Rollup merge of rust-lang#100256 - camelid:typeck-ctxt-doc, r=compiler-errors

Add some high-level docs to `FnCtxt` and `ItemCtxt`

I haven't understood the difference between these before, but
``@compiler-errors`` helped me clear it up. Hopefully this will help other
people who've been confused!

r? `@compiler-errors`

Author: matthiaskrgr

compiler/rustc_typeck/src/check/fn_ctxt/mod.rs

@@ -26,6 +26,17 @@ use rustc_trait_selection::traits::{ObligationCause, ObligationCauseCode};
 use std::cell::{Cell, RefCell};
 use std::ops::Deref;
 
+/// The `FnCtxt` stores type-checking context needed to type-check bodies of
+/// functions, closures, and `const`s, including performing type inference
+/// with [`InferCtxt`].
+///
+/// This is in contrast to [`ItemCtxt`], which is used to type-check item *signatures*
+/// and thus does not perform type inference.
+///
+/// See [`ItemCtxt`]'s docs for more.
+///
+/// [`ItemCtxt`]: crate::collect::ItemCtxt
+/// [`InferCtxt`]: infer::InferCtxt
 pub struct FnCtxt<'a, 'tcx> {
     pub(super) body_id: hir::HirId,
 

compiler/rustc_typeck/src/collect.rs

@@ -94,7 +94,26 @@ pub fn provide(providers: &mut Providers) {
 ///////////////////////////////////////////////////////////////////////////
 
 /// Context specific to some particular item. This is what implements
-/// `AstConv`. It has information about the predicates that are defined
+/// [`AstConv`].
+///
+/// # `ItemCtxt` vs `FnCtxt`
+///
+/// `ItemCtxt` is primarily used to type-check item signatures and lower them
+/// from HIR to their [`ty::Ty`] representation, which is exposed using [`AstConv`].
+/// It's also used for the bodies of items like structs where the body (the fields)
+/// are just signatures.
+///
+/// This is in contrast to [`FnCtxt`], which is used to type-check bodies of
+/// functions, closures, and `const`s -- anywhere that expressions and statements show up.
+///
+/// An important thing to note is that `ItemCtxt` does no inference -- it has no [`InferCtxt`] --
+/// while `FnCtxt` does do inference.
+///
+/// [`FnCtxt`]: crate::check::fn_ctxt::FnCtxt
+///
+/// # Trait predicates
+///
+/// `ItemCtxt` has information about the predicates that are defined
 /// on the trait. Unfortunately, this predicate information is
 /// available in various different forms at various points in the
 /// process. So we can't just store a pointer to e.g., the AST or the