Auto merge of #72905 - JohnTitor:rollup-phtyo5i, r=JohnTitor

Rollup of 10 pull requests

Successful merges:

 - #72775 (Return early to avoid ICE)
 - #72795 (Add a test for `$:ident` in proc macro input)
 - #72822 (remove trivial calls to mk_const)
 - #72825 (Clarify errors and warnings about the transition to the new asm!)
 - #72827 (changed *nix to Unix-like)
 - #72880 (Clean up E0637 explanation)
 - #72886 (Remove allow missing_debug_implementations for MaybeUninit)
 - #72889 (rustc: Remove the `--passive-segments` LLD flag on wasm)
 - #72891 (Add associated consts MIN/MAX for Wrapping<Int>)
 - #72893 (test miri-unleash TLS accesses)

Failed merges:

r? @ghost

README.md

@@ -23,7 +23,7 @@ or reading the [rustc dev guide][rustcguidebuild].
 
 [rustcguidebuild]: https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html
 
-### Building on *nix
+### Building on Unix-like system
 1. Make sure you have installed the dependencies:
 
    * `g++` 5.1 or later or `clang++` 3.5 or later

src/libcore/macros/mod.rs

@@ -1315,7 +1315,7 @@ pub(crate) mod builtin {
     #[unstable(
         feature = "llvm_asm",
         issue = "70173",
-        reason = "LLVM-style inline assembly will never be stabilized, prefer using asm! instead"
+        reason = "prefer using the new asm! syntax instead"
     )]
     #[rustc_builtin_macro]
     #[macro_export]

src/libcore/mem/maybe_uninit.rs

@@ -214,7 +214,6 @@ use crate::mem::ManuallyDrop;
 /// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has
 /// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that
 /// guarantee may evolve.
-#[allow(missing_debug_implementations)]
 #[stable(feature = "maybe_uninit", since = "1.36.0")]
 // Lang item so we can wrap other types in it. This is useful for generators.
 #[lang = "maybe_uninit"]

src/libcore/num/wrapping.rs

@@ -337,14 +337,10 @@ Basic usage:
 #![feature(wrapping_int_impl)]
 use std::num::Wrapping;
 
-assert_eq!(<Wrapping<", stringify!($t), ">>::min_value(), ",
+assert_eq!(<Wrapping<", stringify!($t), ">>::MIN, Wrapping(", stringify!($t), "::MIN));
-"Wrapping(", stringify!($t), "::min_value()));
 ```"),
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                #[inline]
+                pub const MIN: Self = Self(<$t>::MIN);
-                pub const fn min_value() -> Self {
-                    Wrapping(<$t>::min_value())
-                }
             }
 
             doc_comment! {
@@ -358,14 +354,10 @@ Basic usage:
 #![feature(wrapping_int_impl)]
 use std::num::Wrapping;
 
-assert_eq!(<Wrapping<", stringify!($t), ">>::max_value(), ",
+assert_eq!(<Wrapping<", stringify!($t), ">>::MAX, Wrapping(", stringify!($t), "::MAX));
-"Wrapping(", stringify!($t), "::max_value()));
 ```"),
                 #[unstable(feature = "wrapping_int_impl", issue = "32463")]
-                #[inline]
+                pub const MAX: Self = Self(<$t>::MAX);
-                pub const fn max_value() -> Self {
-                    Wrapping(<$t>::max_value())
-                }
             }
 
             doc_comment! {

src/librustc_builtin_macros/asm.rs

@@ -33,7 +33,10 @@ fn parse_args<'a>(
 
     // Detect use of the legacy llvm_asm! syntax (which used to be called asm!)
     if p.look_ahead(1, |t| *t == token::Colon || *t == token::ModSep) {
-        let mut err = ecx.struct_span_err(sp, "legacy asm! syntax is no longer supported");
+        let mut err =
+            ecx.struct_span_err(sp, "the legacy LLVM-style asm! syntax is no longer supported");
+        err.note("consider migrating to the new asm! syntax specified in RFC 2873");
+        err.note("alternatively, switch to llvm_asm! to keep your code working as it is");
 
