diff --git a/src/librustc/hir/map/collector.rs b/src/librustc/hir/map/collector.rs
index e66fa13f4fca..532e61721ca4 100644
--- a/src/librustc/hir/map/collector.rs
+++ b/src/librustc/hir/map/collector.rs
@@ -589,7 +589,7 @@ struct HirItemLike<T> {
     hash_bodies: bool,
 }
 
-impl<'a, 'hir, T> HashStable<StableHashingContext<'hir>> for HirItemLike<T>
+impl<'hir, T> HashStable<StableHashingContext<'hir>> for HirItemLike<T>
     where T: HashStable<StableHashingContext<'hir>>
 {
     fn hash_stable<W: StableHasherResult>(&self,
diff --git a/src/librustc/ich/hcx.rs b/src/librustc/ich/hcx.rs
index 8be610e8bf7a..8454eb75b55c 100644
--- a/src/librustc/ich/hcx.rs
+++ b/src/librustc/ich/hcx.rs
@@ -393,7 +393,7 @@ impl<'a> HashStable<StableHashingContext<'a>> for DelimSpan {
     }
 }
 
-pub fn hash_stable_trait_impls<'a, 'gcx, W>(
+pub fn hash_stable_trait_impls<'a, W>(
     hcx: &mut StableHashingContext<'a>,
     hasher: &mut StableHasher<W>,
     blanket_impls: &[DefId],
diff --git a/src/librustc/ich/impls_ty.rs b/src/librustc/ich/impls_ty.rs
index 563948a63514..c2c0ced07e69 100644
--- a/src/librustc/ich/impls_ty.rs
+++ b/src/librustc/ich/impls_ty.rs
@@ -135,7 +135,7 @@ impl<'gcx> HashStable<StableHashingContext<'gcx>> for ty::BoundVar {
     }
 }
 
-impl<'a, 'gcx, T> HashStable<StableHashingContext<'a>> for ty::Binder<T>
+impl<'a, T> HashStable<StableHashingContext<'a>> for ty::Binder<T>
     where T: HashStable<StableHashingContext<'a>>
 {
     fn hash_stable<W: StableHasherResult>(&self,
@@ -192,7 +192,7 @@ impl<'a> ToStableHashKey<StableHashingContext<'a>> for region::Scope {
     }
 }
 
-impl<'a, 'gcx> HashStable<StableHashingContext<'a>>
+impl<'a> HashStable<StableHashingContext<'a>>
 for ty::TyVid
 {
     fn hash_stable<W: StableHasherResult>(&self,
@@ -204,7 +204,7 @@ for ty::TyVid
     }
 }
 
-impl<'a, 'gcx> HashStable<StableHashingContext<'a>>
+impl<'a> HashStable<StableHashingContext<'a>>
 for ty::IntVid
 {
     fn hash_stable<W: StableHasherResult>(&self,
@@ -216,7 +216,7 @@ for ty::IntVid
     }
 }
 
-impl<'a, 'gcx> HashStable<StableHashingContext<'a>>
+impl<'a> HashStable<StableHashingContext<'a>>
 for ty::FloatVid
 {
     fn hash_stable<W: StableHasherResult>(&self,
@@ -228,7 +228,7 @@ for ty::FloatVid
     }
 }
 
-impl<'a, 'gcx, T> HashStable<StableHashingContext<'a>>
+impl<'a, T> HashStable<StableHashingContext<'a>>
 for ty::steal::Steal<T>
     where T: HashStable<StableHashingContext<'a>>
 {
diff --git a/src/librustc/infer/region_constraints/mod.rs b/src/librustc/infer/region_constraints/mod.rs
index ca766ea724f3..b6fe57636e90 100644
--- a/src/librustc/infer/region_constraints/mod.rs
+++ b/src/librustc/infer/region_constraints/mod.rs
@@ -858,7 +858,7 @@ impl<'a, 'gcx, 'tcx> GenericKind<'tcx> {
     }
 }
 
-impl<'a, 'gcx, 'tcx> VerifyBound<'tcx> {
+impl<'tcx> VerifyBound<'tcx> {
     pub fn must_hold(&self) -> bool {
         match self {
             VerifyBound::IfEq(..) => false,
diff --git a/src/librustc/lib.rs b/src/librustc/lib.rs
index 920f97805439..e2f2799d9634 100644
--- a/src/librustc/lib.rs
+++ b/src/librustc/lib.rs
@@ -30,6 +30,7 @@
 
 #![deny(rust_2018_idioms)]
 #![deny(internal)]
+#![deny(unused_lifetimes)]
 #![allow(explicit_outlives_requirements)]
 
