Merge 5bd2852ee5 into e6228e92b4

chore(from_into): Add missing period in docs

Cargo.lock

@@ -13,47 +13,48 @@ dependencies = [
 
 [[package]]
 name = "anstream"
-version = "0.6.13"
+version = "0.6.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d96bd03f33fe50a863e394ee9718a706f988b9079b20c3784fb726e7678b62fb"
+checksum = "418c75fa768af9c03be99d17643f93f79bbba589895012a80e3452a19ddda15b"
 dependencies = [
  "anstyle",
  "anstyle-parse",
  "anstyle-query",
  "anstyle-wincon",
  "colorchoice",
+ "is_terminal_polyfill",
  "utf8parse",
 ]
 
 [[package]]
 name = "anstyle"
-version = "1.0.6"
+version = "1.0.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8901269c6307e8d93993578286ac0edf7f195079ffff5ebdeea6a59ffb7e36bc"
+checksum = "038dfcf04a5feb68e9c60b21c9625a54c2c0616e79b72b0fd87075a056ae1d1b"
 
 [[package]]
 name = "anstyle-parse"
-version = "0.2.3"
+version = "0.2.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c75ac65da39e5fe5ab759307499ddad880d724eed2f6ce5b5e8a26f4f387928c"
+checksum = "c03a11a9034d92058ceb6ee011ce58af4a9bf61491aa7e1e59ecd24bd40d22d4"
 dependencies = [
  "utf8parse",
 ]
 
 [[package]]
 name = "anstyle-query"
-version = "1.0.2"
+version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e28923312444cdd728e4738b3f9c9cac739500909bb3d3c94b43551b16517648"
+checksum = "ad186efb764318d35165f1758e7dcef3b10628e26d41a44bc5550652e6804391"
 dependencies = [
  "windows-sys 0.52.0",
 ]
 
 [[package]]
 name = "anstyle-wincon"
-version = "3.0.2"
+version = "3.0.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1cd54b81ec8d6180e24654d0b371ad22fc3dd083b6ff8ba325b72e00c87660a7"
+checksum = "61a38449feb7068f52bb06c12759005cf459ee52bb4adc1d5a7c4322d716fb19"
 dependencies = [
  "anstyle",
  "windows-sys 0.52.0",
@@ -61,9 +62,9 @@ dependencies = [
 
 [[package]]
 name = "anyhow"
-version = "1.0.82"
+version = "1.0.86"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f538837af36e6f6a9be0faa67f9a314f8119e4e4b5867c6ab40ed60360142519"
+checksum = "b3d1d046238990b9cf5bcde22a3fb3584ee5cf65fb2765f454ed428c7a0063da"
 
 [[package]]
 name = "assert_cmd"
@@ -82,9 +83,9 @@ dependencies = [
 
 [[package]]
 name = "autocfg"
-version = "1.2.0"
+version = "1.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f1fdabc7756949593fe60f30ec81974b613357de856987752631dea1e3394c80"
+checksum = "0c4b4d0bd25bd0b74681c0ad21497610ce1b7c91b1022cd21c80c6fbdd9476b0"
 
 [[package]]
 name = "bitflags"
@@ -94,9 +95,9 @@ checksum = "bef38d45163c2f1dde094a7dfd33ccf595c92905c8f8f4fdc18d06fb1037718a"
 
 [[package]]
 name = "bitflags"
-version = "2.5.0"
+version = "2.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cf4b9d6a944f767f8e5e0db018570623c85f3d925ac718db4e06d0187adb21c1"
+checksum = "b048fb63fd8b5923fc5aa7b340d8e156aec7ec02f0c78fa8a6ddc2613f6f71de"
 
 [[package]]
 name = "bstr"
@@ -117,9 +118,9 @@ checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
 
 [[package]]
 name = "clap"
-version = "4.5.4"
+version = "4.5.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "90bc066a67923782aa8515dbaea16946c5bcc5addbd668bb80af688e53e548a0"
+checksum = "5db83dced34638ad474f39f250d7fea9598bdd239eaced1bdf45d597da0f433f"
 dependencies = [
  "clap_builder",
  "clap_derive",
@@ -127,9 +128,9 @@ dependencies = [
 
