Add docs

Author: camelid

Diff for: compiler/rustc_ast/src/ast.rs

@@ -124,7 +124,15 @@ impl Path {
         self.segments.first().is_some_and(|segment| segment.ident.name == kw::PathRoot)
     }
 
-    // FIXME: add docs
+    /// Check if this path is potentially a trivial const arg, i.e., one that can _potentially_
+    /// be represented without an anon const in the HIR.
+    ///
+    /// If `allow_mgca_arg` is true (as should be the case in most situations when
+    /// `#![feature(min_generic_const_args)]` is enabled), then this always returns true
+    /// because all paths are valid.
+    ///
+    /// Otherwise, it returns true iff the path has exactly one segment, and it has no generic args
+    /// (i.e., it is _potentially_ a const parameter).
     #[tracing::instrument(level = "debug", ret)]
     pub fn is_potential_trivial_const_arg(&self, allow_mgca_arg: bool) -> bool {
         allow_mgca_arg
@@ -1209,13 +1217,18 @@ pub struct Expr {
 }
 
 impl Expr {
-    // FIXME: update docs
-    /// Could this expr be either `N`, or `{ N }`, where `N` is a const parameter.
+    /// Check if this expression is potentially a trivial const arg, i.e., one that can _potentially_
+    /// be represented without an anon const in the HIR.
+    ///
+    /// This will unwrap at most one block level (curly braces). After that, if the expression
+    /// is a path, it mostly dispatches to [`Path::is_potential_trivial_const_arg`].
+    /// See there for more info about `allow_mgca_arg`.
     ///
-    /// If this is not the case, name resolution does not resolve `N` when using
-    /// `min_const_generics` as more complex expressions are not supported.
+    /// The only additional thing to note is that when `allow_mgca_arg` is false, this function
+    /// will only allow paths with no qself, before dispatching to the `Path` function of
+    /// the same name.
     ///
-    /// Does not ensure that the path resolves to a const param, the caller should check this.
+    /// Does not ensure that the path resolves to a const param/item, the caller should check this.
     /// This also does not consider macros, so it's only correct after macro-expansion.
     pub fn is_potential_trivial_const_arg(&self, allow_mgca_arg: bool) -> bool {
         let this = self.maybe_unwrap_block();

Diff for: tests/ui/feature-gates/feature-gate-min-generic-const-args.rs

@@ -4,7 +4,7 @@ trait Trait {
     const ASSOC: usize;
 }
 
-// FIXME(min_generic_const_args): add suggestion for mgca to this error
+// FIXME(mgca): add suggestion for mgca to this error
 fn foo<T: Trait>() -> [u8; <T as Trait>::ASSOC] {
     //~^ ERROR generic parameters may not be used in const operations
     loop {}