 #![feature(arbitrary_self_types)]
diff --git a/src/librustc/lint/context.rs b/src/librustc/lint/context.rs
index c5c6c4b94470..b51257c52060 100644
--- a/src/librustc/lint/context.rs
+++ b/src/librustc/lint/context.rs
@@ -566,7 +566,7 @@ impl LintPassObject for EarlyLintPassObject {}
 
 impl LintPassObject for LateLintPassObject {}
 
-pub trait LintContext<'tcx>: Sized {
+pub trait LintContext: Sized {
     type PassObject: LintPassObject;
 
     fn sess(&self) -> &Session;
@@ -700,7 +700,7 @@ impl<'a, T: EarlyLintPass> EarlyContextAndPass<'a, T> {
     }
 }
 
-impl<'a, 'tcx> LintContext<'tcx> for LateContext<'a, 'tcx> {
+impl LintContext for LateContext<'_, '_> {
     type PassObject = LateLintPassObject;
 
     /// Gets the overall compiler `Session` object.
@@ -728,7 +728,7 @@ impl<'a, 'tcx> LintContext<'tcx> for LateContext<'a, 'tcx> {
     }
 }
 
-impl<'a> LintContext<'a> for EarlyContext<'a> {
+impl LintContext for EarlyContext<'_> {
     type PassObject = EarlyLintPassObject;
 
     /// Gets the overall compiler `Session` object.
diff --git a/src/librustc/macros.rs b/src/librustc/macros.rs
index f8d7a5e29f6f..b4088201f3bf 100644
--- a/src/librustc/macros.rs
+++ b/src/librustc/macros.rs
@@ -80,7 +80,7 @@ macro_rules! impl_stable_hash_for {
     // We want to use the enum name both in the `impl ... for $enum_name` as well as for
     // importing all the variants. Unfortunately it seems we have to take the name
     // twice for this purpose
-    (impl<$($lt:lifetime $(: $lt_bound:lifetime)? ),* $(,)? $($T:ident),* $(,)?>
+    (impl<$($T:ident),* $(,)?>
         for enum $enum_name:path
         [ $enum_path:path ]
     {
@@ -91,7 +91,7 @@ macro_rules! impl_stable_hash_for {
            $( { $($named_field:ident $(-> $named_delegate:tt)?),* } )?
         ),* $(,)?
     }) => {
-        impl<'a, $($lt $(: $lt_bound)?,)* $($T,)*>
+        impl<$($T,)*>
             ::rustc_data_structures::stable_hasher::HashStable<$crate::ich::StableHashingContext<'a>>
             for $enum_name
             where $($T: ::rustc_data_structures::stable_hasher::HashStable<$crate::ich::StableHashingContext<'a>>),*
@@ -117,13 +117,13 @@ macro_rules! impl_stable_hash_for {
     // Structs
     (struct $struct_name:path { $($field:ident $(-> $delegate:tt)?),* $(,)? }) => {
         impl_stable_hash_for!(
-            impl<'tcx> for struct $struct_name { $($field $(-> $delegate)?),* }
+            impl<> for struct $struct_name { $($field $(-> $delegate)?),* }
         );
     };
-    (impl<$($lt:lifetime $(: $lt_bound:lifetime)? ),* $(,)? $($T:ident),* $(,)?> for struct $struct_name:path {
+    (impl<$($T:ident),* $(,)?> for struct $struct_name:path {
         $($field:ident $(-> $delegate:tt)?),* $(,)?
     }) => {
-        impl<'a, $($lt $(: $lt_bound)?,)* $($T,)*>
+        impl<$($T,)*>
             ::rustc_data_structures::stable_hasher::HashStable<$crate::ich::StableHashingContext<'a>> for $struct_name
             where $($T: ::rustc_data_structures::stable_hasher::HashStable<$crate::ich::StableHashingContext<'a>>),*
         {
@@ -143,12 +143,12 @@ macro_rules! impl_stable_hash_for {
     // We cannot use normal parentheses here, the parser won't allow it
     (tuple_struct $struct_name:path { $($field:ident $(-> $delegate:tt)?),*  $(,)? }) => {
         impl_stable_hash_for!(
-            impl<'tcx> for tuple_struct $struct_name { $($field $(-> $delegate)?),* }
+            impl<> for tuple_struct $struct_name { $($field $(-> $delegate)?),* }
         );
     };
-    (impl<$($lt:lifetime $(: $lt_bound:lifetime)? ),* $(,)? $($T:ident),* $(,)?>
+    (impl<$($T:ident),* $(,)?>
      for tuple_struct $struct_name:path { $($field:ident $(-> $delegate:tt)?),*  $(,)? }) => {
-        impl<'a, $($lt $(: $lt_bound)?,)* $($T,)*>
+        impl<$($T,)*>
             ::rustc_data_structures::stable_hasher::HashStable<$crate::ich::StableHashingContext<'a>> for $struct_name
             where $($T: ::rustc_data_structures::stable_hasher::HashStable<$crate::ich::StableHashingContext<'a>>),*
         {
@@ -170,7 +170,7 @@ macro_rules! impl_stable_hash_for {
 macro_rules! impl_stable_hash_for_spanned {
     ($T:path) => (
 
