`let_chains` and typo of `==` to `=` produce bad diagnostic

[estebank]

I wrote code like

                            let qself_res = if let hir::TyKind::Path(hir::QPath::Resolved(_, ref path)) = qself.kind 
                                && let [segment] = path.segments
                                && kw::SelfUpper = segment.ident.name
                            {

when I meant

                            let qself_res = if let hir::TyKind::Path(hir::QPath::Resolved(_, ref path)) = qself.kind 
                                && let [segment] = path.segments
                                && kw::SelfUpper == segment.ident.name
                            {

and was confused by the rust-analyzer and rustc output:

error: `let` expressions are not supported here
    --> compiler/rustc_typeck/src/astconv/mod.rs:1812:48
     |
1812 | ...                   let qself_res = if let hir::TyKind::Path(hir::QPath::Resolved(_, ref path)) = qself.kind 
     |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: only supported directly in conditions of `if` and `while` expressions
     = note: as well as when nested within `&&` and parentheses in those conditions

error: `let` expressions are not supported here
    --> compiler/rustc_typeck/src/astconv/mod.rs:1813:36
     |
1813 | ...                   && let [segment] = path.segments
     |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: only supported directly in conditions of `if` and `while` expressions
     = note: as well as when nested within `&&` and parentheses in those conditions

error[E0433]: failed to resolve: use of undeclared crate or module `kw`
    --> compiler/rustc_typeck/src/astconv/mod.rs:1814:36
     |
1814 | ...                   && kw::SelfUpper = segment.ident.name
     |                          ^^ use of undeclared crate or module `kw`

error[E0308]: mismatched types
    --> compiler/rustc_typeck/src/astconv/mod.rs:1812:48
     |
1812 |   ...                   let qself_res = if let hir::TyKind::Path(hir::QPath::Resolved(_, ref path)) = qself.kind 
     |  __________________________________________^
1813 | | ...                       && let [segment] = path.segments
1814 | | ...                       && kw::SelfUpper = segment.ident.name
     | |_______________________________________________________________^ expected `bool`, found `()`
     |
help: you might have meant to use pattern matching
     |
1812 |                             let qself_res = if let let hir::TyKind::Path(hir::QPath::Resolved(_, ref path)) = qself.kind 
     |                                                +++

[Lee-Janggun]

I can confirm the following code now produces the more helpful error:
https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=cc13cfcbb4f7aa698bec48eb1c63af52

#![feature(let_chains)]

fn temp(b1 : bool, b2 : bool, o1 : Option<usize>, o2 : Option<usize>) -> Option<usize> {
    let res = if let Some(u1) = o1 
                  && let Some(u2) = o2 
                  && b1 = b2
                {
                    Some(u1 + u2)
                } else {
                    None
                };
                    
    res
}

Compiling playground v0.0.1 (/playground)
error: `let` expressions are not supported here
 --> src/lib.rs:4:18
  |
4 |     let res = if let Some(u1) = o1 && let Some(u2) = o2 && b1 = b2
  |                  ^^^^^^^^^^^^^^^^^
  |
  = note: only supported directly in conditions of `if` and `while` expressions

error: `let` expressions are not supported here
 --> src/lib.rs:4:39
  |
4 |     let res = if let Some(u1) = o1 && let Some(u2) = o2 && b1 = b2
  |                                       ^^^^^^^^^^^^^^^^^
  |
  = note: only supported directly in conditions of `if` and `while` expressions

error[[E0308]](https://doc.rust-lang.org/nightly/error-index.html#E0308): mismatched types
 --> src/lib.rs:4:18
  |
4 |     let res = if let Some(u1) = o1 && let Some(u2) = o2 && b1 = b2
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `bool`, found `()`
  |
help: you might have meant to compare for equality
  |
4 |     let res = if let Some(u1) = o1 && let Some(u2) = o2 && b1 == b2
  |                                                                +

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to 3 previous errors