         // Find the span of the "asm!" so that we can offer an automatic suggestion
         let asm_span = sp.from_inner(InnerSpan::new(0, 4));

src/librustc_codegen_ssa/back/linker.rs

@@ -1010,9 +1010,6 @@ impl<'a> WasmLd<'a> {
         //   sharing memory and instantiating the module multiple times. As a
         //   result if it were exported then we'd just have no sharing.
         //
-        // * `--passive-segments` - all memory segments should be passive to
-        //   prevent each module instantiation from reinitializing memory.
-        //
         // * `--export=__wasm_init_memory` - when using `--passive-segments` the
         //   linker will synthesize this function, and so we need to make sure
         //   that our usage of `--export` below won't accidentally cause this
@@ -1026,7 +1023,6 @@ impl<'a> WasmLd<'a> {
             cmd.arg("--shared-memory");
             cmd.arg("--max-memory=1073741824");
             cmd.arg("--import-memory");
-            cmd.arg("--passive-segments");
             cmd.arg("--export=__wasm_init_memory");
             cmd.arg("--export=__wasm_init_tls");
             cmd.arg("--export=__tls_size");

src/librustc_error_codes/error_codes/E0637.md

@@ -1,6 +1,7 @@
 An underscore `_` character has been used as the identifier for a lifetime.
 
-Erroneous example:
+Erroneous code example:
+
 ```compile_fail,E0106,E0637
 fn longest<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
          //^^ `'_` is a reserved lifetime name
@@ -11,6 +12,7 @@ fn longest<'_>(str1: &'_ str, str2: &'_ str) -> &'_ str {
     }
 }
 ```
+
 `'_`, cannot be used as a lifetime identifier because it is a reserved for the
 anonymous lifetime. To fix this, use a lowercase letter such as 'a, or a series
 of lowercase letters such as `'foo`.  For more information, see [the
@@ -18,6 +20,7 @@ book][bk-no].  For more information on using the anonymous lifetime in rust
 nightly, see [the nightly book][bk-al].
 