-        impl<'a, 'tcx> HashStable<StableHashingContext<'a>> for ::syntax::source_map::Spanned<$T>
+        impl HashStable<StableHashingContext<'a>> for ::syntax::source_map::Spanned<$T>
         {
             #[inline]
             fn hash_stable<W: StableHasherResult>(&self,
diff --git a/src/librustc/mir/interpret/pointer.rs b/src/librustc/mir/interpret/pointer.rs
index a6c47ff5ca0f..26002a411d4b 100644
--- a/src/librustc/mir/interpret/pointer.rs
+++ b/src/librustc/mir/interpret/pointer.rs
@@ -105,7 +105,7 @@ impl From<AllocId> for Pointer {
     }
 }
 
-impl<'tcx> Pointer<()> {
+impl Pointer<()> {
     #[inline(always)]
     pub fn new(alloc_id: AllocId, offset: Size) -> Self {
         Pointer { alloc_id, offset, tag: () }
diff --git a/src/librustc/mir/interpret/value.rs b/src/librustc/mir/interpret/value.rs
index 23c37676f365..f7b3385668f7 100644
--- a/src/librustc/mir/interpret/value.rs
+++ b/src/librustc/mir/interpret/value.rs
@@ -146,7 +146,7 @@ impl<Tag> From<Double> for Scalar<Tag> {
     }
 }
 
-impl<'tcx> Scalar<()> {
+impl Scalar<()> {
     #[inline(always)]
     fn check_data(data: u128, size: u8) {
         debug_assert_eq!(truncate(data, Size::from_bytes(size as u64)), data,
diff --git a/src/librustc/mir/visit.rs b/src/librustc/mir/visit.rs
index b4acc076f3e4..babce812d4a3 100644
--- a/src/librustc/mir/visit.rs
+++ b/src/librustc/mir/visit.rs
@@ -967,7 +967,7 @@ pub enum PlaceContext {
     NonUse(NonUseContext),
 }
 
-impl<'tcx> PlaceContext {
+impl PlaceContext {
     /// Returns `true` if this place context represents a drop.
     pub fn is_drop(&self) -> bool {
         match *self {
diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index 003fdd501a35..6a03aa64d26d 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -785,7 +785,7 @@ macro_rules! options {
         return op;
     }
 
-    impl<'a> dep_tracking::DepTrackingHash for $struct_name {
+    impl dep_tracking::DepTrackingHash for $struct_name {
         fn hash(&self, hasher: &mut DefaultHasher, error_format: ErrorOutputType) {
             let mut sub_hashes = BTreeMap::new();
             $({
diff --git a/src/librustc/traits/auto_trait.rs b/src/librustc/traits/auto_trait.rs
index 7505b3c1be84..9ce35d15d3c3 100644
--- a/src/librustc/traits/auto_trait.rs
+++ b/src/librustc/traits/auto_trait.rs
@@ -270,7 +270,7 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
     // the final synthesized generics: we don't want our generated docs page to contain something
     // like 'T: Copy + Clone', as that's redundant. Therefore, we keep track of a separate
     // 'user_env', which only holds the predicates that will actually be displayed to the user.
-    fn evaluate_predicates<'b, 'gcx, 'c>(
+    fn evaluate_predicates<'b, 'c>(
         &self,
         infcx: &InferCtxt<'b, 'tcx, 'c>,
         trait_did: DefId,
diff --git a/src/librustc/traits/mod.rs b/src/librustc/traits/mod.rs
index c135b0b759c6..fd2d8fd0661a 100644
--- a/src/librustc/traits/mod.rs
+++ b/src/librustc/traits/mod.rs
@@ -1191,7 +1191,7 @@ where
         folder: &mut F,
     ) -> chalk_engine::ExClause<Self>;
 
