A bit more polish on const eval errors

compiler/rustc_mir/src/const_eval/eval_queries.rs

@@ -16,6 +16,7 @@ use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::{self, subst::Subst, TyCtxt};
 use rustc_span::source_map::Span;
 use rustc_target::abi::{Abi, LayoutOf};
+use std::borrow::Cow;
 use std::convert::TryInto;
 
 pub fn note_on_undefined_behavior_error() -> &'static str {
@@ -328,11 +329,22 @@ pub fn eval_to_allocation_raw_provider<'tcx>(
                 ))
             } else {
                 let msg = if is_static {
-                    "could not evaluate static initializer"
+                    Cow::from("could not evaluate static initializer")
                 } else {
-                    "evaluation of constant value failed"
+                    // If the current item has generics, we'd like to enrich the message with the
+                    // instance and its substs: to show the actual compile-time values, in addition to
+                    // the expression, leading to the const eval error.
+                    let instance = &key.value.instance;
+                    if !instance.substs.is_empty() {
+                        let instance = with_no_trimmed_paths(|| instance.to_string());
+                        let msg = format!("evaluation of `{}` failed", instance);
+                        Cow::from(msg)
+                    } else {
+                        Cow::from("evaluation of constant value failed")
+                    }
                 };
-                Err(err.report_as_error(ecx.tcx.at(ecx.cur_span()), msg))
+
+                Err(err.report_as_error(ecx.tcx.at(ecx.cur_span()), &msg))
             }
         }
         Ok(mplace) => {

compiler/rustc_mir/src/monomorphize/collector.rs

@@ -194,6 +194,7 @@ use rustc_middle::mir::mono::{InstantiationMode, MonoItem};
 use rustc_middle::mir::visit::Visitor as MirVisitor;
 use rustc_middle::mir::{self, Local, Location};
 use rustc_middle::ty::adjustment::{CustomCoerceUnsized, PointerCast};
+use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::subst::{GenericArgKind, InternalSubsts};
 use rustc_middle::ty::{self, GenericParamDefKind, Instance, Ty, TyCtxt, TypeFoldable};
 use rustc_middle::{middle::codegen_fn_attrs::CodegenFnAttrFlags, mir::visit::TyContext};
@@ -444,12 +445,10 @@ fn collect_items_rec<'tcx>(
     // defined in the local crate.
     if tcx.sess.diagnostic().err_count() > error_count && starting_point.node.krate() != LOCAL_CRATE
     {
+        let formatted_item = with_no_trimmed_paths(|| starting_point.node.to_string());
         tcx.sess.span_note_without_error(
             starting_point.span,
-            &format!(
-                "the above error was encountered while instantiating `{}`",
-                starting_point.node
-            ),
+            &format!("the above error was encountered while instantiating `{}`", formatted_item),
         );
     }
 

src/test/ui/const-generics/const_evaluatable_checked/from-sig-fail.rs

@@ -2,7 +2,7 @@
 #![allow(incomplete_features)]
 
 fn test<const N: usize>() -> [u8; N - 1] {
-    //~^ ERROR evaluation of constant
+    //~^ ERROR evaluation of `test::<0_usize>::{constant#0}` failed
     todo!()
 }
 

src/test/ui/const-generics/const_evaluatable_checked/from-sig-fail.stderr

@@ -1,4 +1,4 @@
-error[E0080]: evaluation of constant value failed
+error[E0080]: evaluation of `test::<0_usize>::{constant#0}` failed
   --> $DIR/from-sig-fail.rs:4:35
    |
 LL | fn test<const N: usize>() -> [u8; N - 1] {

src/test/ui/const-generics/const_evaluatable_checked/simple_fail.full.stderr

@@ -1,10 +1,10 @@
-error[E0080]: evaluation of constant value failed
-  --> $DIR/simple_fail.rs:9:48
+error[E0080]: evaluation of `test::<0_usize>::{constant#0}` failed
+  --> $DIR/simple_fail.rs:10:48
    |
 LL | fn test<const N: usize>() -> Arr<N> where [u8; N - 1]: Sized {
    |                                                ^^^^^ attempt to compute `0_usize - 1_usize`, which would overflow
 