 Corrected example:
+
 ```
 fn longest<'a>(str1: &'a str, str2: &'a str) -> &'a str {
     if str1.len() > str2.len() {

src/librustc_middle/mir/interpret/error.rs

@@ -523,12 +523,12 @@ impl fmt::Display for UnsupportedOpInfo {
         match self {
             Unsupported(ref msg) => write!(f, "{}", msg),
             ReadForeignStatic(did) => {
-                write!(f, "cannot read from foreign (extern) static {:?}", did)
+                write!(f, "cannot read from foreign (extern) static ({:?})", did)
             }
             NoMirFor(did) => write!(f, "no MIR body is available for {:?}", did),
             ReadPointerAsBytes => write!(f, "unable to turn pointer into raw bytes",),
             ReadBytesAsPointer => write!(f, "unable to turn bytes into a pointer"),
-            ThreadLocalStatic(did) => write!(f, "accessing thread local static {:?}", did),
+            ThreadLocalStatic(did) => write!(f, "cannot access thread local static ({:?})", did),
         }
     }
 }

src/librustc_middle/ty/structural_impls.rs

@@ -1019,7 +1019,11 @@ impl<'tcx> TypeFoldable<'tcx> for &'tcx ty::Const<'tcx> {
     fn super_fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {
         let ty = self.ty.fold_with(folder);
         let val = self.val.fold_with(folder);
-        folder.tcx().mk_const(ty::Const { ty, val })
+        if ty != self.ty || val != self.val {
+            folder.tcx().mk_const(ty::Const { ty, val })
+        } else {
+            *self
+        }
     }
 
     fn fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self {

src/librustc_mir/transform/check_consts/ops.rs

@@ -12,9 +12,6 @@ use super::ConstCx;
 
 /// An operation that is not *always* allowed in a const context.
 pub trait NonConstOp: std::fmt::Debug {
-    /// Whether this operation can be evaluated by miri.
-    const IS_SUPPORTED_IN_MIRI: bool = true;
-
     /// Returns the `Symbol` corresponding to the feature gate that would enable this operation,
     /// or `None` if such a feature gate does not exist.
     fn feature_gate() -> Option<Symbol> {
@@ -356,8 +353,6 @@ impl NonConstOp for StaticAccess {
 #[derive(Debug)]
 pub struct ThreadLocalAccess;
 impl NonConstOp for ThreadLocalAccess {
-    const IS_SUPPORTED_IN_MIRI: bool = false;
-
     fn emit_error(&self, ccx: &ConstCx<'_, '_>, span: Span) {
         struct_span_err!(
             ccx.tcx.sess,

src/librustc_mir/transform/check_consts/validation.rs

@@ -244,11 +244,7 @@ impl Validator<'mir, 'tcx> {
             return;
         }
 
-        // If an operation is supported in miri it can be turned on with
+        if self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
-        // `-Zunleash-the-miri-inside-of-you`.
-        let is_unleashable = O::IS_SUPPORTED_IN_MIRI;
-
-        if is_unleashable && self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
             self.tcx.sess.miri_unleashed_feature(span, O::feature_gate());
             return;
         }

src/librustc_trait_selection/traits/error_reporting/suggestions.rs

@@ -1909,6 +1909,12 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 
                 let self_ty = self.resolve_vars_if_possible(&trait_ref.self_ty());
 
+                // Do not check on infer_types to avoid panic in evaluate_obligation.
+                if self_ty.has_infer_types() {
+                    return;
+                }
+                let self_ty = self.tcx.erase_regions(&self_ty);
+
                 let impls_future = self.tcx.type_implements_trait((
                     future_trait,
                     self_ty,

src/librustc_trait_selection/traits/mod.rs

@@ -540,13 +540,6 @@ fn type_implements_trait<'tcx>(
         trait_def_id, ty, params, param_env
     );
 
-    // Do not check on infer_types to avoid panic in evaluate_obligation.
-    if ty.has_infer_types() {
-        return false;
-    }
-
-    let ty = tcx.erase_regions(&ty);
-
     let trait_ref = ty::TraitRef { def_id: trait_def_id, substs: tcx.mk_substs_trait(ty, params) };
 
     let obligation = Obligation {