-    fn visit_ex_clause_with<'gcx: 'tcx, V: TypeVisitor<'tcx>>(
+    fn visit_ex_clause_with<V: TypeVisitor<'tcx>>(
         ex_clause: &chalk_engine::ExClause<Self>,
         visitor: &mut V,
     ) -> bool;
diff --git a/src/librustc/traits/query/type_op/normalize.rs b/src/librustc/traits/query/type_op/normalize.rs
index e3d7a4d57a56..594f55e52f8e 100644
--- a/src/librustc/traits/query/type_op/normalize.rs
+++ b/src/librustc/traits/query/type_op/normalize.rs
@@ -145,7 +145,7 @@ BraceStructLiftImpl! {
 }
 
 impl_stable_hash_for! {
-    impl<'tcx, T> for struct Normalize<T> {
+    impl<T> for struct Normalize<T> {
         value
     }
 }
diff --git a/src/librustc/traits/select.rs b/src/librustc/traits/select.rs
index 7810d65e88cc..c637fc27cc0c 100644
--- a/src/librustc/traits/select.rs
+++ b/src/librustc/traits/select.rs
@@ -104,7 +104,7 @@ pub enum IntercrateAmbiguityCause {
 impl IntercrateAmbiguityCause {
     /// Emits notes when the overlap is caused by complex intercrate ambiguities.
     /// See #23980 for details.
-    pub fn add_intercrate_ambiguity_hint<'a, 'tcx>(
+    pub fn add_intercrate_ambiguity_hint(
         &self,
         err: &mut errors::DiagnosticBuilder<'_>,
     ) {
@@ -2299,7 +2299,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
     /// candidates and prefer where-clause candidates.
     ///
     /// See the comment for "SelectionCandidate" for more details.
-    fn candidate_should_be_dropped_in_favor_of<'o>(
+    fn candidate_should_be_dropped_in_favor_of(
         &mut self,
         victim: &EvaluatedCandidate<'tcx>,
         other: &EvaluatedCandidate<'tcx>,
@@ -2423,7 +2423,7 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
     // These cover the traits that are built-in to the language
     // itself: `Copy`, `Clone` and `Sized`.
 
-    fn assemble_builtin_bound_candidates<'o>(
+    fn assemble_builtin_bound_candidates(
         &mut self,
         conditions: BuiltinImplConditions<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
diff --git a/src/librustc/traits/structural_impls.rs b/src/librustc/traits/structural_impls.rs
index 4d382d6c45a7..f6108f2a2869 100644
--- a/src/librustc/traits/structural_impls.rs
+++ b/src/librustc/traits/structural_impls.rs
@@ -90,13 +90,13 @@ impl<'tcx, N: fmt::Debug> fmt::Debug for traits::VtableClosureData<'tcx, N> {
     }
 }
 
-impl<'tcx, N: fmt::Debug> fmt::Debug for traits::VtableBuiltinData<N> {
+impl<N: fmt::Debug> fmt::Debug for traits::VtableBuiltinData<N> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "VtableBuiltinData(nested={:?})", self.nested)
     }
 }
 
-impl<'tcx, N: fmt::Debug> fmt::Debug for traits::VtableAutoImplData<N> {
+impl<N: fmt::Debug> fmt::Debug for traits::VtableAutoImplData<N> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(
             f,
diff --git a/src/librustc/ty/fast_reject.rs b/src/librustc/ty/fast_reject.rs
index 59ab4561f2c8..3ac76c384e60 100644
--- a/src/librustc/ty/fast_reject.rs
+++ b/src/librustc/ty/fast_reject.rs
@@ -154,7 +154,7 @@ impl<D: Copy + Debug + Ord + Eq + Hash> SimplifiedTypeGen<D> {
     }
 }
 
-impl<'a, 'gcx, D> HashStable<StableHashingContext<'a>> for SimplifiedTypeGen<D>
+impl<'a, D> HashStable<StableHashingContext<'a>> for SimplifiedTypeGen<D>
     where D: Copy + Debug + Ord + Eq + Hash +
              HashStable<StableHashingContext<'a>>,
 {
diff --git a/src/librustc/ty/instance.rs b/src/librustc/ty/instance.rs
index f0251917074d..cee1359eb366 100644
--- a/src/librustc/ty/instance.rs
+++ b/src/librustc/ty/instance.rs
@@ -442,7 +442,7 @@ fn resolve_associated_item<'a, 'tcx>(
     }
 }
 