-error[E0080]: evaluation of constant value failed
+error[E0080]: evaluation of `Arr::<0_usize>::{constant#0}` failed
   --> $DIR/simple_fail.rs:6:33
    |
 LL | type Arr<const N: usize> = [u8; N - 1];

src/test/ui/const-generics/const_evaluatable_checked/simple_fail.min.stderr

@@ -8,7 +8,7 @@ LL | type Arr<const N: usize> = [u8; N - 1];
    = help: use `#![feature(const_generics)]` and `#![feature(const_evaluatable_checked)]` to allow generic const expressions
 
 error: generic parameters may not be used in const operations
-  --> $DIR/simple_fail.rs:9:48
+  --> $DIR/simple_fail.rs:10:48
    |
 LL | fn test<const N: usize>() -> Arr<N> where [u8; N - 1]: Sized {
    |                                                ^ cannot perform const operation using `N`

src/test/ui/const-generics/const_evaluatable_checked/simple_fail.rs

@@ -3,12 +3,13 @@
 #![cfg_attr(full, feature(const_evaluatable_checked))]
 #![allow(incomplete_features)]
 
-type Arr<const N: usize> = [u8; N - 1]; //[full]~ ERROR evaluation of constant
+type Arr<const N: usize> = [u8; N - 1];
 //[min]~^ ERROR generic parameters may not be used in const operations
+//[full]~^^ ERROR evaluation of `Arr::<0_usize>::{constant#0}` failed
 
 fn test<const N: usize>() -> Arr<N> where [u8; N - 1]: Sized {
 //[min]~^ ERROR generic parameters may not be used in const operations
-//[full]~^^ ERROR evaluation of constant
+//[full]~^^ ERROR evaluation of `test::<0_usize>::{constant#0}` failed
     todo!()
 }
 

src/test/ui/consts/const-eval/issue-50814-2.stderr

@@ -10,7 +10,7 @@ LL |     const BAR: usize = [5, 6, 7][T::BOO];
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
-error[E0080]: evaluation of constant value failed
+error[E0080]: evaluation of `foo::<()>` failed
   --> $DIR/issue-50814-2.rs:19:6
    |
 LL |     &<A<T> as Foo<T>>::BAR

src/test/ui/consts/const-eval/issue-50814.stderr

@@ -10,7 +10,7 @@ LL |     const MAX: u8 = A::MAX + B::MAX;
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
-error[E0080]: evaluation of constant value failed
+error[E0080]: evaluation of `foo::<i32>` failed
   --> $DIR/issue-50814.rs:21:6
    |
 LL |     &Sum::<U8,U8>::MAX

src/test/ui/consts/const-eval/issue-85155.stderr

@@ -1,10 +1,10 @@
-error[E0080]: evaluation of constant value failed
+error[E0080]: evaluation of `post_monomorphization_error::ValidateConstImm::<2_i32, 0_i32, 1_i32>::VALID` failed
   --> $DIR/auxiliary/post_monomorphization_error.rs:7:17
    |
 LL |         let _ = 1 / ((IMM >= MIN && IMM <= MAX) as usize);
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ attempt to divide `1_usize` by zero
 
-note: the above error was encountered while instantiating `fn stdarch_intrinsic::<2_i32>`
+note: the above error was encountered while instantiating `fn post_monomorphization_error::stdarch_intrinsic::<2_i32>`
   --> $DIR/issue-85155.rs:19:5
    |
 LL |     post_monomorphization_error::stdarch_intrinsic::<2>();

src/test/ui/mir/issue-80742.stderr

@@ -1,4 +1,4 @@
-error[E0080]: evaluation of constant value failed
+error[E0080]: evaluation of `Inline::<dyn std::fmt::Debug>::{constant#0}` failed
   --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
    |
 LL |     intrinsics::size_of::<T>()
@@ -35,7 +35,7 @@ LL |   pub trait Debug {
    = note: the following trait bounds were not satisfied:
            `dyn Debug: Sized`
 
-error[E0080]: evaluation of constant value failed
+error[E0080]: evaluation of `Inline::<dyn std::fmt::Debug>::{constant#0}` failed
   --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
    |
 LL |     intrinsics::size_of::<T>()