src/test/ui/asm/rustfix-asm.fixed

@@ -8,9 +8,9 @@ fn main() {
         let x = 1;
         let y: i32;
         llvm_asm!("" :: "r" (x));
-        //~^ ERROR legacy asm! syntax is no longer supported
+        //~^ ERROR the legacy LLVM-style asm! syntax is no longer supported
         llvm_asm!("" : "=r" (y));
-        //~^ ERROR legacy asm! syntax is no longer supported
+        //~^ ERROR the legacy LLVM-style asm! syntax is no longer supported
         let _ = y;
     }
 }

src/test/ui/asm/rustfix-asm.rs

@@ -8,9 +8,9 @@ fn main() {
         let x = 1;
         let y: i32;
         asm!("" :: "r" (x));
-        //~^ ERROR legacy asm! syntax is no longer supported
+        //~^ ERROR the legacy LLVM-style asm! syntax is no longer supported
         asm!("" : "=r" (y));
-        //~^ ERROR legacy asm! syntax is no longer supported
+        //~^ ERROR the legacy LLVM-style asm! syntax is no longer supported
         let _ = y;
     }
 }

src/test/ui/asm/rustfix-asm.stderr

@@ -1,18 +1,24 @@
-error: legacy asm! syntax is no longer supported
+error: the legacy LLVM-style asm! syntax is no longer supported
   --> $DIR/rustfix-asm.rs:10:9
    |
 LL |         asm!("" :: "r" (x));
    |         ----^^^^^^^^^^^^^^^^
    |         |
    |         help: replace with: `llvm_asm!`
+   |
+   = note: consider migrating to the new asm! syntax specified in RFC 2873
+   = note: alternatively, switch to llvm_asm! to keep your code working as it is
 
-error: legacy asm! syntax is no longer supported
+error: the legacy LLVM-style asm! syntax is no longer supported
   --> $DIR/rustfix-asm.rs:12:9
    |
 LL |         asm!("" : "=r" (y));
    |         ----^^^^^^^^^^^^^^^^
    |         |
    |         help: replace with: `llvm_asm!`
+   |
+   = note: consider migrating to the new asm! syntax specified in RFC 2873
+   = note: alternatively, switch to llvm_asm! to keep your code working as it is
 
 error: aborting due to 2 previous errors
 

src/test/ui/consts/miri_unleashed/inline_asm.rs

@@ -1,15 +1,22 @@
 // compile-flags: -Zunleash-the-miri-inside-of-you
 // only-x86_64
-#![feature(llvm_asm)]
+#![feature(asm,llvm_asm)]
 #![allow(const_err)]
 
 fn main() {}
 
 // Make sure we catch executing inline assembly.
-static TEST_BAD: () = {
+static TEST_BAD1: () = {
     unsafe { llvm_asm!("xor %eax, %eax" ::: "eax"); }
     //~^ ERROR could not evaluate static initializer
     //~| NOTE inline assembly is not supported
     //~| NOTE in this expansion of llvm_asm!
     //~| NOTE in this expansion of llvm_asm!
 };
+
+// Make sure we catch executing inline assembly.
+static TEST_BAD2: () = {
+    unsafe { asm!("nop"); }
+    //~^ ERROR could not evaluate static initializer
+    //~| NOTE inline assembly is not supported
+};

src/test/ui/consts/miri_unleashed/inline_asm.stderr

@@ -6,6 +6,12 @@ LL |     unsafe { llvm_asm!("xor %eax, %eax" ::: "eax"); }
    |
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
+error[E0080]: could not evaluate static initializer
+  --> $DIR/inline_asm.rs:19:14
+   |
+LL |     unsafe { asm!("nop"); }
+   |              ^^^^^^^^^^^^ inline assembly is not supported
+
 warning: skipping const checks
    |
 help: skipping check that does not even have a feature gate
@@ -13,8 +19,13 @@ help: skipping check that does not even have a feature gate
    |
 LL |     unsafe { llvm_asm!("xor %eax, %eax" ::: "eax"); }
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: skipping check that does not even have a feature gate
+  --> $DIR/inline_asm.rs:19:14
+   |
+LL |     unsafe { asm!("nop"); }
+   |              ^^^^^^^^^^^^
    = note: this warning originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to previous error; 1 warning emitted
+error: aborting due to 2 previous errors; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0080`.

src/test/ui/consts/miri_unleashed/tls.rs

@@ -0,0 +1,17 @@
+// compile-flags: -Zunleash-the-miri-inside-of-you
+#![feature(thread_local)]
+#![allow(const_err)]
+
+use std::thread;
+
+#[thread_local]
+static A: u8 = 0;
+
+// Make sure we catch accessing thread-local storage.