 [[package]]
 name = "clap_builder"
-version = "4.5.2"
+version = "4.5.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ae129e2e766ae0ec03484e609954119f123cc1fe650337e155d03b022f24f7b4"
+checksum = "f7e204572485eb3fbf28f871612191521df159bc3e15a9f5064c66dba3a8c05f"
 dependencies = [
  "anstream",
  "anstyle",
@@ -139,9 +140,9 @@ dependencies = [
 
 [[package]]
 name = "clap_derive"
-version = "4.5.4"
+version = "4.5.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "528131438037fd55894f62d6e9f068b8f45ac57ffa77517819645d10aed04f64"
+checksum = "c780290ccf4fb26629baa7a1081e68ced113f1d3ec302fa5948f1c381ebf06c6"
 dependencies = [
  "heck",
  "proc-macro2",
@@ -151,15 +152,15 @@ dependencies = [
 
 [[package]]
 name = "clap_lex"
-version = "0.7.0"
+version = "0.7.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "98cc8fbded0c607b7ba9dd60cd98df59af97e84d24e49c8557331cfc26d301ce"
+checksum = "4b82cf0babdbd58558212896d1a4272303a57bdb245c2bf1147185fb45640e70"
 
 [[package]]
 name = "colorchoice"
-version = "1.0.0"
+version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "acbf1af155f9b9ef647e42cdc158db4b64a1b61f743629225fde6f3e0be2a7c7"
+checksum = "0b6a852b24ab71dffc585bcb46eaf7959d175cb865a7152e35b348d1b2960422"
 
 [[package]]
 name = "console"
@@ -176,18 +177,18 @@ dependencies = [
 
 [[package]]
 name = "crossbeam-channel"
-version = "0.5.12"
+version = "0.5.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ab3db02a9c5b5121e1e42fbdb1aeb65f5e02624cc58c43f2884c6ccac0b82f95"
+checksum = "33480d6946193aa8033910124896ca395333cae7e2d1113d1fef6c3272217df2"
 dependencies = [
  "crossbeam-utils",
 ]
 
 [[package]]
 name = "crossbeam-utils"
-version = "0.8.19"
+version = "0.8.20"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "248e3bacc7dc6baa3b21e405ee045c3047101a49145e7e9eca583ab4c2ca5345"
+checksum = "22ec99545bb0ed0ea7bb9b8e1e9122ea386ff8a48c0922e43f36d45ab09e0e80"
 
 [[package]]
 name = "difflib"
@@ -203,9 +204,9 @@ checksum = "fea41bba32d969b513997752735605054bc0dfa92b4c56bf1189f2e174be7a10"
 
 [[package]]
 name = "either"
-version = "1.11.0"
+version = "1.13.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a47c1c47d2f5964e29c61246e81db715514cd532db6b5116a25ea3c03d6780a2"
+checksum = "60b1af1c220855b6ceac025d3f6ecdd2b7c4894bfe9cd9bda4fbb4bc7c0d4cf0"
 
 [[package]]
 name = "encode_unicode"
@@ -221,9 +222,9 @@ checksum = "5443807d6dff69373d433ab9ef5378ad8df50ca6298caf15de6e52e24aaf54d5"
 
 [[package]]
 name = "errno"
-version = "0.3.8"
+version = "0.3.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a258e46cdc063eb8519c00b9fc845fc47bcfca4130e2f08e88665ceda8474245"
+checksum = "534c5cf6194dfab3db3242765c03bbe257cf92f22b38f6bc0c58d59108a820ba"
 dependencies = [
  "libc",
  "windows-sys 0.52.0",
@@ -267,9 +268,9 @@ checksum = "d2fabcfbdc87f4758337ca535fb41a6d701b65693ce38287d856d1674551ec9b"
 
