Prevent explicitly specifiying lifetime arguments to functions when the parameters are not visible in the type signature.

compiler/rustc_hir_analysis/src/hir_ty_lowering/generics.rs

@@ -399,7 +399,7 @@ pub fn check_generic_arg_count_for_value_path(
 
 /// Checks that the correct number of generic arguments have been provided.
 /// This is used both for datatypes and function calls.
-#[instrument(skip(cx, gen_pos), level = "debug")]
+#[instrument(skip(cx, gen_pos), level = "info")]
 pub(crate) fn check_generic_arg_count(
     cx: &dyn HirTyLowerer<'_>,
     def_id: DefId,
@@ -409,6 +409,8 @@ pub(crate) fn check_generic_arg_count(
     has_self: bool,
 ) -> GenericArgCountResult {
     let gen_args = seg.args();
+    let tcx = cx.tcx();
+    let kind = tcx.def_kind(def_id);
     let default_counts = gen_params.own_defaults();
     let param_counts = gen_params.own_counts();
 
@@ -430,7 +432,23 @@ pub(crate) fn check_generic_arg_count(
         prohibit_assoc_item_constraint(cx, c, None);
     }
 
-    let tcx = cx.tcx();
+    // hidden lifetimes may not be specified explicitly.
+    // if it doesn't show up in the function signature,
+    // it can't be written as a lifetime arg.
+    //
+    // While most hidden lifetimes are late-bound (e.g. `fn(_: &u32)` ),
+    // there are some cases (complicated and involve associated types)
+    // where an early-bound lifetime parameter can be hidden from the function signature.
+
+    let hidden_early_lifetimes =
+        gen_params.own_params.iter().filter(|x| x.is_anonymous_lifetime()).count();
+
+    debug!(?hidden_early_lifetimes);
+
+    if kind.is_fn_like() {
+        debug!("we are using fn-like {:?} ({gen_pos:?})", tcx.item_name(def_id));
+    }
+    debug!("# gen_args = {}", gen_args.args.len());
 
     // Suppress this warning for delegations as it is compiler generated and lifetimes are
     // propagated while late-bound lifetimes may be present.
@@ -476,9 +494,15 @@ pub(crate) fn check_generic_arg_count(
         Err(reported)
     };
 
-    let min_expected_lifetime_args = if infer_lifetimes { 0 } else { param_counts.lifetimes };
-    let max_expected_lifetime_args = param_counts.lifetimes;
+    let min_expected_lifetime_args =
+        if infer_lifetimes { 0 } else { param_counts.lifetimes - hidden_early_lifetimes };
+    debug!(?min_expected_lifetime_args);
+
+    let max_expected_lifetime_args = param_counts.lifetimes - hidden_early_lifetimes;
+    debug!(?max_expected_lifetime_args);
+
     let num_provided_lifetime_args = gen_args.num_lifetime_args();
+    debug!(?num_provided_lifetime_args);
 
     let lifetimes_correct = check_lifetime_args(
         min_expected_lifetime_args,
@@ -542,7 +566,7 @@ pub(crate) fn check_generic_arg_count(
                     .map(|param| param.name)
                     .collect();
                 if constraint_names == param_names {
-                    let has_assoc_ty_with_same_name = if let DefKind::Trait = tcx.def_kind(def_id) {
+                    let has_assoc_ty_with_same_name = if let DefKind::Trait = kind {
                         gen_args.constraints.iter().any(|constraint| {
                             traits::supertrait_def_ids(tcx, def_id).any(|trait_did| {
                                 cx.probe_trait_that_defines_assoc_item(

compiler/rustc_middle/src/ty/generics.rs

@@ -105,7 +105,7 @@ impl GenericParamDef {
     }
 }
 
-#[derive(Default)]
+#[derive(Debug, Default)]
 pub struct GenericParamCount {
     pub lifetimes: usize,
     pub types: usize,

tests/ui/lifetimes/turbofishing-invisible-lifetimes-154490.rs

@@ -0,0 +1,68 @@
+trait EvilTrait {
+    type EvilAssoc<'a>;
+    fn evil_assoc_1(_: Option<Self::EvilAssoc<'_>>) -> &i32 {
+        todo!()
+    }
+}
+
+// zero explicit generic lifetimes
+fn evil_early_bound_1<T: EvilTrait>(_: Option<<T as EvilTrait>::EvilAssoc<'_>>) -> &i32 {
+    todo!()
+}
+
+fn evil_early_bound_2<T: EvilTrait>(_: Option<<T as EvilTrait>::EvilAssoc<'_>>) -> &i32 {
+    todo!()
+}
+
+fn evil_multi_bound_3<'b, T: EvilTrait>(
+    _: Option<<T as EvilTrait>::EvilAssoc<'_>>
+) -> (&i32, &'b i64) {
+    todo!()
+}
+
+fn evil_early_bound_4<T: EvilTrait>(_: Option<<T as EvilTrait>::EvilAssoc<'_>>) -> i32 {
+    todo!()
+}
+
+fn normal_early_bound<'eb: 'eb, T: EvilTrait>(_: &'eb i32 ) -> &'eb i32 {
+    todo!()
+}
+
+fn normal_late_bound<'lb, T: EvilTrait>(_: &'lb i32 ) -> &'lb i32 {
+    todo!()
+}
+
+struct LtWrapper<'a>(&'a i32);
+
+fn elide_struct_1(_: &i32) -> LtWrapper {
+    todo!()
+}
+
+fn elide_struct_2(_: &i32) -> LtWrapper {
+    todo!()
+}
+
+fn foo<T: EvilTrait>() {
+    static WHATEVER: i32 = 123;
+
+    evil_early_bound_1::<'static, T>(None);
+    //~^ ERROR: function takes 0 lifetime arguments but 1 lifetime argument was supplied [E0107]
+    // ^ FIXME: should this have a better diagnostic?
+    evil_early_bound_2::<T>(None);
+    evil_multi_bound_3::<'static, 'static, T>(None);
+    //~^ ERROR: function takes 1 lifetime argument but 2 lifetime arguments were supplied [E0107]
+    evil_early_bound_4::<'static, T>(None);
+    //~^ ERROR: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present [E0794]
+    <T as EvilTrait>::evil_assoc_1::<'static>(None);
+    //~^ ERROR: associated function takes 0 lifetime arguments but 1 lifetime argument was supplied [E0107]
+    elide_struct_1(&WHATEVER);
+    elide_struct_2::<'static>(&WHATEVER);
+    //~^ ERROR: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present [E0794]
+    normal_early_bound::<'static, T>(&WHATEVER);
+    // ^ this is fine
+    normal_late_bound::<'static, T>(&WHATEVER);
+    //~^ ERROR: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present [E0794]
+}
+
+
+fn main() {}

