Make a proper suggestion.

compiler/rustc_hir_analysis/src/astconv/generics.rs

@@ -611,7 +611,40 @@ pub(crate) fn prohibit_explicit_late_bound_lifetimes(
             })
             .collect();
 
-        tcx.sess.struct_span_err(spans, msg).span_note(span_late, note).help(help).emit();
+        let mut err = tcx.sess.struct_span_err(spans, msg);
+        err.span_note(span_late, note);
+
+        let mut suggestions = vec![];
+        if let Some(span_ext) = args.span_ext() {
+            if args.num_lifetime_params() == args.args.len()
+                && span_ext.ctxt() == seg.ident.span.ctxt()
+            {
+                // We only specify lifetime args, so suggest to remove everything.
+                suggestions.push((seg.ident.span.shrink_to_hi().to(span_ext), String::new()));
+            } else {
+                for [arg, next_arg] in args.args.array_windows() {
+                    if let hir::GenericArg::Lifetime(lifetime) = arg
+                        && let Some(arg_span) = lifetime.ident.span.find_ancestor_inside(span_ext)
+                        && let Some(next_span_lo) = next_arg.span().shrink_to_lo().find_ancestor_inside(span_ext)
+                    {
+                        suggestions.push((arg_span.with_hi(next_span_lo.lo()), String::new()));
+                    }
+                }
+
+                if let Some(arg) = args.args.last()
+                    && let hir::GenericArg::Lifetime(lifetime) = arg
+                    && let Some(arg_span) = lifetime.ident.span.find_ancestor_inside(span_ext)
+                {
+                    suggestions.push((arg_span, String::new()));
+                }
+            }
+        }
+        if !suggestions.is_empty() {
+            err.multipart_suggestion_verbose(help, suggestions, Applicability::MachineApplicable);
+        } else {
+            err.help(help);
+        }
+        err.emit();
 
         ExplicitLateBound::Yes
     } else {

compiler/rustc_hir_analysis/src/lib.rs

@@ -57,6 +57,7 @@ This API is completely unstable and subject to change.
 
 #![allow(rustc::potential_query_instability)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
+#![feature(array_windows)]
 #![feature(box_patterns)]
 #![feature(control_flow_enum)]
 #![feature(drain_filter)]

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

@@ -9,7 +9,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _: [u8; faz::<'a>(&())];
+LL +     let _: [u8; faz(&())];
+   |
 
 error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:21:23
@@ -22,7 +26,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _: [u8; faz::<'b>(&())];
+LL +     let _: [u8; faz(&())];
+   |
 
 error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:41:24
@@ -35,7 +43,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _: Foo<{ faz::<'a>(&()) }>;
+LL +     let _: Foo<{ faz(&()) }>;
+   |
 
 error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:44:24
@@ -48,7 +60,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _: Foo<{ faz::<'b>(&()) }>;
+LL +     let _: Foo<{ faz(&()) }>;
+   |
 
 error: unconstrained generic constant
   --> $DIR/const-arg-in-const-arg.rs:13:12
@@ -109,7 +125,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _ = [0; faz::<'a>(&())];
+LL +     let _ = [0; faz(&())];
+   |
 
 error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:33:23
@@ -122,7 +142,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _ = [0; faz::<'b>(&())];
+LL +     let _ = [0; faz(&())];
+   |
 
 error: unconstrained generic constant
   --> $DIR/const-arg-in-const-arg.rs:47:19
@@ -151,7 +175,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _ = Foo::<{ faz::<'a>(&()) }>;
+LL +     let _ = Foo::<{ faz(&()) }>;
+   |
 
 error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:55:27
@@ -164,7 +192,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _ = Foo::<{ faz::<'b>(&()) }>;
+LL +     let _ = Foo::<{ faz(&()) }>;
+   |
 
 error: aborting due to 16 previous errors
 

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

@@ -227,7 +227,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _: [u8; faz::<'a>(&())];
+LL +     let _: [u8; faz(&())];
+   |
 
 error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:21:23
@@ -240,7 +244,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _: [u8; faz::<'b>(&())];
+LL +     let _: [u8; faz(&())];
+   |
 
 error[E0747]: unresolved item provided when a constant was expected
   --> $DIR/const-arg-in-const-arg.rs:38:24
@@ -264,7 +272,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _: Foo<{ faz::<'a>(&()) }>;
+LL +     let _: Foo<{ faz(&()) }>;
+   |
 
 error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:44:24
@@ -277,7 +289,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _: Foo<{ faz::<'b>(&()) }>;
+LL +     let _: Foo<{ faz(&()) }>;
+   |
 
 error: constant expression depends on a generic parameter
   --> $DIR/const-arg-in-const-arg.rs:25:17
@@ -309,7 +325,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _ = [0; faz::<'a>(&())];
+LL +     let _ = [0; faz(&())];
+   |
 