 [[package]]
 name = "hashbrown"
-version = "0.14.3"
+version = "0.14.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "290f1a1d9242c78d09ce40a5e87e7554ee637af1351968159f4952f028f75604"
+checksum = "e5274423e17b7c9fc20b6e7e208532f9b19825d82dfd615708b70edd83df41f1"
 
 [[package]]
 name = "heck"
@@ -331,13 +332,19 @@ dependencies = [
 
 [[package]]
 name = "instant"
-version = "0.1.12"
+version = "0.1.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7a5bbe824c507c5da5956355e86a746d82e0e1464f65d862cc5e71da70e94b2c"
+checksum = "e0242819d153cba4b4b05a5a8f2a7e9bbf97b6055b2a002b395c96b5ff3c0222"
 dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "is_terminal_polyfill"
+version = "1.70.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f8478577c03552c21db0e2724ffb8986a5ce7af88107e6be5d2ee6e158c12800"
+
 [[package]]
 name = "itoa"
 version = "1.0.11"
@@ -366,21 +373,21 @@ dependencies = [
 
 [[package]]
 name = "lazy_static"
-version = "1.4.0"
+version = "1.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
+checksum = "bbd2bcb4c963f2ddae06a2efc7e9f3591312473c50c6685e1f298068316e66fe"
 
 [[package]]
 name = "libc"
-version = "0.2.153"
+version = "0.2.155"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9c198f91728a82281a64e1f4f9eeb25d82cb32a5de251c6bd1b5154d63a8e7bd"
+checksum = "97b3888a4aecf77e811145cadf6eef5901f4782c53886191b2f693f24761847c"
 
 [[package]]
 name = "linux-raw-sys"
-version = "0.4.13"
+version = "0.4.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "01cda141df6706de531b6c46c3a33ecca755538219bd484262fa09410c13539c"
+checksum = "78b3ae25bc7c8c38cec158d1f2757ee79e9b3740fbc7ccf0e59e4b08d793fa89"
 
 [[package]]
 name = "log"
@@ -390,9 +397,9 @@ checksum = "90ed8c1e510134f979dbc4f070f87d4313098b704861a105fe34231c70a3901c"
 
 [[package]]
 name = "memchr"
-version = "2.7.2"
+version = "2.7.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6c8640c5d730cb13ebd907d8d04b52f55ac9a2eec55b440c8892f40d56c76c1d"
+checksum = "78ca9ab1a0babb1e7d5695e3530886289c18cf2f87ec19a575a0abdce112e3a3"
 
 [[package]]
 name = "mio"
@@ -418,7 +425,7 @@ version = "6.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6205bd8bb1e454ad2e27422015fb5e4f2bcc7e08fa8f27058670d208324a4d2d"
 dependencies = [
- "bitflags 2.5.0",
+ "bitflags 2.6.0",
  "crossbeam-channel",
  "filetime",
  "fsevent-sys",
@@ -444,9 +451,9 @@ dependencies = [
 
 [[package]]
 name = "num-traits"
-version = "0.2.18"
+version = "0.2.19"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "da0df0e5185db44f69b44f26786fe401b6c293d1907744beaa7fa62b2e5a517a"
+checksum = "071dfc062690e90b734c0b2273ce72ad0ffa95f0c74596bc250dcfd960262841"
 dependencies = [
  "autocfg",
 ]
@@ -495,9 +502,9 @@ dependencies = [
 
 [[package]]
 name = "proc-macro2"
-version = "1.0.81"
+version = "1.0.86"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3d1597b0c024618f09a9c3b8655b7e430397a36d23fdafec26d6965e9eec3eba"
+checksum = "5e719e8df665df0d1c8fbfd238015744736151d4445ec0836b8e628aae103b77"
 dependencies = [
  "unicode-ident",
 ]
@@ -522,9 +529,9 @@ dependencies = [
 
 [[package]]
 name = "regex"
-version = "1.10.4"
+version = "1.10.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c117dbdfde9c8308975b6a18d71f3f385c89461f7b3fb054288ecf2a2058ba4c"
+checksum = "b91213439dad192326a0d7c6ee3955910425f441d7038e0d6933b0aec5c4517f"
 dependencies = [
  "aho-corasick",
  "memchr",
@@ -534,9 +541,9 @@ dependencies = [
 
