learning-journal

name	learning-journal
description	Help record Rust learning progress, mistakes, debug attempts, and concept gaps.

Learning Journal Skill

Use this skill to help the learner keep track of Rust concepts, mistakes, solved puzzles, and debug attempts.

Suggested files

journal/
├── solved-puzzles.md
├── debug-attempts.md
├── mistakes.md
├── rust-concepts-map.md
└── YYYY-MM-DD.md

What to record for forward puzzles

• puzzle id
• time spent
• failing tests
• compiler errors that were confusing
• Rust concepts involved
• final lesson learned
• whether the learner used hints or Agent help

What to record for bug labs

• bug id
• initial symptom
• first hypothesis
• actual root cause
• wrong fix attempted
• repair strategy
• compiler message learned
• test that proved the fix
• concept to review next

Do not

• Shame the learner for mistakes.
• Write vague motivational summaries.
• Record giant transcripts.
• Store private or unrelated content.
• Turn the journal into a hidden solution file.

Good forward entry shape

# 2026-05-09

## p20_borrowed_text_index

Problem:
Returned references were tied to a temporary `String`.

Compiler clue:
`returns a value referencing data owned by the current function`

Concept:
Returned `&str` slices must borrow from the input text, not from a local allocation.

Next review:
Redo a tokenizer puzzle using `&'a str`.

Good debug entry shape

# 2026-05-09

## b20_dangling_str_slice

Initial symptom:
Compiler rejected returning `&str`.

Wrong hypothesis:
Tried to add `'a` to the function signature.

Root cause:
The returned slice pointed into a local `String` created by `to_lowercase()`.

Repair:
Return `String`, because normalization requires allocation.

Lesson:
Lifetime annotations describe relationships. They do not extend the lifetime of owned local values.