>(self) -> &'b u8 { loop {} }
    |                          ^^
 
-error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/method-call-lifetime-args-fail.rs:59:28
    |
 LL |     S::late_unused_early::<'static, 'static, 'static>(S);
@@ -232,4 +232,5 @@ LL |     fn early<'a, 'b>(self) -> (&'a u8, &'b u8) { loop {} }
 
 error: aborting due to 18 previous errors
 
-For more information about this error, try `rustc --explain E0107`.
+Some errors have detailed explanations: E0107, E0794.
+For more information about an error, try `rustc --explain E0107`.