Improve handling of expressions in patterns

[ShE3py]

Closes #112593.

Methodcalls' dots in patterns are silently recovered as commas (e.g. Foo("".len()) -> Foo("", len())) so extra diagnostics are emitted:

struct Foo(u8, String, u8);

fn bar(foo: Foo) -> bool {
    match foo {
        Foo(4, "yippee".yeet(), 7) => true,
        _ => false
    }
}

error: expected one of `)`, `,`, `...`, `..=`, `..`, or `|`, found `.`
 --> main.rs:5:24
  |
5 |         Foo(4, "yippee".yeet(), 7) => true,
  |                        ^
  |                        |
  |                        expected one of `)`, `,`, `...`, `..=`, `..`, or `|`
  |                        help: missing `,`

error[E0531]: cannot find tuple struct or tuple variant `yeet` in this scope
 --> main.rs:5:25
  |
5 |         Foo(4, "yippee".yeet(), 7) => true,
  |                         ^^^^ not found in this scope

error[E0023]: this pattern has 4 fields, but the corresponding tuple struct has 3 fields
 --> main.rs:5:13
  |
1 | struct Foo(u8, String, u8);
  |            --  ------  -- tuple struct has 3 fields
...
5 |         Foo(4, "yippee".yeet(), 7) => true,
  |             ^  ^^^^^^^^ ^^^^^^  ^ expected 3 fields, found 4

error: aborting due to 3 previous errors

This PR checks for patterns that ends with a dot and a lowercase ident (as structs/variants should be uppercase):

error: expected a pattern, found a method call
 --> main.rs:5:16
  |
5 |         Foo(4, "yippee".yeet(), 7) => true,
  |                ^^^^^^^^^^^^^^^ method calls are not allowed in patterns

error: aborting due to 1 previous error

Also check for expressions:

fn is_idempotent(x: f32) -> bool {
    match x {
        x * x => true,
        _ => false,
    }
}

fn main() {
    let mut t: [i32; 5];
    let t[0] = 1;
}

error: expected a pattern, found an expression
 --> main.rs:3:9
  |
3 |         x * x => true,
  |         ^^^^^ arbitrary expressions are not allowed in patterns

error: expected a pattern, found an expression
  --> main.rs:10:9
   |
10 |     let t[0] = 1;
   |         ^^^^ arbitrary expressions are not allowed in patterns

Would be cool if the compiler could suggest adding a guard for matches, but I've no idea how to do it.

---

@rustbot label +A-diagnostics +A-parser +A-patterns +C-enhancement

[rustbot]

r? @compiler-errors

(rustbot has picked a reviewer for you, use r? to override)

[compiler-errors]

This is a draft so I'm marking it as waiting-on-author

@rustbot author

[bors]

☔ The latest upstream changes (presumably #118527) made this pull request unmergeable. Please resolve the merge conflicts.

[ShE3py]

@rustbot ready

[bors]

📌 Commit a13fbaa has been approved by WaffleLapkin

It is now in the queue for this repository.

[matthiaskrgr]

@bors r-
I suspect this needs another rebase :(
#120436

[ShE3py]

Blessed:

 error[E0658]: exclusive range pattern syntax is experimental
   --> $DIR/range_pat_interactions1.rs:30:17
    |
 LL |             y @ ..-7 => assert_eq!(y, -8),
    |                 ^^^^
    |
    = note: see issue #37854 <https://github.com/rust-lang/rust/issues/37854> for more information
    = help: add `#![feature(exclusive_range_pattern)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = help: use an inclusive range pattern, like N..=M

@rustbot ready

[WaffleLapkin]

@bors r+

[bors]

📌 Commit 6f014a8 has been approved by WaffleLapkin

It is now in the queue for this repository.