Rollup merge of #121595 - strottos:issue_116615, r=compiler-errors

Better reporting on generic argument mismatchs

This allows better reporting as per issue #116615 .

If you have a function:
```
fn foo(a: T, b: T) {}
```
and call it like so:
```
foo(1, 2.)
```
it'll give improved error reported similar to the following:
```
error[E0308]: mismatched types
 --> generic-mismatch-reporting-issue-116615.rs:6:12
  |
6 |     foo(1, 2.);
  |     --- -  ^^ expected integer, found floating-point number
  |     |   |
  |     |   expected argument `b` to be an integer because that argument needs to match the type of this parameter
  |     arguments to this function are incorrect
  |
note: function defined here
 --> generic-mismatch-reporting-issue-116615.rs:1:4
  |
1 | fn foo<T>(a: T, b: T) {}
  |    ^^^ -  ----  ----
  |        |  |     |
  |        |  |     this parameter needs to match the integer type of `a`
  |        |  `b` needs to match the type of this parameter
  |        `a` and `b` all reference this parameter T
```

Open question, do we need to worry about error message translation into other languages? Not sure what the status of that is in Rust.

NB: Needs some checking over and some tests have altered that need sanity checking, but overall this is starting to get somewhere now. Will take out of draft PR status when this has been done, raising now to allow feedback at this stage, probably 90% ready.

compiler/rustc_errors/src/lib.rs

@@ -1951,6 +1951,39 @@ pub fn report_ambiguity_error<'a, G: EmissionGuarantee>(
     }
 }
 
+/// Grammatical tool for displaying messages to end users in a nice form.
+///
+/// Returns "an" if the given string starts with a vowel, and "a" otherwise.
+pub fn a_or_an(s: &str) -> &'static str {
+    let mut chars = s.chars();
+    let Some(mut first_alpha_char) = chars.next() else {
+        return "a";
+    };
+    if first_alpha_char == '`' {
+        let Some(next) = chars.next() else {
+            return "a";
+        };
+        first_alpha_char = next;
+    }
+    if ["a", "e", "i", "o", "u", "&"].contains(&&first_alpha_char.to_lowercase().to_string()[..]) {
+        "an"
+    } else {
+        "a"
+    }
+}
+
+/// Grammatical tool for displaying messages to end users in a nice form.
+///
+/// Take a list ["a", "b", "c"] and output a display friendly version "a, b and c"
+pub fn display_list_with_comma_and<T: std::fmt::Display>(v: &[T]) -> String {
+    match v.len() {
+        0 => "".to_string(),
+        1 => v[0].to_string(),
+        2 => format!("{} and {}", v[0], v[1]),
+        _ => format!("{}, {}", v[0], display_list_with_comma_and(&v[1..])),
+    }
+}
+
 #[derive(Clone, Copy, PartialEq, Hash, Debug)]
 pub enum TerminalUrl {
     No,