 [[package]]
 name = "regex-automata"
-version = "0.4.6"
+version = "0.4.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "86b83b8b9847f9bf95ef68afb0b8e6cdb80f498442f5179a29fad448fcc1eaea"
+checksum = "38caf58cc5ef2fed281f89292ef23f6365465ed9a41b7a7754eb4e26496c92df"
 dependencies = [
  "aho-corasick",
  "memchr",
@@ -545,17 +552,17 @@ dependencies = [
 
 [[package]]
 name = "regex-syntax"
-version = "0.8.3"
+version = "0.8.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "adad44e29e4c806119491a7f06f03de4d1af22c3a680dd47f1e6e179439d1f56"
+checksum = "7a66a03ae7c801facd77a29370b4faec201768915ac14a721ba36f20bc9c209b"
 
 [[package]]
 name = "rustix"
-version = "0.38.32"
+version = "0.38.34"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "65e04861e65f21776e67888bfbea442b3642beaa0138fdb1dd7a84a52dffdb89"
+checksum = "70dc5ec042f7a43c4a73241207cecc9873a06d45debb38b329f8541d85c2730f"
 dependencies = [
- "bitflags 2.5.0",
+ "bitflags 2.6.0",
  "errno",
  "libc",
  "linux-raw-sys",
@@ -584,9 +591,9 @@ dependencies = [
 
 [[package]]
 name = "ryu"
-version = "1.0.17"
+version = "1.0.18"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e86697c916019a8588c99b5fac3cead74ec0b4b819707a682fd4d23fa0ce1ba1"
+checksum = "f3cb5ba0dc43242ce17de99c180e96db90b235b8a9fdc9543c96d2209116bd9f"
 
 [[package]]
 name = "same-file"
@@ -599,18 +606,18 @@ dependencies = [
 
 [[package]]
 name = "serde"
-version = "1.0.198"
+version = "1.0.203"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9846a40c979031340571da2545a4e5b7c4163bdae79b301d5f86d03979451fcc"
+checksum = "7253ab4de971e72fb7be983802300c30b5a7f0c2e56fab8abfc6a214307c0094"
 dependencies = [
  "serde_derive",
 ]
 
 [[package]]
 name = "serde_derive"
-version = "1.0.198"
+version = "1.0.203"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e88edab869b01783ba905e7d0153f9fc1a6505a96e4ad3018011eedb838566d9"
+checksum = "500cbc0ebeb6f46627f50f3f5811ccf6bf00643be300b4c3eabc0ef55dc5b5ba"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -619,9 +626,9 @@ dependencies = [
 
 [[package]]
 name = "serde_json"
-version = "1.0.116"
+version = "1.0.118"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3e17db7126d17feb94eb3fad46bf1a96b034e8aacbc2e775fe81505f8b0b2813"
+checksum = "d947f6b3163d8857ea16c4fa0dd4840d52f3041039a85decd46867eb1abef2e4"
 dependencies = [
  "itoa",
  "ryu",
@@ -630,9 +637,9 @@ dependencies = [
 
 [[package]]
 name = "serde_spanned"
-version = "0.6.5"
+version = "0.6.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "eb3622f419d1296904700073ea6cc23ad690adbd66f13ea683df73298736f0c1"
+checksum = "79e674e01f999af37c49f70a6ede167a8a60b2503e56c5599532a65baa5969a0"
 dependencies = [
  "serde",
 ]
@@ -651,9 +658,9 @@ checksum = "7da8b5736845d9f2fcb837ea5d9e2628564b3b043a70948a3f0b778838c5fb4f"
 
