Rollup merge of #70725 - Centril:nix-unwraps, r=estebank

Avoid `.unwrap()`s on `.span_to_snippet(...)`s

First commit fixes https://github.com/rust-lang/rust/issues/70724 and the others fix similar issues found by grepping.

r? @estebank

src/test/ui/issues/issue-70724-add_type_neq_err_label-unwrap.rs

@@ -0,0 +1,10 @@
+fn a() -> i32 {
+    3
+}
+
+pub fn main() {
+    assert_eq!(a, 0);
+    //~^ ERROR binary operation `==` cannot
+    //~| ERROR mismatched types
+    //~| ERROR doesn't implement
+}

src/test/ui/issues/issue-70724-add_type_neq_err_label-unwrap.stderr

@@ -0,0 +1,41 @@
+error[E0369]: binary operation `==` cannot be applied to type `fn() -> i32 {a}`
+  --> $DIR/issue-70724-add_type_neq_err_label-unwrap.rs:6:5
+   |
+LL |     assert_eq!(a, 0);
+   |     ^^^^^^^^^^^^^^^^^
+   |     |
+   |     fn() -> i32 {a}
+   |     {integer}
+   |     help: you might have forgotten to call this function: `*left_val()`
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0308]: mismatched types
+  --> $DIR/issue-70724-add_type_neq_err_label-unwrap.rs:6:5
+   |
+LL |     assert_eq!(a, 0);
+   |     ^^^^^^^^^^^^^^^^^ expected fn item, found integer
+   |
+   = note: expected fn item `fn() -> i32 {a}`
+                 found type `i32`
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0277]: `fn() -> i32 {a}` doesn't implement `std::fmt::Debug`
+  --> $DIR/issue-70724-add_type_neq_err_label-unwrap.rs:6:5
+   |
+LL | fn a() -> i32 {
+   |    - consider calling this function
+...
+LL |     assert_eq!(a, 0);
+   |     ^^^^^^^^^^^^^^^^^ `fn() -> i32 {a}` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
+   |
+   = help: the trait `std::fmt::Debug` is not implemented for `fn() -> i32 {a}`
+   = help: use parentheses to call the function: `a()`
+   = note: required because of the requirements on the impl of `std::fmt::Debug` for `&fn() -> i32 {a}`
+   = note: required by `std::fmt::Debug::fmt`
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0277, E0308, E0369.
+For more information about an error, try `rustc --explain E0277`.