-fn needs_fn_once_adapter_shim<'a, 'tcx>(actual_closure_kind: ty::ClosureKind,
+fn needs_fn_once_adapter_shim(actual_closure_kind: ty::ClosureKind,
                                         trait_closure_kind: ty::ClosureKind)
     -> Result<bool, ()>
 {
diff --git a/src/librustc/ty/layout.rs b/src/librustc/ty/layout.rs
index 8e2c3dd3d8ad..879d03352e65 100644
--- a/src/librustc/ty/layout.rs
+++ b/src/librustc/ty/layout.rs
@@ -1678,7 +1678,7 @@ impl ty::query::TyCtxtAt<'a, 'tcx, '_> {
     }
 }
 
-impl<'a, 'tcx, C> TyLayoutMethods<'tcx, C> for Ty<'tcx>
+impl<'tcx, C> TyLayoutMethods<'tcx, C> for Ty<'tcx>
     where C: LayoutOf<Ty = Ty<'tcx>> + HasTyCtxt<'tcx>,
           C::TyLayout: MaybeResult<TyLayout<'tcx>>,
           C: HasParamEnv<'tcx>
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 69bf05c66f39..aafc510f5b6e 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -203,7 +203,7 @@ impl AssocItem {
 
     /// Tests whether the associated item admits a non-trivial implementation
     /// for !
-    pub fn relevant_for_never<'tcx>(&self) -> bool {
+    pub fn relevant_for_never(&self) -> bool {
         match self.kind {
             AssocKind::Existential |
             AssocKind::Const |
@@ -1614,7 +1614,7 @@ pub struct Placeholder<T> {
     pub name: T,
 }
 
-impl<'a, 'gcx, T> HashStable<StableHashingContext<'a>> for Placeholder<T>
+impl<'a, T> HashStable<StableHashingContext<'a>> for Placeholder<T>
     where T: HashStable<StableHashingContext<'a>>
 {
     fn hash_stable<W: StableHasherResult>(
diff --git a/src/librustc/ty/print/mod.rs b/src/librustc/ty/print/mod.rs
index a7cb7bd3956f..d6bc4e537581 100644
--- a/src/librustc/ty/print/mod.rs
+++ b/src/librustc/ty/print/mod.rs
@@ -11,6 +11,8 @@ pub use self::pretty::*;
 
 pub mod obsolete;
 
+// FIXME(eddyb) false positive, the lifetime parameters are used with `P:  Printer<...>`.
+#[allow(unused_lifetimes)]
 pub trait Print<'gcx, 'tcx, P> {
     type Output;
     type Error;
diff --git a/src/librustc/ty/query/config.rs b/src/librustc/ty/query/config.rs
index 0eda92ea8b82..286894c82b7e 100644
--- a/src/librustc/ty/query/config.rs
+++ b/src/librustc/ty/query/config.rs
@@ -17,6 +17,8 @@ use crate::ich::StableHashingContext;
 
 // Query configuration and description traits.
 
+// FIXME(eddyb) false positive, the lifetime parameter is used for `Key`/`Value`.
+#[allow(unused_lifetimes)]
 pub trait QueryConfig<'tcx> {
     const NAME: QueryName;
     const CATEGORY: ProfileCategory;
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index 5cc46b21fca1..abde4d6de2ae 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -886,7 +886,7 @@ impl<T> Binder<T> {
     }
 
     /// Wraps `value` in a binder, binding higher-ranked vars (if any).
-    pub fn bind<'tcx>(value: T) -> Binder<T> {
+    pub fn bind(value: T) -> Binder<T> {
         Binder(value)
     }
 
@@ -988,7 +988,7 @@ pub struct ProjectionTy<'tcx> {
     pub item_def_id: DefId,
 }
 
-impl<'a, 'tcx> ProjectionTy<'tcx> {
+impl<'tcx> ProjectionTy<'tcx> {
     /// Construct a `ProjectionTy` by searching the trait from `trait_ref` for the
     /// associated item named `item_name`.
     pub fn from_ref_and_name(
diff --git a/src/librustc/util/captures.rs b/src/librustc/util/captures.rs
index 09d576b23c0f..dee9a7c3f4a7 100644
--- a/src/librustc/util/captures.rs
+++ b/src/librustc/util/captures.rs
@@ -3,6 +3,8 @@
 /// Basically a workaround; see [this comment] for details.
 ///
 /// [this comment]: https://github.com/rust-lang/rust/issues/34511#issuecomment-373423999
+// FIXME(eddyb) false positive, the lifetime parameter is "phantom" but needed.
+#[allow(unused_lifetimes)]
 pub trait Captures<'a> { }
 
 impl<'a, T: ?Sized> Captures<'a> for T { }