 [[package]]
 name = "syn"
-version = "2.0.60"
+version = "2.0.68"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "909518bc7b1c9b779f1bbf07f2929d35af9f0f37e47c6e9ef7f9dddc1e1821f3"
+checksum = "901fa70d88b9d6c98022e23b4136f9f3e54e4662c3bc1bd1d84a42a9a0f0c1e9"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -668,18 +675,18 @@ checksum = "3369f5ac52d5eb6ab48c6b4ffdc8efbcad6b89c765749064ba298f2c68a16a76"
 
 [[package]]
 name = "toml_datetime"
-version = "0.6.5"
+version = "0.6.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3550f4e9685620ac18a50ed434eb3aec30db8ba93b0287467bca5826ea25baf1"
+checksum = "4badfd56924ae69bcc9039335b2e017639ce3f9b001c393c1b2d1ef846ce2cbf"
 dependencies = [
  "serde",
 ]
 
 [[package]]
 name = "toml_edit"
-version = "0.22.11"
+version = "0.22.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "fb686a972ccef8537b39eead3968b0e8616cb5040dbb9bba93007c8e07c9215f"
+checksum = "f21c7aaf97f1bd9ca9d4f9e73b0a6c74bd5afef56f2bc931943a6e1c37e04e38"
 dependencies = [
  "indexmap",
  "serde",
@@ -696,15 +703,15 @@ checksum = "3354b9ac3fae1ff6755cb6db53683adb661634f67557942dea4facebec0fee4b"
 
 [[package]]
 name = "unicode-width"
-version = "0.1.11"
+version = "0.1.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e51733f11c9c4f72aa0c160008246859e340b00807569a0da0e7a1079b27ba85"
+checksum = "0336d538f7abc86d282a4189614dfaa90810dfc2c6f6427eaf88e16311dd225d"
 
 [[package]]
 name = "utf8parse"
-version = "0.2.1"
+version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "711b9620af191e0cdc7468a8d14e709c3dcdb115b36f838e601583af800a370a"
+checksum = "06abde3611657adf66d383f00b093d7faecc7fa57071cce2578660c9f1010821"
 
 [[package]]
 name = "wait-timeout"
@@ -743,37 +750,15 @@ dependencies = [
  "winsafe",
 ]
 
-[[package]]
-name = "winapi"
-version = "0.3.9"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5c839a674fcd7a98952e593242ea400abe93992746761e38641405d28b00f419"
-dependencies = [
- "winapi-i686-pc-windows-gnu",
- "winapi-x86_64-pc-windows-gnu",
-]
-
-[[package]]
-name = "winapi-i686-pc-windows-gnu"
-version = "0.4.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ac3b87c63620426dd9b991e5ce0329eff545bccbbb34f3be09ff6fb6ab51b7b6"
-
 [[package]]
 name = "winapi-util"
-version = "0.1.6"
+version = "0.1.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f29e6f9198ba0d26b4c9f07dbe6f9ed633e1f3d5b8b414090084349e46a52596"
+checksum = "4d4cc384e1e73b93bafa6fb4f1df8c41695c8a91cf9c4c64358067d15a7b6c6b"
 dependencies = [
- "winapi",
+ "windows-sys 0.52.0",
 ]
 
-[[package]]
-name = "winapi-x86_64-pc-windows-gnu"
-version = "0.4.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
-
 [[package]]
 name = "windows-sys"
 version = "0.48.0"
@@ -915,9 +900,9 @@ checksum = "bec47e5bfd1bff0eeaf6d8b485cc1074891a197ab4225d504cb7a1ab88b02bf0"
 
 [[package]]
 name = "winnow"
-version = "0.6.6"
+version = "0.6.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f0c976aaaa0e1f90dbb21e9587cdaf1d9679a1cde8875c0d6bd83ab96a208352"
+checksum = "59b5e5f6c299a3c7890b876a2a587f3115162487e704907d9b6cd29473052ba1"
 dependencies = [
  "memchr",
 ]

Cargo.toml

@@ -9,17 +9,17 @@ authors = [
 edition = "2021"
 