tests/ui/lifetimes/turbofishing-invisible-lifetimes-154490.stderr

@@ -0,0 +1,82 @@
+error[E0107]: function takes 0 lifetime arguments but 1 lifetime argument was supplied
+  --> $DIR/turbofishing-invisible-lifetimes-154490.rs:48:5
+   |
+LL |     evil_early_bound_1::<'static, T>(None);
+   |     ^^^^^^^^^^^^^^^^^^   ------- help: remove the lifetime argument
+   |     |
+   |     expected 0 lifetime arguments
+   |
+note: function defined here, with 0 lifetime parameters
+  --> $DIR/turbofishing-invisible-lifetimes-154490.rs:9:4
+   |
+LL | fn evil_early_bound_1<T: EvilTrait>(_: Option<<T as EvilTrait>::EvilAssoc<'_>>) -> &i32 {
+   |    ^^^^^^^^^^^^^^^^^^
+
+error[E0107]: function takes 1 lifetime argument but 2 lifetime arguments were supplied
+  --> $DIR/turbofishing-invisible-lifetimes-154490.rs:52:5
+   |
+LL |     evil_multi_bound_3::<'static, 'static, T>(None);
+   |     ^^^^^^^^^^^^^^^^^^          --------- help: remove the lifetime argument
+   |     |
+   |     expected 1 lifetime argument
+   |
+note: function defined here, with 1 lifetime parameter: `'b`
+  --> $DIR/turbofishing-invisible-lifetimes-154490.rs:17:4
+   |
+LL | fn evil_multi_bound_3<'b, T: EvilTrait>(
+   |    ^^^^^^^^^^^^^^^^^^ --
+
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+  --> $DIR/turbofishing-invisible-lifetimes-154490.rs:54:26
+   |
+LL |     evil_early_bound_4::<'static, T>(None);
+   |                          ^^^^^^^
+   |
+note: the late bound lifetime parameter is introduced here
+  --> $DIR/turbofishing-invisible-lifetimes-154490.rs:23:75
+   |
+LL | fn evil_early_bound_4<T: EvilTrait>(_: Option<<T as EvilTrait>::EvilAssoc<'_>>) -> i32 {
+   |                                                                           ^^
+
+error[E0107]: associated function takes 0 lifetime arguments but 1 lifetime argument was supplied
+  --> $DIR/turbofishing-invisible-lifetimes-154490.rs:56:23
+   |
+LL |     <T as EvilTrait>::evil_assoc_1::<'static>(None);
+   |                       ^^^^^^^^^^^^----------- help: remove the unnecessary generics
+   |                       |
+   |                       expected 0 lifetime arguments
+   |
+note: associated function defined here, with 0 lifetime parameters
+  --> $DIR/turbofishing-invisible-lifetimes-154490.rs:3:8
+   |
+LL |     fn evil_assoc_1(_: Option<Self::EvilAssoc<'_>>) -> &i32 {
+   |        ^^^^^^^^^^^^
+
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+  --> $DIR/turbofishing-invisible-lifetimes-154490.rs:59:22
+   |
+LL |     elide_struct_2::<'static>(&WHATEVER);
+   |                      ^^^^^^^
+   |
+note: the late bound lifetime parameter is introduced here
+  --> $DIR/turbofishing-invisible-lifetimes-154490.rs:41:22
+   |
+LL | fn elide_struct_2(_: &i32) -> LtWrapper {
+   |                      ^
+
+error[E0794]: cannot specify lifetime arguments explicitly if late bound lifetime parameters are present
+  --> $DIR/turbofishing-invisible-lifetimes-154490.rs:63:25
+   |
+LL |     normal_late_bound::<'static, T>(&WHATEVER);
+   |                         ^^^^^^^
+   |
+note: the late bound lifetime parameter is introduced here
+  --> $DIR/turbofishing-invisible-lifetimes-154490.rs:31:22
+   |
+LL | fn normal_late_bound<'lb, T: EvilTrait>(_: &'lb i32 ) -> &'lb i32 {
+   |                      ^^^
+
+error: aborting due to 6 previous errors
+
+Some errors have detailed explanations: E0107, E0794.
+For more information about an error, try `rustc --explain E0107`.