+static TEST_BAD: () = {
+    unsafe { let _val = A; }
+    //~^ ERROR could not evaluate static initializer
+    //~| NOTE cannot access thread local static
+};
+
+fn main() {}

src/test/ui/consts/miri_unleashed/tls.stderr

@@ -0,0 +1,17 @@
+error[E0080]: could not evaluate static initializer
+  --> $DIR/tls.rs:12:25
+   |
+LL |     unsafe { let _val = A; }
+   |                         ^ cannot access thread local static (DefId(0:4 ~ tls[317d]::A[0]))
+
+warning: skipping const checks
+   |
+help: skipping check that does not even have a feature gate
+  --> $DIR/tls.rs:12:25
+   |
+LL |     unsafe { let _val = A; }
+   |                         ^
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0080`.

src/test/ui/feature-gates/feature-gate-asm.rs

@@ -5,6 +5,6 @@ fn main() {
         asm!("");
         //~^ ERROR inline assembly is not stable enough
         llvm_asm!("");
-        //~^ ERROR LLVM-style inline assembly will never be stabilized
+        //~^ ERROR prefer using the new asm! syntax instead
     }
 }

src/test/ui/feature-gates/feature-gate-asm.stderr

@@ -7,7 +7,7 @@ LL |         asm!("");
    = note: see issue #72016 <https://github.com/rust-lang/rust/issues/72016> for more information
    = help: add `#![feature(asm)]` to the crate attributes to enable
 
-error[E0658]: use of unstable library feature 'llvm_asm': LLVM-style inline assembly will never be stabilized, prefer using asm! instead
+error[E0658]: use of unstable library feature 'llvm_asm': prefer using the new asm! syntax instead
   --> $DIR/feature-gate-asm.rs:7:9
    |
 LL |         llvm_asm!("");

src/test/ui/feature-gates/feature-gate-asm2.rs

@@ -5,6 +5,6 @@ fn main() {
         println!("{:?}", asm!(""));
         //~^ ERROR inline assembly is not stable enough
         println!("{:?}", llvm_asm!(""));
-        //~^ ERROR LLVM-style inline assembly will never be stabilized
+        //~^ ERROR prefer using the new asm! syntax instead
     }
 }

src/test/ui/feature-gates/feature-gate-asm2.stderr

@@ -7,7 +7,7 @@ LL |         println!("{:?}", asm!(""));
    = note: see issue #72016 <https://github.com/rust-lang/rust/issues/72016> for more information
    = help: add `#![feature(asm)]` to the crate attributes to enable
 
-error[E0658]: use of unstable library feature 'llvm_asm': LLVM-style inline assembly will never be stabilized, prefer using asm! instead
+error[E0658]: use of unstable library feature 'llvm_asm': prefer using the new asm! syntax instead
   --> $DIR/feature-gate-asm2.rs:7:26
    |
 LL |         println!("{:?}", llvm_asm!(""));

src/test/ui/proc-macro/auxiliary/test-macros.rs

@@ -108,5 +108,6 @@ pub fn print_attr(_: TokenStream, input: TokenStream) -> TokenStream {
 
 #[proc_macro_derive(Print, attributes(print_helper))]
 pub fn print_derive(input: TokenStream) -> TokenStream {
-    print_helper(input, "DERIVE")
+    print_helper(input, "DERIVE");
+    TokenStream::new()
 }

src/test/ui/proc-macro/dollar-crate.rs

@@ -1,3 +1,4 @@
+// check-pass
 // edition:2018
 // aux-build:test-macros.rs
 // aux-build:dollar-crate-external.rs
@@ -23,7 +24,7 @@ mod local {
             struct A($crate::S);
 
             #[derive(Print)]
-            struct D($crate::S); //~ ERROR the name `D` is defined multiple times
+            struct D($crate::S);
         };
     }
 
@@ -33,7 +34,7 @@ mod local {
 mod external {
     use crate::dollar_crate_external;
 
-    dollar_crate_external::external!(); //~ ERROR the name `D` is defined multiple times
+    dollar_crate_external::external!();
 }
 
 fn main() {}

src/test/ui/proc-macro/dollar-crate.stderr

@@ -1,30 +0,0 @@
-error[E0428]: the name `D` is defined multiple times
-  --> $DIR/dollar-crate.rs:26:13
-   |
-LL |             struct D($crate::S);
-   |             ^^^^^^^^^^^^^^^^^^^^
-   |             |
-   |             `D` redefined here
-   |             previous definition of the type `D` here
-...
-LL |     local!();
-   |     --------- in this macro invocation
-   |
-   = note: `D` must be defined only once in the type namespace of this module
