Show inlay hints for type placeholders

With the extra InferenceResult that maps type placeholders to their
inferred type, we can now easily display inlay hints for them.

Author: nicolas-guichard

src/tools/rust-analyzer/crates/ide/src/inlay_hints.rs

@@ -40,6 +40,7 @@ mod implicit_static;
 mod implied_dyn_trait;
 mod lifetime;
 mod param_name;
+mod placeholders;
 mod ra_fixture;
 mod range_exclusive;
 
@@ -291,6 +292,10 @@ fn hints(
                     implied_dyn_trait::hints(hints, famous_defs, config, Either::Right(dyn_));
                     Some(())
                 },
+                ast::Type::InferType(placeholder) => {
+                    placeholders::type_hints(hints, famous_defs, config, display_target, placeholder);
+                    Some(())
+                },
                 _ => Some(()),
             },
             ast::GenericParamList(it) => bounds::hints(hints, famous_defs, config,  it),

src/tools/rust-analyzer/crates/ide/src/inlay_hints/placeholders.rs

@@ -0,0 +1,76 @@
+//! Implementation of type placeholder inlay hints:
+//! ```no_run
+//! let a = Vec<_> = vec![4];
+//!           //^ = i32
+//! ```
+
+use hir::DisplayTarget;
+use ide_db::famous_defs::FamousDefs;
+use syntax::{
+    AstNode,
+    ast::{InferType, Type},
+};
+
+use crate::{InlayHint, InlayHintPosition, InlayHintsConfig, InlayKind, inlay_hints::label_of_ty};
+
+pub(super) fn type_hints(
+    acc: &mut Vec<InlayHint>,
+    famous_defs @ FamousDefs(sema, _): &FamousDefs<'_, '_>,
+    config: &InlayHintsConfig<'_>,
+    display_target: DisplayTarget,
+    placeholder: InferType,
+) -> Option<()> {
+    if !config.type_hints {
+        return None;
+    }
+
+    let syntax = placeholder.syntax();
+    let range = syntax.text_range();
+
+    let ty = sema.resolve_type(&Type::InferType(placeholder))?;
+
+    let mut label = label_of_ty(famous_defs, config, &ty, display_target)?;
+    label.prepend_str("= ");
+
+    acc.push(InlayHint {
+        range,
+        kind: InlayKind::Type,
+        label,
+        text_edit: None,
+        position: InlayHintPosition::After,
+        pad_left: true,
+        pad_right: false,
+        resolve_parent: None,
+    });
+    Some(())
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{
+        InlayHintsConfig,
+        inlay_hints::tests::{DISABLED_CONFIG, check_with_config},
+    };
+
+    #[track_caller]
+    fn check_type_infer(#[rust_analyzer::rust_fixture] ra_fixture: &str) {
+        check_with_config(InlayHintsConfig { type_hints: true, ..DISABLED_CONFIG }, ra_fixture);
+    }
+
+    #[test]
+    fn inferred_types() {
+        check_type_infer(
+            r#"
+struct S<T>(T);
+
+fn foo() {
+    let t: (_, _, [_; _]) = (1_u32, S(2), [false] as _);
+          //^ = u32
+             //^ = S<i32>
+                 //^ = bool
+                                                   //^ = [bool; 1]
+}
+"#,
+        );
+    }
+}