 error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:33:23
@@ -322,7 +342,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _ = [0; faz::<'b>(&())];
+LL +     let _ = [0; faz(&())];
+   |
 
 error[E0747]: unresolved item provided when a constant was expected
   --> $DIR/const-arg-in-const-arg.rs:49:27
@@ -346,7 +370,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _ = Foo::<{ faz::<'a>(&()) }>;
+LL +     let _ = Foo::<{ faz(&()) }>;
+   |
 
 error: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
   --> $DIR/const-arg-in-const-arg.rs:55:27
@@ -359,7 +387,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL | const fn faz<'a>(_: &'a ()) -> usize { 13 }
    |              ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     let _ = Foo::<{ faz::<'b>(&()) }>;
+LL +     let _ = Foo::<{ faz(&()) }>;
+   |
 
 error: aborting due to 36 previous errors
 

tests/ui/const-generics/issues/issue-83466.stderr

@@ -9,7 +9,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL |     fn func<'a, U>(self) -> U {
    |             ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     S.func::<'a, 10_u32>()
+LL +     S.func::<10_u32>()
+   |
 
 error[E0747]: constant provided when a type was expected
   --> $DIR/issue-83466.rs:11:18

tests/ui/issues/issue-60622.stderr

@@ -9,7 +9,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL |     fn a(&self) {}
    |          ^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     b.a::<'_, T>();
+LL +     b.a::<T>();
+   |
 
 error[E0107]: method takes 0 generic arguments but 1 generic argument was supplied
   --> $DIR/issue-60622.rs:10:7

tests/ui/issues/issue-72278.stderr

@@ -9,7 +9,11 @@ note: the late bound lifetime parameter is introduced here
    |
 LL |     fn func<'a, U>(&'a self) -> U {
    |             ^^
-   = help: remove the explicit lifetime argument
+help: remove the explicit lifetime argument
+   |
+LL -     S.func::<'a, U>()
+LL +     S.func::<U>()
+   |
 
 error: aborting due to previous error
 

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

@@ -0,0 +1,75 @@
+// run-rustfix
+#![allow(dead_code)]
+
+struct S;
+
+impl S {
+    fn late<'a, 'b>(self, _: &'a u8, _: &'b u8) {}
+    fn late_implicit(self, _: &u8, _: &u8) {}
+    fn early<'a, 'b>(self) -> (&'a u8, &'b u8) { loop {} }
+    fn late_early<'a, 'b>(self, _: &'a u8) -> &'b u8 { loop {} }
+    fn late_implicit_early<'b>(self, _: &u8) -> &'b u8 { loop {} }
+    fn late_implicit_self_early<'b>(&self) -> &'b u8 { loop {} }
+    fn late_unused_early<'a, 'b>(self) -> &'b u8 { loop {} }
+    fn life_and_type<'a, T>(self) -> &'a T { loop {} }
+}
+
+fn method_call() {
+    S.early(); // OK
+    S.early::<'static, 'static>();
+    //~^ ERROR method takes 2 lifetime arguments but 1 lifetime argument
+    S.early::<'static, 'static, >();
+    //~^ ERROR method takes 2 lifetime arguments but 3 lifetime arguments were supplied
+    let _: &u8 = S.life_and_type::<'static>();
+    S.life_and_type::<u8>();
+    S.life_and_type::<'static, u8>();
+}
+
+fn ufcs() {
+    S::late(S, &0, &0); // OK
+    S::late(S, &0, &0);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+    S::late(S, &0, &0);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+    S::late(S, &0, &0);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+    S::late_early(S, &0); // OK
+    S::late_early(S, &0);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+    S::late_early(S, &0);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+
+    S::late_implicit(S, &0, &0); // OK
+    S::late_implicit(S, &0, &0);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+    S::late_implicit(S, &0, &0);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+    S::late_implicit(S, &0, &0);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+    S::late_implicit_early(S, &0); // OK
+    S::late_implicit_early(S, &0);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+    S::late_implicit_early(S, &0);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+    S::late_implicit_self_early(&S); // OK
+    S::late_implicit_self_early(&S);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+    S::late_implicit_self_early(&S);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+    S::late_unused_early(S); // OK
+    S::late_unused_early(S);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+    S::late_unused_early(S);
+    //~^ ERROR cannot specify lifetime arguments explicitly
+
+    S::early(S); // OK
+    S::early::<'static, 'static>(S);
+    //~^ ERROR method takes 2 lifetime arguments but 1 lifetime argument
+    S::early::<'static, 'static, >(S);
+    //~^ ERROR method takes 2 lifetime arguments but 3 lifetime arguments were supplied
+    let _: &u8 = S::life_and_type::<'static>(S);
+    S::life_and_type::<u8>(S);
+    S::life_and_type::<'static, u8>(S);
+}
+
+fn main() {}

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

@@ -1,3 +1,6 @@
+// run-rustfix
+#![allow(dead_code)]
+
 struct S;
 
 impl S {