-   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error[E0428]: the name `D` is defined multiple times
-  --> $DIR/dollar-crate.rs:36:5
-   |
-LL |     dollar_crate_external::external!();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |     |
-   |     `D` redefined here
-   |     previous definition of the type `D` here
-   |
-   = note: `D` must be defined only once in the type namespace of this module
-   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0428`.

src/test/ui/proc-macro/input-interpolated.rs

@@ -0,0 +1,25 @@
+// Check what token streams proc macros see when interpolated tokens are passed to them as input.
+
+// check-pass
+// aux-build:test-macros.rs
+
+#[macro_use]
+extern crate test_macros;
+
+macro_rules! pass_ident {
+    ($i:ident) => {
+        fn f() {
+            print_bang!($i);
+        }
+
+        #[print_attr]
+        const $i: u8 = 0;
+
+        #[derive(Print)]
+        struct $i {}
+    };
+}
+
+pass_ident!(A);
+
+fn main() {}

src/test/ui/proc-macro/input-interpolated.stdout

@@ -0,0 +1,69 @@
+PRINT-BANG INPUT (DISPLAY): A
+PRINT-BANG RE-COLLECTED (DISPLAY):  A 
+PRINT-BANG INPUT (DEBUG): TokenStream [
+    Group {
+        delimiter: None,
+        stream: TokenStream [
+            Ident {
+                ident: "A",
+                span: #0 bytes(402..403),
+            },
+        ],
+        span: #3 bytes(269..271),
+    },
+]
+PRINT-ATTR INPUT (DISPLAY): const A: u8 = 0;
+PRINT-ATTR RE-COLLECTED (DISPLAY): const A : u8 = 0 ;
+PRINT-ATTR INPUT (DEBUG): TokenStream [
+    Ident {
+        ident: "const",
+        span: #0 bytes(0..0),
+    },
+    Ident {
+        ident: "A",
+        span: #0 bytes(0..0),
+    },
+    Punct {
+        ch: ':',
+        spacing: Alone,
+        span: #0 bytes(0..0),
+    },
+    Ident {
+        ident: "u8",
+        span: #0 bytes(0..0),
+    },
+    Punct {
+        ch: '=',
+        spacing: Alone,
+        span: #0 bytes(0..0),
+    },
+    Literal {
+        kind: Integer,
+        symbol: "0",
+        suffix: None,
+        span: #0 bytes(0..0),
+    },
+    Punct {
+        ch: ';',
+        spacing: Alone,
+        span: #0 bytes(0..0),
+    },
+]
+PRINT-DERIVE INPUT (DISPLAY): struct A {
+}
+PRINT-DERIVE RE-COLLECTED (DISPLAY): struct A { }
+PRINT-DERIVE INPUT (DEBUG): TokenStream [
+    Ident {
+        ident: "struct",
+        span: #0 bytes(0..0),
+    },
+    Ident {
+        ident: "A",
+        span: #0 bytes(0..0),
+    },
+    Group {
+        delimiter: Brace,
+        stream: TokenStream [],
+        span: #0 bytes(0..0),
+    },
+]

src/test/ui/suggestions/issue-72766.rs

@@ -0,0 +1,20 @@
+// edition:2018
+// compile-flags: -Cincremental=tmp/issue-72766
+
+pub struct SadGirl;
+
+impl SadGirl {
+    pub async fn call(&self) -> Result<(), ()> {
+        Ok(())
+    }
+}
+
+async fn async_main() -> Result<(), ()> {
+    // should be `.call().await?`
+    SadGirl {}.call()?; //~ ERROR: the `?` operator can only be applied to values
+    Ok(())
+}
+
+fn main() {
+    let _ = async_main();
+}

src/test/ui/suggestions/issue-72766.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/issue-72766.rs:14:5
+   |
+LL |     SadGirl {}.call()?;
+   |     ^^^^^^^^^^^^^^^^^^
+   |     |
+   |     the `?` operator cannot be applied to type `impl std::future::Future`
+   |     help: consider using `.await` here: `SadGirl {}.call().await?`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `impl std::future::Future`
+   = note: required by `std::ops::Try::into_result`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/tools/clippy/clippy_lints/src/utils/mod.rs

@@ -323,6 +323,11 @@ pub fn implements_trait<'a, 'tcx>(
     trait_id: DefId,
     ty_params: &[GenericArg<'tcx>],
 ) -> bool {
+    // Do not check on infer_types to avoid panic in evaluate_obligation.
+    if ty.has_infer_types() {
+        return false;
+    }
+    let ty = cx.tcx.erase_regions(&ty);
     let ty_params = cx.tcx.mk_substs(ty_params.iter());
     cx.tcx.type_implements_trait((trait_id, ty, ty_params, cx.param_env))
 }