More informative error message for E0015

[fee1-dead]

Helps with #92380

[rust-highfive]

r? @cjgillot

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[fee1-dead]

r? @oli-obk

[fee1-dead]

cc @rust-lang/wg-const-eval

[oli-obk]

It is a bit surprising to users to see things that aren't mentioned in the code, can we get a span that knows that it's expanded code?

[bors]

☔ The latest upstream changes (presumably #91656) made this pull request unmergeable. Please resolve the merge conflicts.

[fee1-dead]

Note: This is still not ready for review, but I added a lot of cases for desugarings which can help the users. Examples for the current change:

error[E0015]: cannot call non-const fn `<[closure@$DIR/issue-28113.rs:4:5: 4:19] as Fn<()>>::call` in constants
  --> $DIR/issue-28113.rs:4:5
   |
LL |     || -> u8 { 5 }()
   |     ^^^^^^^^^^^^^^^^
   |
   = note: calls in constants are limited to constant functions, tuple structs and tuple variants

error[E0015]: cannot call non-const fn `Foo::{constant#0}::Foo::<17_usize>::value` in constants
  --> $DIR/nested-type.rs:15:5
   |
LL |     Foo::<17>::value()
   |     ^^^^^^^^^^^^^^^^^^
   |
   = note: calls in constants are limited to constant functions, tuple structs and tuple variants

error[E0015]: cannot perform deref coercion on `A` in statics
  --> $DIR/issue-25901.rs:4:24
   |
LL | static S: &'static B = &A;
   |                        ^^
   |
   = note: attempting to deref into `B`
note: deref defined here
  --> $DIR/issue-25901.rs:10:5
   |
LL |     type Target = B;
   |     ^^^^^^^^^^^^^^^^
note: impl defined here, but it is not `const`
  --> $DIR/issue-25901.rs:9:1
   |
LL | impl Deref for A {
   | ^^^^^^^^^^^^^^^^
   = note: calls in statics are limited to constant functions, tuple structs and tuple variants

error[E0015]: cannot convert `std::ops::Range<i32>` into an iterator in constants
  --> $DIR/const-for.rs:5:14
   |
LL |     for _ in 0..5 {}
   |              ^^^^
   |
note: impl defined here, but it is not `const`
  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
   |
LL | impl<I: Iterator> IntoIterator for I {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: calls in constants are limited to constant functions, tuple structs and tuple variants

error[E0015]: cannot call non-const fn `<std::ops::Range<i32> as Iterator>::next` in constants
  --> $DIR/const-for.rs:5:14
   |
LL |     for _ in 0..5 {}
   |              ^^^^
   |
   = note: calls in constants are limited to constant functions, tuple structs and tuple variants

error[E0015]: cannot call non-const operator in constant functions
  --> $DIR/call-generic-method-fail.rs:5:5
   |
LL |     *t == *t
   |     ^^^^^^^^
   |
   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
help: consider further restricting this bound
   |
LL | pub const fn equals_self<T: PartialEq + ~const std::cmp::PartialEq>(t: &T) -> bool {
   |                                       ++++++++++++++++++++++++++++

[oli-obk]

@bors r+

[bors]

📌 Commit ea0f26a has been approved by oli-obk

[fee1-dead]

@bors r-

failed in a rollup

[fee1-dead]

@oli-obk: please review the two new commits. I improved diagnostics even further by reporting the selection error.

@rustbot ready

[oli-obk]

Oh that is a great idea and looks great!

@bors r+

[fee1-dead]

@rustbot ready

[bors]

☔ The latest upstream changes (presumably #93691) made this pull request unmergeable. Please resolve the merge conflicts.

[oli-obk]

r=me after a rebase

[fee1-dead]

@bors r=oli-obk

[bors]

📌 Commit 88d433e has been approved by oli-obk