 [dependencies]
-anyhow = "1.0.82"
-clap = { version = "4.5.4", features = ["derive"] }
+anyhow = "1.0.86"
+clap = { version = "4.5.7", features = ["derive"] }
 console = "0.15.8"
 indicatif = "0.17.8"
 notify-debouncer-mini = "0.4.1"
-serde_json = "1.0.116"
-serde = { version = "1.0.198", features = ["derive"] }
+serde_json = "1.0.118"
+serde = { version = "1.0.203", features = ["derive"] }
 shlex = "1.3.0"
-toml_edit = { version = "0.22.11", default-features = false, features = ["parse", "serde"] }
+toml_edit = { version = "0.22.14", default-features = false, features = ["parse", "serde"] }
 which = "6.0.1"
-winnow = "0.6.6"
+winnow = "0.6.13"
 
 [[bin]]
 name = "rustlings"

exercises/19_smart_pointers/refcell1.rs

@@ -0,0 +1,52 @@
+// refcell1.rs
+//
+// Interior mutability is a design pattern in Rust that allows you to mutate 
+// data even when there are immutable references to that data; 
+// normally, this action is disallowed by the borrowing rules.  
+
+// The RefCell<T> type represents single ownership over the data it holds.
+// Recall the borrowing rules in Rust:
+//      1. At any given time, you can have either (but not both) one mutable 
+//      reference or any number of immutable references.
+//      2. References must always be valid.
+
+// With references and Box<T>, the borrowing rules’ invariants are enforced at 
+// compile time. With RefCell<T>, these invariants are enforced at runtime. 
+// With references, if you break these rules, you’ll get a compiler error. 
+// With RefCell<T>, if you break these rules, your program will panic and exit.
+// The RefCell<T> type is useful when you’re sure your code follows the 
+// borrowing rules but the compiler is unable to understand and guarantee that.
+
+// I AM NOT DONE
+
+use std::cell::RefCell;
+
+#[derive(Debug)]
+#[allow(dead_code)]
+struct User {
+    name: RefCell<String>,
+}
+
+#[allow(dead_code)]
+impl User {
+    fn name(&self) -> String {
+        self.name.borrow().to_string()
+    }
+
+    // Note: do not use &mut self!
+    fn set_name(&self, name: String) {
+        todo!()
+    }
+}
+
+fn main() {
+    let u = User {
+        name: RefCell::new("Alice".to_string()),
+    };
+    println!("My name is {}!", *u.name.borrow());
+
+    let new_name = "Bob".to_string();
+    u.set_name(new_name.clone());
+
+    println!("My name is {}!", *u.name.borrow());
+}

exercises/23_conversions/from_into.rs

@@ -39,7 +39,7 @@ impl Default for Person {
 // 5. Extract the other element from the split operation and parse it into a
 //    `usize` as the age.
 // If while parsing the age, something goes wrong, then return the default of
-// Person Otherwise, then return an instantiated Person object with the results
+// Person. Otherwise, then return an instantiated Person object with the results
 
 // I AM NOT DONE
 

info.toml

@@ -318,7 +318,7 @@ name = "move_semantics1"
 path = "exercises/06_move_semantics/move_semantics1.rs"
 mode = "test"
 hint = """
-So you've got the "cannot borrow immutable local variable `vec` as mutable"
+So you've got the "cannot borrow `vec` as mutable, as it is not declared as mutable"
 error on the line where we push an element to the vector, right?
 
 The fix for this is going to be adding one keyword, and the addition is NOT on
@@ -1103,6 +1103,20 @@ Check out https://doc.rust-lang.org/std/borrow/enum.Cow.html for documentation
 on the `Cow` type.
 """
 
+[[exercises]]
+name = "refcell1"
+path = "exercises/19_smart_pointers/refcell1.rs"
+mode = "compile"
+hint = """
+Remember that RefCell<T> allows for an immutable object to be modified.
+
+Use the .borrow_mut() method on the RefCell to get a mutable reference to
+the underlying data.
+
+See https://doc.rust-lang.org/book/ch15-05-interior-mutability.html for more
+information on RefCell.
+"""
+
 # THREADS
 
 [[exercises]]